The Universe in the Classroom

No. 12 - Winter 1988-89

© 1989, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112

The Moon: It's Just a Phase It's Going Through...

Editor's Note: The 20th anniversary of the first manned landing on the Moon--when Neil Armstrong and Edwin Aldrin became the first members of our species to walk on another world--will be celebrated this July 20th (1989). To prepare for this epochal event, the next two issues of The Universe in the Classroom will be devoted to the Moon. In this issue, we'll take a look at the Moon in our sky--its phases and some ways in which students can enjoy (and learn from) its ever-changing appearance. Our next issue will consider the Moon as a world it its own right--its craters, plains, and mountains--and present a short history of lunar exploration.

Our planet's large natural satellite, the Moon, is the easiest (and cheapest!) astronomical object to observe. The only "scientific instrument" you'll need at first is a pair of eyes. The Moon is the only thing in the sky (other than the Sun) that doesn't look like just a pinpoint of light or an indistinct fuzzy patch as seen with the unaided eye. Even better, the way the Moon looks to us is continually changing; keeping track of its appearance from night to night (or day to day) is a fascinating and easy way to get acquainted with the rhythms of change in the sky.

The Moon's Orbit

The Moon orbits around the Earth, taking about a month to go all the way around our planet. Its orbit is very nearly circular; it stays about 380,000 kilometers away from us as it moves counterclockwise (as viewed from a northern hemisphere perspective). It also stays fairly close to the Earth's equatorial plane (an imaginary extension of Earth's equator out into space); the point on the Earth directly under the Moon is never more than 29° north or south of the equator.

Demonstrating the Moon's Motion

Over the millenia, the Moon has become "locked" into a special kind of motion around the Earth. It rotates on its axis at the same pace as it revolves around the Earth. As a result, the Moon keeps the same face toward us throughout its orbit.

Younger students can enjoy demonstrating this motion by "orbiting" around another student seated in the center of the classroom. Have the "Moon" student begin by facing the "Earth" student (the one seated in the center). Then have the "Moon" student "orbit" — circle around the student in the center. The "orbiting" student will find that if he carefully swivels his body 1/4 turn for each 1/4 "orbit" around the room, then he will always be facing inward as he completes a full circle. (His back will never be visible to the seated "Earth" student in the center.) This is why astronomers speak of the "nearside" and "farside" of the Moon.

Phases of the Moon

Since the Moon shines by reflecting sunlight, what it looks like to us at any particular time depends on the angle at which the Sun's light is hitting the Moon. For example, if sunlight is coming from the right, the right half of the Moon's globe will be bright and the left will be dark. If sunlight is coming from behind the Moon as we look at our satellite, then its farside will be illuminated, and we will see a dark globe or "new moon."

Moon phases

FIGURE 1: The pictures are shown from a northern hemisphere perspective. Those of us who live in the northern hemisphere (outside the tropics) have to look generally south to see the Moon when it's highest in the sky, and we wee the right-hand side illuminated at first quarter phase, for example. People who live in temperate or high latitudes south of the equator must look toward the north; to them the first quarter moon has its left side illuminated.

The diagram is not to scale. In reality, the Moon is 1/4 the diameter of the Earth and its orbit's width is about 60 times the Earth's diameter.

Figure 1 shows the Moon during one orbit around the Earth from a vantage point far above Earth's North Pole. It also shows what the Moon would look like when it is highest in the sky at eight times during the month.

Let's look at these phases of the Moon one by one:

New Moon New Moon

At new moon, the Sun and the Moon are quite close to each other in the sky. (In fact, the Moon in its new phase will sometimes block all or part of out view of the Sun. This event is called a solar eclipse. The side toward us is in shadow and is dark. Moreover, since the Moon and Sun are so near each other on the sky, they are above the horizon at the same time. Thus the Moon is only in the sky during the day, when the sky is bright--which means a new moon cannot be seen from Earth!

Waxing Crescent Waxing Crescent

As time passes after new moon, the Moon--orbiting counterclockwise around the Earth--moves away from the Sun toward the east from our vantage point, traveling about 12 degrees per day toward the left, as seen from Earth's northern hemisphere. (To help you get a feel for measuring sizes on the sky using degrees, note that the full moon is about half a degree wide. So 12 degrees is about 24 times the Moon's width on the sky.) Two or three days after new moon, a waxing crescent can be seen just to the east (left) of the Sun. It is most easily seen just after sunset, following the Sun closely down toward the western horizon. Notice that the "horns" of a crescent moon always point away from the Sun (alternatively, you can think of a crescent moon as a bow about to shoot an arrow toward the Sun). Also, since the Moon in a crescent phase--either waxing or waning--is close to the Sun on the sky, it can't be above the horizon at night unless it's shortly after sunset or before sunrise. (An exception to this is if you are near the Earth's North or South poles. At those places, near the time of the "midnight sun," it is possible to see the crescent moon above the horizon near midnight.)

