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"The one in the Campus was put to use in a remarkable way by August of Revered Memory so as to mark the sun's shadow and thereby the lengths of days and nights. A pavement was laid down for a distance appropriate to the height of the obelisk so that the shadow cast at noon on the shortest day of the year might exactly coincide with it. Bronze rods let into the pavement were meant to measure the shadow day by day as it gradually became shorter and then lengthened again....He placed on the pinnacle a gilt ball, at the top of which the shadow would be concentrated, for otherwise the shadow cast by the tip of the obelisk would have lacked definition. He is said to have understood the principle from observing the shadow cast by the human head. The readings thus given have for about thirty years past failed to correspond to the calendar, either because the course of the sun itself is anomalous and has been altered by some change in the behavior of the heavens or because the whole earth has shifted slightly from its central position, a phenomenon which, I hear, has been detected also in other places. Or else earth-tremors in the city may have brought about a purely local displacement of the shaft or floods from the Tiber may have caused the mass to settle, even thought the foundations are said to have been sunk to a depth equal to the height of the load they have to carry."
Pliny, Natural History (XXXVI.72-73)
In 10 BC, Augustus brought the first two obelisks to Rome (Strabo, Geography, XVII.27; Ammianus Marcellinus, XVII.4.12), where, as identical inscriptions indicate, they were set up as monuments to his conquest of Egypt (twenty years before) and dedicated to the Sun. One, which now is in the Piazza del Popolo, was placed on the spina of the Circus Maximus, which, in the etymology of Tertullian, was thought to derive its name from Circe, the daughter of Helios, who organized the first games there in his honor (On the Spectacles, VIII). The other slightly smaller obelisk (although still the fourth largest in Rome), which had been erected at Heliopolis by Psammetichus II in the sixth-century BC, was the gnomon (indicator) of the Horologium Augusti. It now is in front of the Palazzo Montecitorio.
"You [Capricorn] shorten the nights you have brought to their greatest length and give birth to a new year by enlarging the daylight hours."
Manilus, Astronomica (IV.254ff)
A gilt bronze globe surmounted the obelisk on the spina of the Vatican Circus (Circus of Gaius and Nero), erected there by Caligula (Pliny, XVI.201, XXXVI.74; Tacitus, Annals, XIV.14). In 1586, Sixtus V had the obelisk moved several hundred yards to its present location in St. Peter's Square. Symbolizing the terrestrial orb and rule over a pacified world, the sphere itself now is in the Palazzo dei Conservatori Museum (Rome).
In 1976, Buchner hypothesized that the Horologium was a colossal sundial, its gnomon casting a shadow over a vast pavement of travertine calculated to have measured some 525 by 246 feet. As the day advanced, the shadow moved from west to east along the equinox and from north to south along the meridian through the passage of the seasons, beginning with the winter solstice under Capricorn, the day of Augustus' conception (as well as the birth sign of Apollo, his patron deity), and ending with the summer solstice under Cancer. It then returned northward for the remaining six months of the year. Nine months later on the fall equinox (September 23), the date claimed by Augustus as his birthday, the shadow of the gnomon extended directly to the east and, as the sun set, into the Ara Pacis itself, symbolizing that Augustus was one who had been born for peace.
In 1980, Buchner discovered what he thought to be this sundial. But then, ten years later, Schütz convincingly argued that, rather than an elaborate solarium, as Buchner had contended, the Horologium of Augustus was a solar meridian designed to indicate the progress of the year as the sun moved through the zodiac from solstice to solstice. Its purpose was just as Pliny had said: "to measure the shadow day by day as it gradually became shorter and then lengthened again." Instead of the imagined expanse of travertine, there was only this single longitudinal line, demarcating the furthest extent of the shadow at noontime on the winter solstice. In 1750, two years after the obelisk had been excavated, Angelo Maria Bandini published De obelisco Caesaris Augusti, to which the English antiquarian James Stuart contributed. It, too, concluded that the Horologium was a solar meridian.
