Floor Plan of the Pantheon, Rome

Floor Plan of the Pantheon, Rome

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Floor Plan of the Pantheon, Rome - History

Having been built between 118-128 AD, the Pantheon possesses architectural features that were popular during its construction, while also maintaining its own uniqueness. The porch and the intermediate block assume a Greek style, with an entablature resting on sixteen columns. After passing through the portico, one encounters the large rotunda that follows a Roman style because the large dome is supported by exerting strain on the walls of the cylinder on which it rests. The particular design of the Pantheon, including the unification of Greek and Roman style, has led to speculation as to who the architect of the Pantheon was. While conclusive evidence of the identity of the architect has yet to be found, some believe Hadrian might have designed the entire building. Hadrian had a strong interest in architecture, and had a love for both Greek and Roman culture. Thus, the Pantheon symbolizes his attempt to combine both cultures’ architectural styles in one building.

The building of the Pantheon would have been a huge undertaking. In total, five thousand tons of concrete were used to build the rotunda by pouring successive rings of concrete into a previously constructed wooden framework. The walls of the cylinder were six meters wide to support the strain of the entire dome on the foundation. The height and diameter of the interior rotunda both measure 43.3 m the implication of this is that a perfect sphere with a same diameter would fit just perfectly inside the rotunda. The oculus, or opening at the top of the dome, measures 8.8 m across and significantly lightens the load on the foundation of the structure. It is also in keeping with the belief that there should not be a roof on a Roman temple. Serving as the major source of light in the Pantheon, the oculus also allows in rain and snow, setting a different atmosphere throughout the seasons. The floor is sloped towards drains that are present to collect rain. Blind windows line the rotunda, probably meant to let light into the extensive network of passageways that are used by maintenance crews. William MacDonald, a Pantheon expert, believes that the windows also allow the building to breathe by circulating air to prevent moisture collection that could cause cracks in the cast cement. The marble work on the floor containing patterns of circles and squares is a 19th century accurate reproduction of the original floor.

When observing the Pantheon from outside, the columns play a significant role in adding to the grandeur. The sixteen monolithic columns are made of red and gray granite and the shafts stand 40 Roman feet tall. Carved in eastern Egypt, the transport of the columns to the construction site required them to be floated up the Nile River on a barge, through Mediterranean Sea and up the Tiber River. Once they reached Rome, they were carried down the streets of the city and then erected. Three of the columns on the east side of the building fell, and were replaced by the Pope Urban VIII and Alexander VII. The columns of the Pantheon have prompted a lot of discussion because scholars believe that if the columns had only been 10 Roman feet taller, they would have allowed for continuity between the porch and intermediate block that is lacking in the current structure. Certainly 50 Roman foot monolithic columns were considerably more difficult to acquire it is quite possible that the larger columns were instead used for the Temple of Trajan, which was being built by Hadrian around the same time for his adoptive father. Problems with obtaining larger columns may have thus prompted the architect of the Pantheon to compromise and use smaller columns. Politically, it would have been important for Hadrian to devote the larger columns to the Temple of Trajan to show respect for Trajan, especially because the size of the columns was very important to the Temple of Trajan since it dictated the size of the entire building, whereas it was not as crucial to the structure of the Pantheon.

When it was first built, the entire exterior of the dome, as well as the interior of the coffered ceiling, would have been covered in bronze. However, some of the bronze was removed to make the 80 cannons at Castel Santपngelo, Hadrian’s mausoleum, but was eventually returned when it was melted down for the tomb of Vittorio Emanuele II, which now rests at the Pantheon. Some more of the bronze was pilfered by the Goths, but most of it was taken by the Barberini Pope Urban VIII, prompting the expression “what the barbarians did not do, the Barberini did.”

Many aspects of the exterior as evident today would have been very different when the Pantheon was first built. The brickwork covering the outer wall of the rotunda would have been covered in stucco, marble paneling, or even travertine. Currently, the Pantheon sits somewhat sunken into the floor because the street level has risen around the building. Originally, the Pantheon would have sat high above street level, with five steep stairs used to reach it.

The UW KnowledgeWorks software used to create this site was developed by the The Program for Educational Transformation Through Technology at the University of Washington.

Dimensions of the Pantheon

The giant dome that dominates the interior is 43.30 meters or 142 feet in diameter (for comparison, the White House dome is 96 feet in diameter). The Pantheon stood as the largest dome ever until Brunelleschi's dome at the Florence Cathedral of 1420-36. It's still the largest masonry dome in the world. The Pantheon is made perfectly harmonious by the fact that the distance from the floor to the top of the dome is exactly equal to its diameter. Adytons (shrines recessed into the wall) and coffers (sunken panels) cleverly reduce the weight of the dome, as did a lightweight cement made of pumice used in the upper levels. The dome gets thinner as it approaches the oculus, the hole in the top of the dome used as a light source for the interior. The thickness of the dome at that point is only 1.2 meters.

The oculus is 7.8 meters in diameter. Yes, rain and snow occasionally fall through it, but the floor is slanted and drains cleverly remove the water if it manages to hit the floor. In practice, rain seldom falls inside the dome.

The massive columns supporting the portico weigh 60 tons. Each was 39 feet (11.8 m) tall, five feet (1.5 m) in diameter and made from stone quarried in Egypt. The columns were transported by wooden sledges to the Nile, barged to Alexandria, and put on vessels for a trip across the Mediterranean to the port of Ostia. From there the columns came up the Tiber by barge.


Front, exterior view of the Pantheon. Shows architecture influenced by traditional Greek temples, including colonnade holding up the triangular pediment.

At the front of the Pantheon, sixteen, monolithic columns form the monument’s well-known portico. The shafts (cylindrical part of the column) are made of Egyptian granite, while the capitals (decorative top of the column) and bases were carved from white Greek marble. The Corinthian capital gives the structure an intricate, decorative quality that starkly contrasts with the smooth, heavy shaft below.

Interestingly enough, studies showed that the columns’ heights and widths vary from column to column. Research has proved that the difference in measurements were most likely because they outsourced materials and labor from several locations. The columns, which had to be shipped overseas, ended up shorter by 10 feet and had to be accommodated by a lower porch. 1 Since the projects were distributed to different constructors, builders expected to accommodate some inconsistencies. For the Pantheon, such adjustments included varying shaft heights and widths. 2 After the drum was constructed, the rectangular intermediate block was created to connect the circular part of the structure to the temple-like porch. 3 In antiquity, construction involved refinement and compensation, especially when creating a monumental structure like the Pantheon.

Drawing showing the exterior and cross-section of the Pantheon.

