Ancient testimony

1. Plin NH 36.72f.

   "Transporting obelisk to Rome by sea was a more difficult task by far. The ships attracted much interest. The late emperor Augustus dedicated the ship that carried the first obelisk and preserved it in a permanent dock at Puteoli to mark this marvelous feat. Cement caissons were installed on board at Puteoli. The vessel was then towed to Ostia and scuttled to help construct the port. Augustus used the obelisk in the Campus Martius in a remarkable way, namely to cast a shadow and thus mark the length of days and nights. A paved area was laid out to commensurate with the height of the monolith in such a way that the shadow at noon on the shortest day might extend to the end of the paving. As the shadow gradually grew shorter and longer again it was measured by bronze rods fixed in the paving. This device deserves study; it was the result of a brain wave Facundus Novius. Novius placed a gilded ball on the apex of the monolith otherwise the shadow cast would have been very indistinct. He got this idea, so it is said, from seeing the shadow cast by a man's head. These measurements, however, have not agreed with the calendar from some 30 years. Either the sun itself is out of phase or has been altered by some change in the behavior of the heavens, or the whole earth has moved slightly off center. I hear this phenomenon has been observed in other places."

2. Suet. Aug 100. silvae et ambulationes north of the solarium
3. Vitruvius on arch. 9.7.1

   But we must explain the principles of the shortening and lengthening of the day and separate them from these previously explained astronomical observations. For the sun at equinoctial time, turning through Aries (spring equinox) and Libra (autumn equinox), makes the gnomon cast a shadow 8-9ths of its own length at the latitude of Rome .

4. Dedications: solarium / obelisk / inscription: the solarium (sic) was a gift to the Roman people to celebrate his conquest of Egypt.

Overview of the monument --from top down. Note that the solarium had at least two construction phases the first under Augustus (12 BC), a second under Domitian (81-96) and then again in the mid second century that affected the surroundings (a water basin or "canopus" was added). The Domitianic remodel raised the level of the face of the solarium by at least 1.5 m. and probably reused significant amounts of the original material making it difficult to reconstruct the original with certainty.

1. the sphere: diameter = .5 width of obelisk just below pyramid (1.55 m) = 0.77m; size of shadow cast at winter solstice: 0.07 m.; at summer solstice: 0.28 m.
2. pyramid: base and height =1.55 m. (p 126)
3. obelisk itself
   1. Buchner argues that the Augustan version of the obelisk stood about  34-38 cm short of exactly 100 Roman feet, so concludes that there must have been a sphere to bring it to exactly 100 Roman feet. This  model is compatible with  other "columna centenaria" that are found in Rome (e.g., the columns of Trajan and of Marcus Aurelius).
   2. dimensions: 21.79 m. high; 1.55 m wide at the base of the pyramid; 3.0 m wide at the base. Stands on 0.34 m. metal "feet". And on a dedication base of 4.36 m. high and 2.7m x 2.65m. the height of the base is exactly one-fifth of the height of the obelisk.
   3. orientation: off-set from true north and line of equinox so that the SW corner forms a right angle to the middle of the mausoleum of Augustus and the entrance to the ara pacis Abb. 19 (p 364)
4. meridian:
   1. travertine stone; also marble and rose colored granite;
   2. meridian itself was 58 m long (to winter solstice) 2.5m wide, but with the names of the months created a band 7 m. wide.
   3. with bronze letters and lines to name months of year in Greek and Latin, and to mark the individual days. perhaps indications of subdivisions of the hour as the meridian approaches the winter solstice.
   4. a triangular shaped incision with the apex at the winter solstice marked successive increments of additional daylight as one moves from the winter to the summer (5 additional "hours" of sunlight at the summer solstice)
5. bench to south in curve Abb 15
6. water basin added much later and off to west
7. the face of the solarium. In the shape of a trapezoid (abb. 12; 41) 75 m at the north, 58 m. at the base; 75 on the NS axis.
   1. Formal construction / stone facing covering the "hours" from 7am to 5 pm.
   2. As the marking for the 11th hour would extend over the Via Flaminia lata, it is unlikely that there was any formal construction that far east or west. Note too that the Ara Pacis is also "outside" this scheme.
   3. Not clear whether the whole surface in this trapezoid was faced with stone. Drillings to virgin soil toward the extremes of the parallelogram do not indicate any stonework, but stone could have been reused.
   4. The ratio of the obelisk to the shadow. Obelisk was 100 Roman ft high and the shadow at winter solstice was 220 ft (at the latitude of Rome)
8. surroundings: benches to south in curve; silvae et ambulationes to the north.

Benches:

The Analemma

This very unusual photograph was recorded by Dennis di Cicco on a single piece of film that was exposed on 45 different dates throughout an entire year in a permanently mounted camera (44 images were of the sun, with an additional exposure to include the house and tree). It shows the sun's position in the sky at the same time of day on each date. The figure-8 loop of sun images is well known to fanciers of sundials or old terrestrial globes - a graphic representation of the changing seasons and the equation of time.

To see how this figure-8 pattern originates, let's consider how the making of this picture has been affected by the annual apparent motion of the sun along the ecliptic. For simplicity, we'll suppose that all the exposures are made at exactly 8:30 am Eastern standard time.

For the moment, suppose that the sun's annual path around the sky is along the celestial equator instead, and that this motion is at a uniform rate. If that were true, than all 44 solar images in the picture would coincide. Actually, the ecliptic motion is tilted 23.5 degrees to the celestial equator, so that in late June the sun is 23.5 degrees north of the equator and in late December 23.5 degrees south of it. This annual north-south oscillation of the sun's declination angle is responsible for the lengthwise extension of the analemma pattern.

Because the ecliptic is tilted to the equator, the sun's motion relative to the stars is due east only in late June and late December. Hence the sun's eastward advance per day is greatest at those times, and least in March and September when the ecliptic crosses the equator slantingly. This means that when the sun is photographed at 8:30 am on different dates of the year, it generally is some minutes ahead of or behind clock time. The effect of this is to give an east-west spread to the pattern of solar images. We would get a photograph in which the two loops of the figure-8 were equal in size - provided that the sun's motion along the ecliptic were uniform (which would be the case if the earth's orbit were circular). Actually, the earth's orbit is somewhat elliptical, and the effect is to distort the figure-8 to the shape seen in the photograph.

Set the analemma curve upright, and it becomes a miniature almanac. The vertical coordinate of each point on it gives the sun's declination on a particular day of the year, while the horizontal coordinate tells how much the sun is ahead of or behind clock time on that day. (For further explanation of the analemma, see Bernard Oliver's article in Sky and Telescope for July, 1972, page 20.)