Although a sundial and a meridian both measure the position of the sun's shadow and the changing length of the day, a meridan marks only noon (meridianus). It is then, when the sun is at its highest point in the sky, that a shadow falls due north of the object that casts it, becoming increasingly shorter as the sun rises through the seasons. On the summer solstice, the longest day of the year, the shadow has moved completely down the line of the meridian. Then, as the sun declines lower on the horizon, its noontime shadow begins to lengthen and move up the meridan until it can grow no longer, thereby marking the winter solstice, the shortest day of the year.
There is another criticism of Buchner's reconstruction. Tacitus relates that Claudius "enlarged the pomerium, in consonance with the old custom, by which an expansion of the empire confers the right to extend similarly the boundaries of the city" (Annals, XII.23). Half a century before and on the same street as Buchner's discovery, two stone cippi had been found marking the pomerium (sacred boundary) of Rome. One was from the time of Vespasian and both cut through the western part of the sundial. If a travertine pavement had existed, the presumption is that the pomerium would have gone around it.
Buchner also claimed that the equinoctial line of the sundial marked the path of the sun's shadow as it passed through the middle of the Ara Pacis on Augustus' birthday. Ironically, the discovery of the meridian proved that his original calculations for the height and position of the obelisk had been incorrect (although the differences tend to cancel one another out). More telling, the shadow cast by the gnomon along the equinox would have become diffuse and disappeared altogether before it ever reached the Ara Pacis. Too, the shadow would have pointed in the direction of the Altar every day of the year, not just Augustus' natal day..
To be sure, there is a topographical relationship between the Horologium, Mausoleum, and Ara Pacis. But urban planning alone can account for the orientation of the principal monuments of the Campus Martius and do not require the complex mathematical calculations proposed by Buchner. The gnomon, for example, is not directly aligned with the Mausoleum. Stuart had measured the angel of the base when it first was excavated and found that it diverged by several degrees. Rather, the obelisk paralleled the course of the Via Flaminia and formed a right triangle with the Ara Pacis.
The Ara Pacis was dedicated on January 30, 9 BC, the same year that the obelisk was inaugurated, perhaps even on the same occasion (this was the date of Livia's birthday). In 12 BC, Augustus had been made Pontifex Maximus and was responsible for the Roman calendar. The obelisk was erected in 10 BC, and in 9 BC the calendar was found to be in error, every third year having been made a leap year, not every fourth, as the Julian calendar had dictated. It was then that the calendar was corrected and the meridian constructed, which would have been visible proof that the civil calendar was in harmony with the progress of the sun. Given the inscription on the base of the obelisk, with its emphasis on "Augustus" and a separate line for "Pontifex Maximus," it very well may have been erected, even if symbolically, in the context of that new role.
Even though Pliny says that the foundation of the obelisk was equal to its height, earthquakes or floods must have caused the alluvial soil of the Campus Martius to sink and, by the time his Natural History was dedicated to Titus in AD 77/78, the sundial had not been accurate for thirty years. Stuart found that the obelisk and surrounding pavement had been raised by about three feet to adjust for this subsidence. (The Ara Pacis sank as well and eventually had to have a retaining wall built around it.) More than two centuries later, Buchner discovered that the level had been raised more than five feet, likely by Domitian, who used the original pavement and lettering to restore the Horologium to accuracy, if only for a short while. And, like Augustus, for whom August was named, so Domitian renamed the months of September and October after himself (Germanicus and Domitianus, Suetonius, XIII.3).
"Let a marble slab be fixed level in the centre of the space enclosed by the walls, or let the ground be smoothed and levelled, so that the slab may not be necessary. In the centre of this plane, for the purpose of marking the shadow correctly, a brazen gnomon must be erected. The Greeks call this gnomon skiotheres. The shadow cast by the gnomon is to be marked about the fifth ante-meridional hour, and the extreme point of the shadow accurately determined. From the central point of the space whereon the gnomon stands, as a centre, with a distance equal to the length of the shadow just observed, describe a circle. After the sun has passed the meridian, watch the shadow which the gnomon continues to cast till the moment when its extremity again touches the circle which has been described. From the two points thus obtained in the circumference of the circle describe two arcs intersecting each other, and through their intersection and the centre of the circle first described draw a line to its extremity: this line will indicate the north and south points. One-sixteenth part of the circumference of the whole circle is to be set out to the right and left of the north and south points, and drawing lines from the points thus obtained to the centre of the circle, we have one-eighth part of the circumference for the region of the north, and another eighth part for the region of the south. Divide the remainders of the circumference on each side into three equal parts, and the divisions or regions of the eight winds will be then obtained: then let the directions of the streets and lanes be determined by the tendency of the lines which separate the different regions of the winds."