The Pantheon’s drum, is a massive, cylindrical structure that forms the bulk of the monument. Though made of concrete, it is deceivingly light — the thicker sections of the wall accommodate empty, semicircular spaces. 4 The drum’s solid appearance conceals its hollow interiors while maintaining its strength. The Romans invented and utilized a system of interlocking brick arches, vaults, and piers to enable the drum’s even weight distribution and support. From the outside, the monument seems dense but allows a hollow interior: this Roman technique can be seen in Trajan’s Column, which accommodates a spiral staircase within its shaft. Some historians have used this parallel to Trajan’s Column as support for the Pantheon’s stronger association with Trajan, rather than Hadrian. 5

The dome itself is created by overlapping barrel vaults over the third-story chambers this technique relates back to the aforementioned octagonal plan of Nero’s palace. The drum and step-rings (the stacked, shallow cylinders at the base of the dome) support around 65 percent of the dome’s weight. 6 The dome’s careful weight distribution minimized the amount of its unsupported weight, strengthening and stabilizing the overall structure. The dome is still covered in plaster, but the exterior, which was originally protected with bronze plates, was eventually replaced with lead. 7 The only remaining bronze functions as a protective support around the oculus.

1 Mark Jones, "Building on Adversity," in The Pantheon, ed. Marder and Jones, (New York, NY: Cambridge UP), 220 .

2 Lothar Haselberger. "The human eye and 3D laser scanning: the Pantheon's facade and its capitals," in Journal of Roman Archaeology, (Rhode Island: Journal of Roman Archaeology, 2015), 54.

4 Giangiacomo Martines, "The Conception and Construction of the Drum and Dome," in The Pantheon, ed. Marder and Jones, (New York, NY: Cambridge UP), 106 .

The Pantheon

This famous building stands in the business district of Rome--much as it was built some 18 centuries ago. Amazingly, it has withstood the ravages of both the elements and war permitting a firsthand view of a unique product constructed by Roman hands. Now, it is exposed to acid rain and fumes from passing automobiles and overshadowed by buildings of inferior taste but, with trust in the future, the Pantheon will survive.

Unrecognized, the design of this ancient concrete building reveals unparalleled features not encountered in modern design standards. Recent studies reveal several major cracks in the dome, but it still functions unimpaired. This condition will surely excite the curiosity of our structural engineers. The building was built entirely without steel reinforcing rods to resist tensile cracking, so necessary in concrete members, and for this concrete dome with a long span to last centuries is incredible. Today, no engineer would dare build this structure without steel rods! Modern codes of engineering practice would not permit such mischief. No investor with knowledge of concrete design would provide the funding. Additional constraints when attempting to build a structure as large as the Pantheon will be discussed later, but briefly they include the use of inadequate hand tools and unsafe lifting devices. I believe we can learn from this activity. Workers can build from a plan and can successfully use their proven practices only if construction quality controls are maintained.

History tells us that the Pantheon is a Greek word meaning to honor all Gods (particularly the Olympian divinities). It is ironic that our building has existed throughout many wars while being dedicated to all Gods one can readily perceive this to be a temple for our one God. And, the Church has claimed this holy structure as a resting place for its most famous Popes, so we continue to honor its magnificent divinity.

The first incarnation of this ancient temple was built by Agrippa, the son-in-law of the Roman Emperor Augustus, about 27 B.C. Today, above the entrance carved in stone are the words "M. AGRIPPA L. F. COS. TERTIUM FECIT" which is translated, "Marcus Agrippa, son of Lucius, in his third consulate, made it." Indeed, it is worth mentioning that Agrippa's engineering talents were used in building the famous Pont de Gard aqueduct in France.

As with many cities, tragedy in the form of large firessuch as those of 60, 64, 79, 100 and 110 A.D.seemed to strike Rome. Originally, many Roman buildings contained travertine (limestone rock) which easily cracked in fires. The first Pantheon was severely damaged and required replacement except for some parts of the lower porch section and foundation.

The Pantheon was rebuilt by the Emperor Hadrian during the period 118 to 128 A.D. (a time given by Ward-Perkins). 2 But the Ward-Perkins's period is disputed by, Lugli who said the building was started sometime after 123 A.D. and was finished by Emperor Pius about 140 A.D. 3 However, most of the bricks were made and placed in the Pantheon in 123 A.D., a date that the maker stamped on his bricks. This was discovered in 1892 by the French archaeologist, George Chedanne. It appears the construction of the rotunda walls took a period of 4 to 5 years, and the dome required a like period because of its height and the meager tools the Romans used. This long construction period was fortunate as it gave this pozzolan concrete ample time to cure and gain strength.

Was the second temple like the first? Yes, the fundamental principle of the old Roman religion required that the temples be rebuilt without changes in original form. Tradition required that the main entrance face north, and thus the whole building was oriented on the north-south axis of the building.

A description of its structural features is separated into the configuration, foundation ring, circular walls, and dome to more clearly define various components. How these pieces are unique in view of today's design requirements will be discussed shortly.

Michelangelo the great painter of the Sistine chapel once described the design of the Pantheon as an "Angelic and not human design." 4 Rightly so, for it is indeed one of the most unusual structures ever built by human hands. The ancient Roman's ability to draw the intricate plans and select only the most successful time-proven construction techniques made this complex building possible. Again, it is truly a credit to their mental prowess and organizational skills. The following pictures show the beautiful interior.

The building design is one of a large round shape very much like a large barrel with a dome covering the top. There is a light-well in the center of the dome. Layers of beautiful thin brickwork cover the outside, round walls. Small access holes appear occasionally in the wall which were used during construction to frame interior voids. The main entrance is thoroughly impressive: double bronze doors 21 feet high (6.4 meters), a lasting and fitting contribution from their metal smiths. These doors are protected by a high, broad porch, made with 16 well arranged granite columns supporting a gable styled roof. The beams in the roof structure of the porch are wooden. They were substituted for bronze members stripped-out by those in later years needing metal for their canons. Professional Roman surveyors located the inlaid marble floor to conform with a convex contour which drained away the rain from the oculus for these hundreds of years.

In the following descriptions, some general dimensions are given to indicate the magnitude of this undertaking by the Romans. The rotunda has a rather awesome inner diameter of 142.4 feet (43.4 m), made mostly of concrete. Comparatively speaking, this distance represents about one half the length of our football field. And from the floor to the top of the opening in the dome is the same distance. As a matter of fact, we could think of the design of this building as one that could contain a theoretical ball some 143 feet in diameter. The design is not entirely unusual because there are other Roman buildings which have a similar configuration, but the size is unusual. Other buildings such as the Temple of Mercury (71 feet/21.5 m diameter) at Baiae and Domitian Nympheaum at Albano (51 feet/15.6 m diameter) have domes of this type. The Pantheon still has the longest span constructed before the 19th Century.