First Quarter First Quarter

About a week after new moon, the Moon has moved about 90 degrees away from the Sun, a quarter of the way around the sky toward the east (left). At this time, its right-hand half is illuminated by sunlight. This phase is called first quarter because it occurs when the Moon has completed the first quarter of its orbit from the previous new moon phase. Since it is 90 degrees (1/4 of a full circle) away from the Sun on the sky toward the East, the Moon will lag behind the Sun by about 6 hours (1/4 of 24 hours): it will rise at about noon, be at its highest in the sky at about sunset, and so on. During first quarter and waxing gibbous phases, the Moon is quite easily seen in the blue eastern sky on a clear afternoon. It's also easy to see in the western sky on clear mornings at waning gibbous and last quarter phases. so the old song's assertion "I got the Sun in the mornin' and the Moon at night" isn't always true--at least for the Moon.

Waxing Gibbous Waxing Gibbous

During the following week, the Moon continues to further distance itself from the Sun toward the East by about 12 degrees per day. As it does so, it waxes "fatter" as more and more of the side facing us is illuminated by sunshine. This phase is called waxing gibbous.

Full Moon Full Moon

When the Moon has completed exactly half of its trip around the Earth from the previous new moon, it is on the opposite side of the Earth from the Sun; the face toward us is fully illuminated, and full moon has arrived.

Strictly speaking, "full moon" occurs only for an instant: the time when the Moon is as nearly opposite the Sun on the sky as it will be during its current orbit. however, the Moon will look only imperceptibly different from full for a night or two around that time.

The Moon's orbit around the Earth is tilted by about 5 degrees relative to our planets orbit around the Sun. (If you think of the two orbits as hoops, the two hoops are tilted relative to each other.) This means that full moon doesn't always bring the Sun, Earth and Moon into perfect alignment. If it did, the Moon would move directly into the Earth's shadow, which extend straight back, away from the Sun, at full moon. This does happen occasionally though; the phenomenon is call a lunar eclipse.

Near the time of full phase, the Moon is opposite the Sun; thus it rises around sunset, is high in the sky around midnight, and sets around sunrise, providing light that's bright enough to read by all night long when the weather is clear. (We're not sure your opthamologist would actually recommend reading by the light of the Moon, but astronomers like to point out that it is possible.)

Interestingly enough, though, the Moon is actually one of the darker objects in the solar system. it reflects only 7% of the sunlight that strikes it, absorbing the rest. For comparison, the Earth reflects 37%, Venus 65%, and Mars 15%.

Blue Moon

Since the time between one full moon and the next is 29-1/2 days, it is possible for a calendar month to have two full moons — one at the beginning and one at the end of the month. When this happens, the second full moon of the month is called a "blue moon" — although its color isn't really affected at all. These calendrical coincidences happens every few years.

Waning Gibbous Waning Gibbous

During a week after full moon, as the Moon continues to move eastward in its orbit about 12 degrees per day, the Moon rises later and later and the side we see is illuminated by less and less sunlight — its light wanes. While the Moon is in this waning gibbous phase, its east (left) side remains fully sunlit, while darkness creeps in from the right.

It is during this time that the delay of moonrise from one night to the next is most easily noticed. As the Moon moves farther toward the east from night to night the Earth has to turn a little farther toward the east each night for us to see it. On the average, the Moon rises about 50 minutes later each night, but this can vary significantly depending on the time of year and your latitude north or south of the equator.

Last Quarter Last Quarter

As last quarter phase — when the Moon enters the last quarter of its orbit back to new moon — the eastern (left) half of its disk is illuminated. Now, the Moon has moved to 90 degrees west of the Sun, and precedes it by about 6 hours in their daily race across the sky: first quarter moon rises at about midnight, is highest in the sky at sunrise, and sets at about noon. Thus, the last quarter Moon is easy to see in the western sky on clear mornings.

Waning Crescent Waning Crescent

Finally, during the last week of its monthly orbit the Moon draws closer and closer to the Sun on the sky, approaching it from the west (right). The waning crescent's horns extend toward the west, away from the Sun, and add a pretty touch to the pre-sunrise sky, low in the east.

A day or two after this phase, the Moon will be new again, and the cycle begins anew...

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