The Moon Distance tab displays a variety of data related to the size of the Moon.
Everything is calculated for the selected location and/or date/time.
The distance from the center of the Earth to the center of the Moon (geocentric distance).
The distance from the observer’s location to the center of the Moon (“ topocentric ” distance)
The apparent size of the Moon as a percentage relative to its average geocentric distance.
Angular Size
Depending on the time during the lunar cycle, the Moon may be closer or farther from Earth. As it moves closer in its orbit, its angular size increases. The average angular size, like that of the Sun, is around half a degree, but it varies by location and time. You can obtain an accurate value on this tab.
Apparent Size
Photo: © Fred Greco. Used with permission.
Apparent Size is very much a terrestrial concern. It’s not about the Moon appearing larger or smaller in reality, but instead about the impression of the Moon’s size when juxtaposed against terrestrial features such as a building or mountain summit.
Simple geometry dictates that the farther you are from a feature, the larger the Moon will appear relative to it.
A simple thought experiment illustrates the principle: if you are one mile from the Chrysler Building in New York and the Moon is 228,000 miles away, and you then catch the train into New Jersey, another 13 miles away, you may end up 228,013 miles away from the Moon. The Chrysler Building appears significantly farther away. The Moon, not so much. As a result, the Moon looks relatively larger when juxtaposed behind the Chrysler Building.
This effect increases with the distance from the observer (or camera) to the subject. When you see a genuine “ giant ” Moon next to a building, the shot was necessarily taken at a significant distance from the building using a telephoto lens.
Distance Control
You can dial in the distance from the observer or camera to the subject using the dial control. Use your finger to rotate the dial - the faster you go, the more responsive the control. If you want a faster response, double-tap to increase the response speed tenfold.
The apparent size of the Moon, that is, how large it appears relative to a subject at the specified distance, is calculated based on your setting and the current lunar distance.
Why would you want to know this? If, for example, you’re trying to align the Full Moon behind a skyscraper, if you know the height of the building, knowing the apparent Moon size tells you how large it will appear relative to the build. If your building is 800 ft tall, then if the apparent size of the Moon is 400 ft, then it will match half the building height when juxtaposed against it.