To provide details on this complex configuration, the following figures show the building with its two-ring foundation, voids in the walls, and the step-ring and coffer arrangement in the dome.

Pantheon Sections (left picture:Ward-Perkins 6 , right picture: MacDonald 7 )

The Pantheon was built on marshy, unstable earth which gave a serious supporting problem to its builders. The Jutland Archaeological Society described in detail various aspects of the ring foundation they found it rested on a bed of bluish colored river clay. 8 This condition invited disaster, and in the final construction phase, the foundation cracked at the two ends of the North-South axis. 9

As you can imagine, if one section of a building settles slightly faster and lower than an adjacent section, very large bending stresses are initiated at a point between these two sections which can crack the concrete. And uneven settling was the problem given to the builders. The present-day engineering solution to this type of foundation problem is to drive piles through the clay to bedrock so the building will be firmly supported all the way around. The Roman builders chose a different approach. They built a second ring to hold the first ring from cracking further and to give the clay more area to support the structure. It worked because the building has lasted over 1800 years.

In addition to keeping the crack from extending, the builders placed buttress walls on the south side opposite the massive porch. This acted as a clamping device and although the structural projection appears to be an additional room, it only serves the purpose of being part of the clamp.

Initially, the width of this ring foundation was 23'-7" (7.2 m) wide, only about 3 feet (0.9 m) greater than the walls it supported. The second ring that binds the original together is 10 feet (3.0 m) wide making the total width of the foundation about 34 feet. From the floor level to the bottom of the foundation is 15'-4" (4.7 m). 10

These rings are made of pozzolan concrete consisting of travertine pieces in layers held together by a mortar of lime and pozzolan. This will be discussed later in this work. Interestingly enough, the Jutland Society's investigation showed the foundation material had become "rock hard," 11 a case we might expect when we study the chemistry of pozzolanic reaction under these conditions.

The round wall may best be described as one containing many cavities and chambers on different levels. There is no evidence that a staircase system existed between these upper chambers, and we can assume their function along with other niches was to reduce construction materials together with the weight. This wall can be thought of structurally as a series of concrete piers separated at floor level by 8 very large niches equally spaced along the inner perimeter. The thick wall acts much like a buttress in supporting a thrust from the dome.

To locate these niches, view the circular plan of the rotunda with a set of axes at the major compass points, one of these niches is at each end of a major axis (4 in number). They are semi-circular in shape except the one at the main door which is somewhat square. The other 4 niches are located at the ends of the diagonal set of axes. These are a large rectangular shape with the long side following the curvature of the wall. Two granite columns help support the ceiling in the niches. It is interesting to note that within these niches lie great kings of Italy, important popes, and at one time the famous painter Raphael.

The niches, as well as all other wall openings, have an archway of bricks, known as a relieving arch, to support the upper wall over the openings. The relieving arch is a semicircle of thin bricks standing radially on end extending in the concrete wall. This arch distributes upper loads to the piers during the long time the pozzolan concrete is curing, but after curing, it becomes an integral part of the wall. This archway of bricks was only part of the wall and did not extend into the dome. This type of arch is customary with Roman construction for that period. It is shown together with the niches and their columns in the following figure:

To dimension the wall is not an easy task. First, the standard overall width at the piers is about 20'-4" (6.2 m), but the curtain wall on the side of the large niches is reduced to 7'- 4" (2.2 m) thick. Inside the piers there are small cavities which are semi-circular in shape having a radius of 7'-8" (2.3 m). The logic behind this shape is unknown, but curved surfaces reduce concentration of stresses that are objectionable in structures. The entrance to the cavities is through a 3'-6" (1.1 m) passageway from the outside.

The outside height of the circular wall is 104 feet (31.7 m) which seems awesome when viewed from the door step. It is the height of about a 7-story office building. The top cornice on the wall has an overhang of about 3'-8" (1.1 m) serving as an effective rain shield for the brick facing. The cornice is made of marble and has weathered well. This round wall is divided by two lower cornices. One is at 40'-4" (12.8 m) above the floor, and the other is higher at 71'-6" (21.8 m) from the floor. The latter serves as the spring line for the dome. The wall section becomes much thicker above the second cornice as the dome departs from the wall line.

Characteristic of all Roman walls of that time, the wall was tied together with a special horizontal layer of brickwork every 3'-11" (1.2 m). These bonding courses are made of tile-like bricks called bipedales (about 2 feet/0.6 m square) which extended completely through the wall. Brickwork on both sides of the wall was brought up with the placement of the concrete. This will be explained in later sections.

The composition of the wall has been documented by the Jutland Archaeological Society 13 and by Lugli 14 they agree quite reasonably. The lower section near floor level consists of alternate layers of travertine fragments and fragments of tufa (the caementae) in a mortar of lime and pozzolana. The middle placement of the wall was alternate layers of pieces of tufa and broken tiles or bricks also in the same mortar. The uppermost level of the wall consists of concrete predominantly of broken bricks in mortar. The wall was made lighter as it was made higher, a remarkable example of gradation in their engineering planning.

The dome is an interesting and difficult feature to describe because its configuration is so unusual on both sides. The radii of the dome is 71'-2" (21.7 m) which serves as the basis for the original design. However, G. Cozzo (an Italian engineer) cast doubts on this figure and claimed it to be more like 82 feet (25.0 m). 15 This is pointed out to show there are conflicts among the specialists who continue to study the Pantheon. In this case the former figure appears adequate. The relative thickness of the dome is reduced from 19'-8" (5.9 m) at the base to nearly 5 feet (1.5 m) at the top. 16

On the outside surface, there is a series of seven step- rings half way up the dome, and then the dome line changes into a circular line. On the inside surface the dome contains a series of 5 bands made of waffle-like depressions called coffers. There are 140 coffers which required special forming for the waffle shape. At mid-point the dome contour changes from these coffers to a circular line. In the center of the dome is a large opening, the oculus.

The outside rings are not uniform, there are 7 rings, and the measurements scaled from drawings of the dome are meant to be purely descriptive. The first ring has its outside edge resting on the center of the main wall. It appears to be some 7.5 feet (2.3 m) thick with a horizontal distance to the next ring about this same distance. The remaining 6 step-rings are stepped inward much like placing a series of machine washers, one above the other with their diameters decreasing as they are stacked. The height of these 6 rings vary, and they are estimated to be 2'- 6" (0.8 m) on the average. The horizontal distance to the next of these smaller rings is estimated to be 4 feet (1.2 m). There is an exterior stairway leading through these rings to the oculus.

