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Astronomy 2021: Closer to Home – The Solar System

“Uncle Bob”

Earth's Moon

credit: Lick Observatory/ESA/Hubble 
Editor: Yvette Smith

Before we get to the scientific details I have a few Moon-related treats for you. Many of you were not around on
20 July 1969 when Apollo 11 and the first astronauts landed on the Moon. I was in a rec room at Fort Gordon, Georgia, and watched it happen. My heart rises in my chest when I see it even today. You can watch it here.

Also, to set the Moon mood, I’ve dug out a few classic Moon tunes for you. No one in my opinion, not even Neil Armstrong, is more closely identified with the Moon than the Velvet Fog Mel Torme – by those my age anyway. Here are two from Mel sandwiching a jazzy flight up there played by Beegie Adair.

Now the Facts – Caution: a quiz will follow – no joke.
The Audubon Guide reports the Moon’s rotation and sidereal period in synch at 27.32 days. The synodic period, the time between full moons, is 29.53 days. These data are more fully explained below. The Moon is approximately three-fifths as dense as Earth and is very roughly one-64th the volume of Earth.

The Moon Rotates
But not so that we would notice. The Moon’s rotation is in synch with its revolution about the Earth, the sidereal period, so we always see the same face. A bit of wobble called libration allows us to see slightly more than half of the lunar surface as regions near the edge go in and out of view. The far side is sometimes called the dark side, but it is fully or partially exposed to the sun except in our full phase. The Sun “sees” half a sphere always.

So the Moon goes once around us in 27.32 days, but the time between full moon phases is the synodic period or 29.53 days. Why? Well, in one month’s time the Earth has progressed to a new position in its orbit and the Moon needs two extra days to get fully behind us again.

The New Moon is more or less between Earth and Sun, but not exactly except for the times of solar eclipses; the Moon is up during the day and not seen from Earth. In the days following a crescent may be seen in the west, growing (waxing) each evening, and setting about 50 minutes later each night.

After a week the Moon is in 1st Quarter phase. We see half the moon face lit by the Sun which is 90 degrees to the west. The Moon will be due south at sunset. Keep in mind that the Sun is “seeing” a fully lit face, half of it on the far side we never see.

After two weeks. The Moon has waxed to its full phase. It is 180 degrees away from the Sun and located due south at midnight. Lunar eclipses can occur at these phases when the Earth is directly between the two. The Moon now begins waning.

After three weeks the Moon is at Last Quarter phase and rises around midnight. The Sun is now 90 degrees to the east. In another week the New Moon phase will begin another lunar month.

High or Low?
The full Moon, being on the other side of Earth from the Sun, is high in the sky when the Sun is low. That’s winter. Earth’s axis points away from the Sun and exposes a more elevated full moon to the northern hemisphere.

As Earth travels in its orbit its axis tilt largely maintains its direction. In (northern hemisphere) summer, when the Sun is high, the midnight full Moon may shine in your south-facing windows and keep you awake. In spring and autumn, both Sun, and Full Moon are at moderate elevations.

What about first and last quarter phases? The Illustration helps us to see why 1st quarter moons are high in the spring. Earth’s axis is pointing toward that phase. Note also that the last quarter Moon in spring will be low in the sky, shining into your south-facing windows in the wee hours, due to that same tilt.

Ready for the Moon Quiz?
Answers can be found in solutions, but try to work these out based on the information given, or from your own research.

1. Given the Moon's synodic period and the Earth’s year of 365.25 days, on average, how many full moons do we have in one year?
2. Is it possible that a month of February could be bereft of a full moon. Why or why not?
3. Using the very approximate density and volume ratios given above, estimate what percent of Earth’s mass would we expect the Moon to have?
4. Characterize the time and elevation of the first quarter Moon around the time of the autumnal equinox. In the same way, describe the last quarter phase at summer solstice.
5. Do we know why the Moon’s rotation and its revolution are synchronized?


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