# When can I see the Moon through the hole in the Pantheon?

I am an artist who is very interested in Astronomy but does not know very much about it. I very much enjoyed reading your extremely helpful website.

I would like to know, as precisely as possible, when the moon will shine at its brightest through the hole in the Pantheon in Rome. The Pantheon has a circular hole of 9m across at the centre of it's dome. as shown here.

As your website suggests I have visited the U.S. Naval Observatory Data Services website but the closest I can get is Rome, Italy. What will I need to do to be 100% certain that the date and time given on this website will allow the bright moon to be seen through this narrow opening in the Pantheon roof?

Thanks for your question to curious@astro.cornell.edu. What follows is a rather lengthy and technical answer. I guess I got interested in figuring out the best answer to your problem (presumably that's why I am an Astronomer!), although as a caveat I want to note that it's actually not that easy of a question to answer, so I don't promise to have thought of all the best solutions. Anyway here goes....

Rome has a latitude of 41.9 degrees North (longitude 12.45 W). An object which passes through the zenith at this location therefore must have a declination of 41.9N (right acsension and declination are a co-ordinate system used for objects in the sky which is referenced to the equator and poles of the Earth). This declination is not possible for the Moon, which can only ever have declinations between 28.5S and 28.5N. The highest the Moon will ever get in the sky above Rome is therefore 13.5 deg off zenith, at which point it would illuminate part of the Pantheon, just at an oblique angle (ie. part would be in "Moon shadow").

A Google search gives 43.3m as the height of the interior of the Pantheon dome. A person standing directly below the hole therefore can see an area of the sky around the zenith of radius 5.9 deg (arctan(4.5/43.3)) So the full Moon (diameter 0.5 deg) can be seen in full from the floor of the Pantheon directly below the hole if it has an altitude between 84.3 and 90 degrees (ie. within 5.7 deg of directly overhead). As described above this cannot happen. However, when the Moon reaches 13.5 degrees off Zenith (ie. ALT=76.5) moonlight will fall on a spot about 6m from the centre of the Pantheon. At lower altitudes the moonlight will fall further and further from the centre until it starts falling on the walls and finally cannot be seen directly through the hole in the roof at all.

The highest position of the Full Moon occurs in the winter months when the Sun is at it's lowest in the sky - the Full Moon is exactly opposite the Sun in the sky....

Date of Full Moons in 2006:
 Highest point of Moon Local Time ALT AZ Jan 14: 0:00 74.9 179.4 Dec 4: 23:50 74.9 176.7 Dec 5: 0:00 74.9 185.0 Jan 2 (2007): 23:40 76.1 179.5 Jan 3: 0:00 75.6 197.0

You'll notice that these high points all happen in the middle of the night - perhaps not ideal for an art exhibition. This is because the Full Moon always rises at sunset, therefore reaches its highest point in the middle of the night. You'll also notice that the altitudes are not equal to the theoretical maximum. That's because none of these dates fall exactly on the winter solstice.

For the Moon to be at it's highest in the evening, it has to be First quarter. In this phase, the Moon reaches its highest points at the Spring Equinox.

First quarter Moons in 2006
 Highest point of Moon Local Time ALT AZ Percent illuminated Feb 5: 18:20 69.7 178.2 0.55 Mar 6: 18:20 74.9 194.5 0.49 Apr 5: 18:40 75.1 181.9 0.52

Which date/time will give the best illumiation is not something which is very easy to answer. You need to reach a comprimise between the amount of the Moon which is illuminated verses how late at night it reaches it's highest point. Also note that the Full Moon doesn't fall exactly on the winter solstice in 2006, neither does the First quarter Moon fall exactly on the Spring equinox...

In summary:

1. The Moon never passes directly overhead in Rome, Italy.
2. The Full Moon reaches its highest point at ~midnight in Winter months (near the Winter solstice).
3. First quarter Moon reaches its highest point at ~6pm in Spring months
4. In very early spring/late winter the phase of the Moon between Full and 1st quarter reaches its highest point in the late evening (between 6pm and midnight).

I think I would pick a Moon in a phase between Full and First quarter, in early Spring, for example Feb 9th or March 10th when the Moon is about 70% illuminated and reaches its highest point around 9pm local time. If you're OK with an exhibition at midnight you can do it on a Full Moon in the middle of winter.

As a final note, Azimuth refers to the angle around the horizon. 180 degs is due South, so to be in "direct moonlight" on these nights, you need to be on the Northern side of the Pantheon, about 6-8m from the central point.

Good luck with the exhibition.

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