Digressing for a moment, I can perceive the ancient construction practices applied to building this dome. It is known that the very old Mycenaean tombs in Greece were made by corbeling stone slabs over one another. Following this example in history, it is likely that the Romans used this principle in placing one step-ring on another in building this section of the dome. This work took a long time. The cementing materials properly cured and gained strength to support the next upper ring. The smaller step-rings are faced with semilateres (bricks) 16 which gives credibility to the corbeling method. Each ring was built like a low Roman wall. The circular part of the upper dome was likely placed by using wooden scaffolding.

The compression ring (oculus) at the center of the dome is 19'-3" (5.9 m) in diameter and 4'-7" (1.4 m) thick. The ring is made of 3 horizontal rings of tile, set upright, one above the other the ring is 2 bricks thick.16/17 This ring is effective in properly distributing the compression forces at this point. There is a bronze ring covering the lip dating back to the original construction, but other bronze plates on top of the roof have been removed and later replaced with lead plates.

According to the Jutland Archaeological Society investigations, the lower section of the dome is made of concrete with alternating layers of bricks and tufa both have good affinity with the lime-pozzolan mortar which filled the voids. The upper dome above the step-rings (the top 30 feet/9.1 m) is concrete comprising about 9 inch lumps of light tufa and porous volcanic slag in alternating layers bonded with mortar. 18 It was customary for the Romans to use larger stones in the dome concrete than in the walls. Selecting light stones for the aggregate is another case of gradation to get light-weight concrete, a process that seems to have been evolved about the middle of the first century B.C.

The following figures show the various features such as the step-rings, dome stairway, coffers, lead plates.


The challenge of determining stresses within various sections of the Pantheon has always excited both architects and engineers who are interested in the building. Technical design people recognized that the long 143 foot span of the ancient dome could have critical stress concentrations leading to a catastrophic failure of the structure, but this has not happened.

Nothing in life seems perfect, and this is the case with the Pantheon. The dome and walls have cracked. Concrete cracks under excessive tensile stress as viewed in a hoop condition. A. Terenzio, an Italian superintendent of monuments, documented cracking in the walls and dome during his inspection of the Pantheon in 1930. This occurrence was referenced in a design study of the Pantheon by Mark and Hutchinson as follows:

Terenzio also identifies fractures `reaching from the base of the rotunda to the summit of the dome' that he thought were brought about by differential settlement from uneven loading of the wall, particularly near the entrance of the rotunda in the principal niche. Rather than finding vertical differential settlement, we have observed only traces of lateral opening across the cracks-- corresponding to the effect of hoop tension. 21

Terenzio believed cracking occurred shortly after construction because of dated brick repairs. His sketches of the cracking is shown:

Pantheon Cracking (Terenzio 22 )

The Mark and Hutchinson study showed that meridional cracking in the dome was in the lower half extending up to about 57 degrees from the horizontal on the spring line.22 An earlier stress analysis of this dome by Cowan theoretically placed this point at 37 degrees 36'.23

This is the point where hoop stresses in the dome change from tension to compression presenting a point of weakness within the unreinforced concrete dome. This theoretical point is in reasonable agreement with the actual end of meridional cracking. The Mark and Hutchinson study located the cracks as occurring generally at the openings within the upper cylindrical wall which increased local tensile hoop stresses. In addition to the dome, Terenzio mentioned that cracks in the walls extended upward from 24.6 feet (7.5 m) above the floor.

Mark and Hutchinson have professionally met the challenge of defining the stresses in the Pantheon. Their computer analysis used a three-dimensional, finite-element modeling code to review eight conditions of the dome two of these included cracking. Some design parameters on one of the cracked models were: 1) A solid wall 18.0 feet (5.5 m) wide was used in place of the original wall containing bays 2) coffering was disregarded due to its minor volume 3) a dome thickness of 4.9 feet (1.5 m) was used without step-rings and 4) the weights were 99.8 lb/ft 3 (1600 kg/m 3 ) for lower dome, 84.2 lb/ft 3 (1350 kg/m 3 ) for upper dome, and 109.2 lb/ft 3 (1750 kg/m 3 ) for walls. The Romans decreased the weight of the aggregate as the height was increased. Interestingly, the analysis showed if a concrete--137.3 lb/ft 3 (2200 kg m 3 ) would have been used, the stresses would have been 80 percent higher, so the Romans were knowledgeable and cautious. 24

The cracking pattern of the concrete in the Pantheon provides an unique stress configuration acting in the dome and walls. Mark and Hutchinson describe this picture as one in which the major internal forces in the cracked dome are only in the meridional direction, and this region serves as a series of arches which bears a common compression keystone in the form of the uncracked upper dome. The cracked walls serve as a series of independent piers to support these arches.

Upon modeling this configuration a maximum tensile bending stress of 18.5 psi (1.3 kg/cm 2 ) occurred at the pont where the dome joins the raised outer wall. 25 No tensile test results are available on the Pantheon. However, Cowan discussed tests on ancient concrete from Roman ruins in Libya which gave a compressive strength of 2.8 ksi (200 kg/cm 2 ). An empirical relationship gives a tensile strength of 213 psi (15 kg/cm 2 ) for this specimen. 26 I conclude that the outstanding design work of Mark and Hutchinson places the stresses in the Pantheon within a safe design limit.

Perhaps as insurance against some future dislocation, should we add a steel band around a step-ring? Although the building has survived centuries, this valuable, cracked landmark of Roman history should be protected against future earthquakes at a small cost.

Pantheon (Rome): Plan

Description of work: Commissioned by Hadrian the building has captivated Western architects for generations. In 608 it was one of the first Roman temples to be converted into a church, Santa Maria Rotonda, and it has never been a ruin. "It compromises two elements: the first a conventional but deep porch supported by unfluted granite columns, its plinth originally approached via a flight of steps. This crudely abuts and provides the entrance to the second: the highly unconventional circular temple with its hemispherical dome. The dome springs from a drum whose height is exactly that of the radius of the dome (43.2 meters, 142 feet). " (Woodward, Christopher. The Buildings of Europe: Rome. Manchester University Press, 1995. p 40.)

