The Shriking Moon

The Shrinking Moon

Everyone has no doubt seen a large orange glowing disk hiding behind the trees and buildings on the horizon in the fall. The Harvest Moon, probably the most famous Moon, is an excellent example of the apparent change in size and color the moon goes through as it rises above the horizon, and then above us into the night sky. On the horizon, the Moon appears to be much larger and has a deeper orange tint than the usual white it gives off when directly above us in the night sky. Is this apparent change in size a trick or optical illusion, or is there some outside force at work that causes the Moon to move closer on the horizon? This is the question that this paper will attempt to test, discuss, and solve.

When rising just above the horizon, the Moon appears to be larger than when it is directly above us on the celestial sphere. However, it is really the same actual size as it is at any other point in the sky. The Moon’s orbit is nearly a perfect circle with an eccentricity of .05, and is an average 384,000 kilometers away. Therefore it is impossible for the Moon to be closer or further in a matter of hours.

In order to prove this hypothesis, the angular sizes of the full Moon will be taken when the Moon is directly above the horizon, and thus at the point where it looks the largest to the human eye, and then again when the Moon is nearly directly above us in the sky, and thus at the point where it looks smallest.

To get the angular sizes of the moon, the use of an “Angletron” was needed. An “Angletron” is a made up device used to measure the angular size of the moon by sectioning off small angles on paper using trigonometry. This was developed by University California at Berkeley as an easy way for student with little access to fine instruments to measure angular sizes of celestial objects (Angle 1). The basic idea behind the Angletron is to use a basic small angle formula (Size/Distance X 57.3 degrees) to divide a piece of paper into ј degree margins used to measure the angular size of a celestial object, in this case, the Moon. I measured the moon at the horizon line, or near it, due to objects blocking the horizon, and again when the moon appeared to be near the top of its