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The thousand faces of the full moon

You might think that when we look through the telescope to see or photograph the full moon we always see the same image, but the fact is that the full moon is different every time we look at it as it varies in size (up to 14%), in brightness (in the order of 30%) and in the visible area (the hidden area is not so hidden and we see 59% instead of 50% as if we saw only half of it). The variations in size and brightness are due to the change in distance between the Earth and the Moon, and the additional 9% of visibility is due to the libration, an oscillation caused by the tilt of the Moon's axis of rotation.

The photo below is from the April 2020 Supermoon. Supermoons are full moons that occur near the perigee, when the Moon is closest to the Earth in its elliptical orbit around the Earth. Formally, it is considered a Supermoon when the relative distance between the full moon and the Earth is 0.9 or more.

The relative distance is defined as (Da-Dfm)/(Da-Dp) where Da is the distance at apogee, Dp at perigee and Dfm is the distance from the full moon. If the full moon coincides with the perigee, the relative distance is 1.


The photo corresponds to the full moon of April 8, 2020, which occurred only a few hours after the perigee, so the relative distance is 0.997, a very true Supermoon. But can we appreciate the 14% diameter increase with the naked eye? If we extend our arm, the little finger will occupy approximately 60 minutes of angular measurement above the sky, twice the diameter of the moon which is about 30 minutes. But during the Supermoon, the diameter of the moon will be about 4 minutes longer, that is, instead of half the little finger it will be half plus a small fraction that we will not be able to see with the naked eye. Possibly the feeling we have that the Supermoon is much larger than other full moons is because we look for it or photograph it more often, its greater brightness makes it more prominent, and when it is close to the horizon a visual effect gives us the impression of seeing it much larger.

However, when we photograph the Moon, it may be that depending on the field of view resulting from the telescope's focal point and the size of the CCD, the difference in apparent size is critical. This is my case, in the perigee I cannot fit the whole full Moon in the field of view, which however enters with a slightly smaller apparent size. The following picture is taken in November 2019 with the same camera and a focal point almost identical to the previous one (a small difference due to the use of different filters). The effect of the libration can also be clearly seen, showing a part of the hidden face that was not seen in the previous photo.



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