Work type: Architecture and Landscape

Style of work: Ancient: Roman

Source: Blomfield, Reginald. A history of French Architecture: From the death of Mazarin till the death of Louis XV, 1661-1774. 2 vols. London: G. Bell and sons, ltd, 1921. (Vol. 2 plate CVII)

Floor Plan of the Pantheon, Rome - History

Kim Williams, Architect
Via Mazzini 7
50054 Fucecchio (Firenze) Italy
e-mail: [email protected]

W hat does the seventeenth-century Rundetarn (Round Tower) of Copenhagen have in common with the thirteenth-century Leaning Tower of Pisa? Or Houston's Astrodome, the first indoor baseball stadium built in the United States, with the vast dome of the Pantheon in Rome? Or a Chinese pagoda (fig. 1) with the Sydney Opera house (fig. 2) ? A first response might be "shape" but a more accurate answer would be "symmetry". Each of these strange couples of buildings share a different kind of symmetry that links them, in spite of their temporal and cultural differences. As Magdolna and István Hargittai have noted, symmetry, in architecture as in other arts, is "a unifying concept".[1]

Architecture, as any compositional art, makes extensive use of symmetry. Across all cultures and in all time periods, architectural compositions are symmetrically arranged. There are so many kinds of symmetry, so many kinds of architecture, and so many ways of viewing architecture, that the argument threatens to become so generalized that it loses all meaning. The general exposition of symmetry types found in architecture has been admirably presented in recent work.[2] While I wish to review symmetry types in architecture briefly in order to provide as wide an overview as possible within the limits of this paper, my ultimate object is to explore why an architect might choose a given symmetry type, and thus to provide insight into the design process from the point of view of symmetry.

The special case of architecture

A rchitecture differs fundamentally from other arts because of its spatiality. Identifying a type of symmetry in a two-dimensional composition is relatively straightforward the identification of symmetry types in a three-dimensional object such as a sculpture is somewhat more complicated because our perception of the object changes as we move around it. In the case of architecture, we not only move around it, but we move through it as well. This means that architecture provides us with a special opportunity to experience symmetry as well as to see it. This is possible because architecture consists of two distinct components: solid and void. Architecture is most frequently characterized by the nature of its elements: we recognize a Greek temple by its portico and pediments a Gothic cathedral is characterized by its pointed arches and flying buttresses. These are the elements that make up the solid component of architecture, and it is likely that it is with this solid component the lay person has the most experience. Naturally in the composition of these elements that one would expect to find various kinds of symmetry relations, and this, the symmetry that we see, is what I will be examining in the first part of this paper.
On the other hand, all these solid elements constitute an envelope around what we experience when we move through a building, that is, the void, or architectural space. In a very real way, the true work of the architect is to shape the void, which becomes the theater of the actions that take place in the building. This architectural space is most likely characterized by symmetry as well, though it is perhaps less familiar, and it is a symmetry which we experience. This is what I will examine in the second part of this paper.

An Overview of Symmetry types in Architecture

S ymmetry types are divided into two categories: point groups and space groups. Point groups are characterized by their relationship to at least one important reference point space groups lack such a specific reference point. Both point groups and space groups are found in architecture.

B ilateral symmetry is by far the most common form of symmetry in architecture, and is found in all cultures and in all epochs. In bilateral symmetry, the halves of a composition mirror each other. It is found in the facade of the Pantheon in Rome some 1700 years later on a continent undreamed of when the Pantheon was built, we find the same symmetry in the mission-style architecture of the Alamo in San Antonio, Texas. Bilateral symmetry is present also not only on the scale of a single building, but on an urban scale. An example of this is found in the design of the PraHo do Comercio in Lisbon, Portugal, where three urban elements (a major public square, a monumental gate and the wide commercial street beyond the gate) are symmetrical with respect to a long horizontal axis that governs our visual perspective.
The popular of bilateral symmetry is probably an expression of our experience of nature, and in particular with our experience of our own bodies. As many cultures believe that God created man in His own image, architecture has in turn probably been created in the image of man. Not all bilateral symmetry is of equal value in architecture, however. Two schemes for facades are shown in fig. 3. In one, there are an unequal number of bays in the other, there are an equal number of bays. The first is an example of "orthodox" bilateral symmetry, where the facade is divided into two equal halves but in the second, the axis of symmetry that divides the facade into two equal and independent halves creates a dualism. If it is true, as Dagobert Frey maintains, that bilateral symmetry represents "rest and binding"[3], then dualism represents divisibility. Traditionally, dualism in architecture has been considered something to be avoided. The temples of ancient Greece, for example, always had an even number of columns so that there was never a column on the central axis of the facade. The avoidance of the dual by classical architects probably stems from the ambiguity frequently attributed to the number 2, regarding with suspicion from the time of Pythagoras. The number 2 was considered untrustworthy (a female number) because it could be divided into halves, in contrast to the number 3 (a male number) which was not divisible into two parts. Even in modern architectural theory, dualism in architecture is considered a "classical and elementary blunder" and identified with the "amorphous or ambiguous".[4] These reservations not withstanding, dualism does exist in architecture. The fourteenth-century Oratory of Orsanmichele in Florence is an example (fig.4). It has a dual function: an oratory on the ground floor and a granary above. It has an unusual two-aisled plan. It has two altars. The difficulty of the dual on the level of architectural experience is best exemplified by the problem of the two altars. Where does one stand in the church? One is forced to make a decision whether to stand in front of one altar or the other. It is comparable to a house with two front doors. Where is the entrance? Usually this kind of decision is made for the spectator by the architect, who places one altar in a central position, or one prominent front door on the facade of a house. Thus, dualism in architecture presents a kind of a challenge to both the spectator and the architect.

R otation and reflection provide a sense of movement and rhythm in architectural elements and an emphasis on the central point of the architectural space. The Sacristy of the basilica of S. Spirito in Florence, designed by Giuliano da San Gallo in the last years of the fifteenth century, is octagonal in plan and both the architecture and the distinctive pavement design exhibit rotational and reflection (fig. 5). Domes, whether hemispherical such as that of the Pantheon or octagonal such as the great cupola of the Cathedral of Florence designed by Filippo Brunelleschi, also exhibit both rotation and reflection.

C ylindrical symmetry is that found in towers and columns Verticality in towers represents a defiance of gravity. Rare examples of spherical symmetry may also be found in architecture, though the sphere is a difficult form for the architect because human beings move about on a horizontal plane. The project for a cenotaph for Isaac Newton, designed by Etienne-Louis Boulée in 1784, demonstrates spherical symmetry.

C hiral symmetry is perhaps less well-known than other types of symmetry but frequently effectively used in architecture. Chiral symmetry is found in two objects which are each other's mirror image and which cannot be superimposed, such as our hands. The two opposing colonnades designed by Gianlorenzo Bernini that surround the elliptical piazza in front of St. Peter's in Rome exhibit chiral symmetry (fig.6). In Budapest, the two Klotid Palaces that tower above Felszabadul<s Square, each with asymmetrically placed towers and facade embellishments, are examples of chiral symmetry. A very subtle form of chiral symmetry is presented by the two leaning towers of the newly-completed Puerta de Europa in Madrid, designed by architect John Burgee in collaboration with Philip Johnson. Chiral symmetry in architecture is another way to place visual emphasis on the central element of a composition. In the case of the Puerta de Europa, for example, the two inclined towers emphasize the broad boulevard that passes between them, aptly forming a "gateway to Europe".

