Monday, October 18, 2021

The Motion of the Moon

Looking high in the east at 11 PM September 28th. A nearly Full Moon is known as gibbous. - Submitted Photo

Looking high in the east at 11 PM September 28th. A nearly Full Moon is known as gibbous. – Submitted Photo


By Mike P. Usher

We don’t often discuss the Moon in this stargazing column because, frankly, it’s a real pain to stargazers worldwide – wiping out all but the brightest stars with its own brilliance. The Moon seems bright mainly by contrast; in reality the rock that makes up the Moon is not much more reflective than coal.

The Moon is a fascinating object in its own right, various civilizations have amused themselves by finding patterns like the Man in the Moon, or a rabbit in the dark areas called Maria. The most notable thing about the Moon to a naked eye observer is its phase; over the course of 29.5 days (approximately) it goes from fully lit, to invisible and back to fully lit in a very predictable pattern.

When the Moon is in the direction of the Sun the phase is referred to as New. The crescent Moon is first seen in the west just after sunset a couple of days after the New Moon. Over the next week the Moon moves further east and appears higher and higher in the sky at sunset until the Moon is at the meridian when the Sun sets. When the face of the Moon is 50% lit on the west side it is known as the First Quarter Moon and happens about seven days after the New Moon.

The Moon continues eastwards and at each succeeding sunset appears lower and lower in the eastern sky until it rises at the same time as the Sun sets. The Moon’s disk is fully illuminated and is now known as the Full Moon. Each day after the Full Moon, it rises about 50 minutes later than the day before. ( By no coincidence at all the tides rise 50 minutes later each day also). Three weeks after the New Moon it sits on the meridian at sunrise, is 50% lit and is known as the Last (or Third) Quarter Moon.

As one might expect, at sunrise each day it can be seen that the Moon draws closer to the Sun, shrinks towards a crescent and 29.5 days after the last New Moon the cycle begins again.

All this was well known to the ancients and not really difficult to understand, but the Moon has additional subtle motions that just drove the ancients nuts. For example, the 29.5 day cycle is not really constant – it can vary by some hours. Also more troubling (for the ancients anyway) is the Moon does not follow the same path through the sky each month but varies in a complex way and repeats itself each 18.6 years. Also during the course of a single month the Moon speeds up in its orbit and at other times slows down.

All these motions had to await Isaac Newton’s Theory of Gravity for proper explanation and even he had trouble with them. He later said working out the motion of the Moon was the only problem that ever made his head ache!

See you next time!

Mr. Usher is President of the Everglades Astronomical Society which meets the second Tuesday each month at 7:00PM in the Norris Center, Cambier Park, Naples.

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