Vitruvius, On Architecture (I.6.6-7)
The solstice is one of two times during the year when the Sun is at its greatest distance above or below the celestial equator. It is this "turning" that marks the tropics of Capricorn and Cancer. In the Northern Hemisphere, the winter solstice nominally occurs on December 21, when the sun shines directly overhead at noon on the Tropic of Capricorn, the summer solstice on June 21, when the sun is overhead on the Tropic of Cancer. It is then that the sun appears to stand still, the meaning of "sostice," its noontime elevation seeming not to change.
Just as the shadow of a gnomon falls due north when the Sun is at its apogee, so does the Sun rise exactly in the east and set exactly in the west on the equinoxes, March 21 and September 23.
In this detail (top) from the base of the Column of Antoninus (Vatican Museums), which stood a bit south of the Horologium, the personification of the Campus Martius holds the obelisk that served as the gnomon.
The Einsiedeln Itinerary, which provided pilgrims an itinerary of Rome with directions and a list of sights, indicates that it still was standing in the eighth-century AD. But at some later date, the obelisk fell and broke into five pieces, possibly due to the earthquake that struck Rome in AD 849 or, because the shaft had been damaged by fire from the sack of the city by the Norman duke Robert Guiscard in 1084. It was discovered in 1512 but not excavated until 1748 at the instigation of Benedict XIV (as a plaque over the doorway of the building at Piazza del Parlamento, No. 3 attests).
The obelisk finally was erected by Pius VI in the Piazza di Montecitorio in 1792, with sections restored with red granite taken from the badly damaged Column of Antoninus.
Because there is a slight wobble in the earth's axis, this precession causes the poles (and the stars to which they appear to point) to transcribe a small circle in the heavens every twenty-six thousand years. Two thousand years ago, the celestial north pole that now points to Polaris (at the end of the handle of the Little Dipper), pointed closer to the bowl. This same wobbling also causes the apparent position of the sun to precess westward through the signs of the twelve astrological constellations by one degree every seventy-two years, or one constellation every 2160 years (72 x 30 degrees). In 150 BC, for example, the vernal equinox occurred when the sun appeared to be in Aries (which still is its conventional date in horoscopes). Now the sun is almost through Pisces (one constellation to the west of Aries) and soon will enter Aquarius, a difference of almost two constellations between the actual location of the sun in the sky and the zodiac of the astrologer. Someone born under the sign of Capricorn (Capricornus) about the time of the Third Punic War now would be under the sign of Sagittarius.
References: Die Sonnenuhr des Augustus (1982) by Edmund Buchner reprints two articles by the author from Römische Mitteilungen: "Solarium Augusti und Ara Pacis" (1976), 83, 319-365 and "Horologium Solarium Augusti" (1980), 85, 355-373, and updates his work. His main critic is Michael Schütz, "Zur Sonnenuhr des Augustus auf dem Marsfeld" (1990), Gymnasium, 97, 432-457. These articles are in German, which makes the following critiques of Buchner especially useful for the English speaker: "The Sundial of Emperor Augustus: Rise and Decline of a Hypothesis" (2005) by Frans W. Maes, The Compendium: Journal of the North American Sundial Society, 12(3), 13-27 and "Augustus, Domitian and the So-called Horologium Augusti" (2007) by Peter Heslin, Journal of Roman Studies, 97, 1-20; Imperium and Cosmos: Augustus and the Northern Campus Martius (2006) by Paul Rehak; James "Athenian" Stuart, 1713-1788: The Rediscovery of Antiquity (2006) edited by Susan Weber Soros; Augustus and Capricorn: Astrological Polyvalency and Imperial Rhetoric" (1995) by Tamsyn Barton, The Journal of Roman Studies, 85, 33-51. The illustration, which is based on drawings from Buchner, is in Empires Ascendant: Time Frame 400 BC-AD 200 (1987) by the Editors of Time-Life Books.
See also James Stuart and Ara Pacis.
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