S imilarity symmetry is currently receiving a great deal of attention and is best known for its identification with fractals. Similarity symmetry is found where repeated elements change in scale but retain a similar shape, such as in the layered roofs on a pagoda (see fig. 1 above), the forms of which diminish in size but retain their form as they get closer to the top of the building. Another example of similarity symmetry is found in the nestled shells of the Sydney Opera House, designed by Joern Utzon in 1959 (see fig. 2 above). The shells are all segments of a sphere, thus similar in shape while differing in size and inclination. Another example of similarity symmetry is found in the Castel del Monte in Apulia in Italy, built by Friedrich II at the end of the first millennium. The basic form of the octagonal outer walls of the fort is repeated at a smaller scale in the interior courtyard, and again in the smaller towers which are added to each apex of the main octagon.[5] Similarity symmetry is also often used where it is least obvious, as in the relationships between room sizes. Frank Lloyd Wright used a kind of similarity symmetry in his design for the Palmer House in Ann Arbor, Michigan, in 1950-51.[6] In this case, Wright chose an equilateral triangle as a planning module, repeated at a number of levels and sizes to organize the design of the house. Similarity symmetry, whether visually apparent or not, results in a high degree of order within an architectural design, and lends unity to a composition.

S piral or helical symmetry may be thought of as a special kind of similarity symmetry. Helixes and spirals in architecture often represent continuity. In spiral staircases, the unbroken form expresses the continuity of space from level to level throughout the building. In the fantastic twisted spires of Copenhagen or of Borromini's S. Ivo alla Sapienza in Rome, the theme of continuity is expressed by the unbroken upward progression of the form. Frank Lloyd Wright used the helix in his 1946 design of the Guggenheim Museum of New York. In this case, the exterior of the building reflects the form of the giant helical ramp on the interior. The gallery spaces are arranged along one side of the ramp. The museum visitor takes the elevator to the top floor of the space, then spirals his way down the ramp to the bottom, admiring the art on display along the way. Criticism of the building focused on the fact that the downward spiral forced the visitors to hurry through the museum, unconsciously rushed by the pull of gravity. Legend has it that Wright, who placed greater value on architecture than on art, deliberately designed the building in order to get the visitor out as quickly as possible! In reality, however, the helical ramp once again expresses spatial continuity.

T ranslational symmetry falls in the category of space group symmetry, and is, after bilateral symmetry, the most common kind of symmetry found in architecture. Translation of elements in one direction is found in solemn rows of soldier-like columns, or in the springing succession of arches in an aqueduct. Translation of elements in two directions is present in the wallpaper-like patterns of the curtainwall facades of many modern buildings. Translation may also involve the repetition of entire pieces of buildings, especially in our own century, and may be one reason by modern architecture is so often referred to as boring or monotonous. Translational symmetry seems to carry with it an emphasis on a superlative quality in architecture: the longest, the broadest, the tallest.

This concludes my survey of types of symmetry found in arrangements of architectural elements. For the architect, the knowledge of symmetry types is a powerful tool, for it provides him not with a means for precisely describing a building, but with a range of expressive possibilities. We will learn more about the expressive possibilities of symmetry when we look at the use of symmetry in architectural space. However, before turning to this, I should emphasize another aspect of symmetry in architecture that makes it a special case in the study of symmetry.

Multiple Symmetries in Architecture

I n choosing the examples of various symmetry types for the previous section of this paper, I purposely focused on one aspect or part of a building that exhibits a single kind of symmetry. However, in most buildings we find more than one kind of symmetry. For example, in the Chinese pagoda, we can see at the same time both the cylindrical symmetry inherent in the building's organization about the vertical axis, and the similarity symmetry of the diminishing sizes of the layered roofs. A colonnaded temple facade may demonstrate bilateral symmetry, but it also demonstrates translation. These are examples of multiple symmetries that can be observed without requiring us to change our viewpoint of the building. We also perceive multiple symmetries when we change our position relative to the building, as for example, when we move from outside to inside. Domes are a very good example of this. From the outside, domes appear to be organized about a vertical axis (as they indeed are). When viewed from the inside, however, they appear to be organized about a central point.
Multiple symmetries also arise when a building is composed of multiple elements, some or all of which having its own symmetry. The symmetry type that we identify at any given moment, then, is a result of our physical position in relation to the building. It is important to make this point about multiple symmetries, because most architecture of any complexity at all is designed as a series of spaces that are meant to be experienced sequentially, as though the architect is telling us a story. Changing symmetries can be as important to the unfolding of the story as any of the other devices an architect has at his service. A closer examination of the Pantheon will illustrate the experience of an architectural "story".
The Pantheon in Rome is an excellent example of the experience of multiple symmetries that is common in architecture. When we stand in the piazza in front of the Pantheon, we notice right away the bilateral symmetry of the principle facade. Moving around the building, we discover that the Pantheon is composed of three easily-identified elements: the columned porch, a small intermediate block, and the great rotunda (fig. 7). The three are arranged in sequence: here is the beginning of the "plot" of the story. As we enter the Pantheon, we see that the three elements are arranged with respect to a common horizontal axis it is this axis that gives rise to the bilateral symmetry. However, once inside, the horizontal axis that we have followed to gain entrance into the rotunda disappears. It is replaced by a vertical axis that runs from the center of the pavement up to and through the oculus of the dome above. Thus the dominant symmetry is no longer bilateral. The lower zone exhibits cylindrical symmetry, while the hemispherical dome above exhibits rotation and reflection . The reason for the change in symmetries is that, when we enter into the rotunda we leave behind the zone of the terrestrial, represented by the horizontal axis, and experience the zone of the celestial, symbolized by the vertical axis. The Pantheon is a temple dedicated to all the gods the universe itself is represented in the rotunda by the form of the sphere, half of which is actually present in the coffered cupola which crowns the space, while the other half is only made implicit in the proportions of the space (as mentioned before, the sphere is problematic in architecture because human beings require a horizontal plane). The sphere contains an infinite number of planes of reflection and rotation its infinity symmetry makes it an apt symbol for the cosmos.

Symmetry in Architectural Space

H aving examined how symmetry is found in the parts of a building that we see, we may take a look at how symmetry relates to the part of the building we don't see, which is the void that is the architectural space. Two concepts are fundamental in describing architectural space: center and path. Center relates to a single important place within the larger architectural space, such as the altar in a church. Path relates to the spectator's movement through the space. Christian Norberg-Schulz writes that ". centre and path are present in any church, but their relationship differs."[7] This relationship actually determines how we perceive the architectural space of any given time period. In terms of symmetry, center may be thought of as "point" and path, as "axis." The following, very brief, examination of some 1500 years of architectural history hopes to demonstrate that as architectural space evolved through the centuries, so did the dominant symmetries.
In Roman architecture, strictly observed axial symmetry gives rise to spaces that are monumental and static, that is, generally embodying a sense of equilibrium rather than expressing a sense of dynamic movement.[8]

Consider the symmetry relations of the plan of a Roman basilica, a secular building type used as a court of law (fig.8). It is rectangular, with an apse on each end of the major axis and a doorways on each end of the minor axis. The architectural elements are always arranged so that like elements are always opposite: apse to apse, column to column, doorway to doorway. Excavations have brought to light the remains of the pavements used in basilicas they underline the sense of balance and equilibrium that characterize the architecture, as frequently they are based on patterns described by translational symmetry in two directions, rather than by any other kind of more dynamic symmetry type such as rotation. This same static arrangement of architectural elements is found in the rotunda of the Pantheon, Rome. Here the plan is a circle, with eight reflection planes and one four-fold axis of rotation (to be precise, the symmetry is approximate because the entrance is opposite a large round apse). Again we find oppositions: apse to apse, aedicule (a canopied niche flanked by colonnettes) to aedicule, niche to niche, column to column. The strict axial symmetry establishes the sense of equilibrium within the space that is characteristic of Roman architecture. It is interesting to note, however, that neither the axes nor the center point is made explicit through the pavement design of the rotunda, which is like that of the basilica based upon translation in two directions. Thus the symmetry was an organizing device for the architecture, but does not determine the movement of the spectator within the space. This is one characteristic that distinguishes Roman architecture from that of later periods, in which we will see how axes and centers are used to provide a specific dynamic emphasis and encourage movement.
After the legalization of Christianity in the fourth century, Christian architects chose to adapt the Roman basilica to their own ecclesiastical needs. To do so, they removed the entrances from the minor axis and placed a principal entrance on one end of the major axis, placing the altar in the remaining apse (fig. 9).[9]

Thus the symmetry was radically altered, there remaining only a single plane of reflection and no planes of rotation: the plan of the Christian basilica is bilaterally symmetrical. The axis of symmetry takes on an all-important symbolic role: it becomes a path, symbolizing the earthly pilgrimage of the Christian making his way towards the Kingdom of God. The pavement designs of many of these churches make explicit the axis that governs the architecture. Bilateral symmetry is favored over all other symmetry types during the Early Christian, Romanesque and Gothic periods, spanning from 300 to 1300 AD, because it best expressed the Christian ideal. It is the necessity of expressing the concept of pilgrimage, and not only that of expressing order as suggested by Hermann Weyl, that gave rise to the bilateral symmetry that dominated Christian architecture up until the Renaissance.[10] In addition to bilateral symmetry in plan, the sense of movement along a path is underlined by the translation of elements in a horizontal direction parallel to the dominant longitudinal axis. It is this kind of dynamic indication of direction that is lacking in Roman architecture.
As architectural and philosophical ideals changed in the Renaissance, so did the type of symmetry most frequently used. Sacred architecture was intended as a model of the cosmos created by God. To this notion, Humanism added the concept that, because man is God's most important creation in the cosmos, his place is in its center. The centrally-planned building was favored as best reflecting the perfection of the cosmos, thus rotational and reflectional symmetries were particularly favored during this period. The center point is usually made explicit in the pavement design: this particular emphasis on the center point induces the spectator to place himself there.
Pavement designs from the fifteenth, sixteenth and seventeenth centuries use rotation, reflection and similarity symmetry to emphasize the center. The rosette is a motif that often appears in pavement designs of this time, as for example, in the octagonal Sacristy of the basilica of S. Spirito in Florence (see fig.5 above). Here the rosette is formed from sections of a logarithmic spiral. To create the curvilinear checkerboard motif, a logarithmic segment is rotated a given number of times about the center in one direction, forming a fan pattern, then the direction of the segment is reversed and rotated about the center the same number of times in the opposite direction. The resulting rosette pattern has modules that increase in size but maintain their proportional similarity as they move farther from the center, and is therefore characterized by similarity symmetry as well as rotation and reflection. Another example of paving patterns from this period may be seen in the Cathedral of Florence, S. Maria del Fiore, in which trapezoid-shaped modules increase in size as they move away from the pattern's center, again demonstrating reflection, rotation and similarity symmetry. These patterns were no doubt favored because the perspective illusion they create is an excellent means of emphasizing the central point of the design, and through this, the central point of the architectural space.
Thus we see that in this arc of architectural history, the dominant symmetry evolved from a generalized axial symmetry in the Roman age, to bilateral symmetry in the Paleo-Christian, Romanesque and Gothic ages, to rotational and reflectional symmetry in the Renaissance. Our recognition of the symmetry in an architectural space is one step towards our understanding of the architecture, a means we are given to interpret the architectural "story" we are experiencing.

At this point I draw to a close this discussion of architecture and symmetry. I hope that the wide variety of symmetry types and their various combinations as well as the use of symmetry to define space has been made clear. However, the topic of symmetry in architecture is far from exhausted. There are some further aspects of the subject that I am now studying but about which I am not yet in a position to draw conclusions, and for which this present paper forms a background.
One of these aspects has to do with "broken symmetries". The Pantheon in Rome provides one example of a symmetry break: the cylindrical lower zone of the rotunda is characterized by four planes of reflection and fourfold rotation, while the hemispherical dome above is characterized by twenty-seven-fold rotation. Four and twenty-seven have no common divisors, thus the symmetry "break." Another example of broken symmetries is found between horizontal tiers of the Baptistery of Pisa, which are alternately based on rotations of twelve and twenty.[11] These of course, are historical examples. Many other examples are present in modern architecture.
A second, very important question concerning the architecture today is this, "Why have contemporary architects deliberately chosen to disregard traditional types of symmetry in their architecture? The designs of Richard Meyer and Frank Gehry in the United States come to mind. The advantage of examining contemporary architecture lies in the fact that the architects are most often still living, and while we can never ask the architect of the Pantheon why he broke the symmetry of the rotunda, we can ask Frank Gehry why the design of Guggenheim Museum in Bilbao apparently throws a consideration of symmetry to the wind. I say apparently, because I would want to ask the architect for an explanation before hazarding any judgement of my own. So I hope in a future paper to be able to present the fruits of this current research, and shed even more light on the uses of symmetry, both apparent and otherwise, in architecture.

This paper developed from a lecture I gave in April 1998 at the Department of Mathematics of the University of Milan. I wish to thank Simonetta di Sieno and Liliana Curcio for the invitation to undertake this study.

1.Cf. István Hargittai and Magdolna Hargittai, Symmetry: A Unifying Concept (Bolinas, California: Shelter Publications, 1994). return to text

2.Cf. "The Universality of the Symmetry Concept", Nexus: Architecture and Mathematics, Kim Williams, ed. (Fucecchio, Florence: Edizioni dell'Erba, 1996), 81-95. return to text

3.Cf. Dagobert Frey, "On the Problem of Symmetry in Art" quoted in Hermann Weyl, Symmetry (Princeton: Princeton University Press, 1989), 16. return to text

4.Cf. Sinclair Gauldie, Architecture (London, 1969), 16. Gauldie considers the unresolved dual a "classic and elementary" error. return to text

5.Cf. Heinz Gotze, "Friedrich II and the Love of Geometry", Nexus: Architecture and Mathematics, 67-79. return to text

6.Cf. Leonard K. Eaton, "Fractal Geometry in the Late Work of Frank Lloyd Wright: The Palmer House", Nexus II: Architecture and Mathematics , Kim Williams, ed. (Fucecchio, Florence: Edizioni dell'Erba, 1998), 23-38. return to text

7.Christian Norberg-Schulz, Meaning in Western Architecture (New York: Praeger Publishers, 1975), 145. return to text

8.Cf. Bruno Zevi, Saper vedere l'architettura (Turin: Einaudi Editori, 1948) 57. "Impera negli ambienti circolari e rettangolari la simmetria. una grandiosità duplicement assiale. " ("Symmetry reigns in circular and rectangular environments, based on dual axes. " --translation by Kim Williams). return to text

9.Ibid., 59. " La basilica romana è simmetrica rispetto ai due assi: colonnati contro colonnati, abside di fronte ad abside. Essa crea quindi uno spazio che ha un centro preciso ed unico, funzione dell'edificio, non del cammino umano. Che cosa fa l'architetto cristiano? Praticamente due cose: 1) sopprime un'abside, 2) sposta l'entrata sul lato minore. In questo modo, spezza la doppia simmetria del rettangolo, lascia il solo asse longitudianle e fa di esso la direttrice del cammino dell'uomo. " (The Roman basilica is symmetric with respect to the two axes: colonnade opposite colonnade, apse opposite apse. This creates a space which has a precise and unique center, a function of the building, not of man's movement. What did the Christian architect do? Essentially two things: !) suppressed an apse, 2) moves the entrance to the shorter side. Thus he breaks the dual symmetry of the rectangle, leaving only the longitudinal axis, which he makes the directrix of man's movement.--translated by Kim Williams.) return to text

10.Cf. Hermann Weyl, Symmetry , 16. return to text

11.Cf. David Speiser, "The Symmetries of the Baptistery and the Leaning Tower of Pisa", Nexus: Architecture and Mathematics , 135-146. return to text

Nexus Network Journal: Architecture and Mathematics Online

Symmetry: Symmetry online featuring Symmetry: A Unifying Concept by Magdolna and Istvan Hargittai

Construction Technique

Figure 10. Pantheon interior, light from the oculus illuminating the hole left by the cutting of Brunelleschi’s sample

The Pantheon is a marvel of construction ingenuity- the result of a century of experimentation with the use of advanced building elements such as the relieving arch, vaulting rib, lightweight caementa, and step rings. What is particularly unique to the Pantheon however is the method by which these elements were incorporated into a structural system that has allowed the largest unreinforced concrete dome ever built to stand for almost two millennia.

Until the 20th century, the Pantheon was the largest concrete structure in the world. And it remains the world’s largest unreinforced concrete dome. [17] An engineering marvel, the dome’s components are a tribute to the creativity of the design. For example, the oculus (otherwise known as the “open eye”) serves to reduce loading at the top.

Otherwise, the dome still stands at 142 feet high and wide under a circular rotunda for additional reasons. According to the analysis of Filippo Brunelleschi, an engineer and architect of 1377, a material sample taken from the Pantheon’s dome, to the right of the entrance, shows that the concrete composition of the structure was non-homogenous. (Figure 10.) The construction technique applied to the dome involved applying thinner and lighter concrete at greater heights- the highest part incorporating volcanic pumice as aggregate.

What's the Difference Between the Pantheon and the Parthenon?

If a friend who was about to go off on a European adventure told you they were going to visit the Pantheon, would you immediately picture ruins of ancient white marble columns? What if that same friend told you they would also be stopping by the Parthenon. Would you also picture a similar scene in your head?

The point is, the Parthenon and the Pantheon are often confused as being the same thing. And that's no surprise because the names are super similar. But the two are very different they're not even located in the same country. The Parthenon, for instance, is in Athens, Greece, and the Pantheon is in Rome, Italy. And aside from both being made of marble and sharing a similar etymology — both names are derived from the Greek word parthenos, which is an epithet of the Greek goddess Athena, meaning "virgin" — these two famous buildings of the ancient world actually have very little in common.

We spoke with Christopher Ratté, a classical archaeologist and professor at the University of Michigan and Dr. C. Brian Rose, the curator-in-charge of the Mediterranean Section at the Penn Museum and archaeologist who's been digging in the field for more than 40 years, to find out exactly what makes these two ancient ruins so different.

1. They Were Built in Different Centuries

The Parthenon and the Pantheon are two of the most famous temples ever built in ancient Athens and ancient Rome. The Pantheon was constructed in the second century A.D., while the Parthenon we know today was built much earlier around 447 B.C.E. However, neither, as they say, was built in a day.

The Pantheon is one of today's best-preserved ruins from ancient Rome. It was built sometime between 126 and 128 A.D. during the reign of Emperor Hadrian, who was emperor from 117 to 138 A.D. "It was a reign largely marked by peace . there was plenty of money throughout the empire," Rose says. "Economically it was a very prosperous time and you see that reflected in the building program. [The Pantheon] is primarily made of concrete, but the inside is lined with marble imported from Egypt, Greece, Asia Minor and North Africa these international materials bolster the Pantheon as a symbol of the extent of the Roman Empire."

The Parthenon, on the other hand, took 15 years to build, Rose says. It was built between 447 and 432 B.C.E. during the aftermath of the Persian Wars to highlight the victory of the Greeks over the Persians. At the time, the Greeks were led by (or controlled by, depending on who you talk to) Athens, which was being controlled by a commander named Pericles. Athens had access to a treasury that could pay for additional arms conflict if the Persians came back. This treasury helped to fund the construction of the Parthenon. The goddess Athena was credited with steering the Greeks toward victory, which is why, had you visited the site at the time, you would've found a statue of her in the temple's main room (more on that next).

2. They Honor Different Gods

While both were built to honor gods, the Parthenon was built to honor Athena and the Pantheon was built to honor all of the Greek gods.

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