Phases of the Moon

The Moon's monthly cycle is due to its orbital motion about the Earth. By tracking the Moon and its phases, we can begin to see the sky in three dimensions.

Background Reading: Stars & Planets, p. 302 to 305 (The Moon).


The Moon is a ball of rock lit by the Sun. As it orbits the Earth, different parts of its surface are illuminated, and we see the Moon go through a cycle of phases from new to full and back to new again. To understand the Moon's phases, you need to understand the play of light and shadow on its surface.

LIGHT AND SHADOW

A ball illuminated by a single distant source of light will show phases much like the Moon's. Imagine a white ball hanging in space some distance in front of you. We will call the side of the ball which faces you the ``visible'' surface, even though part of this side may be in shadow, depending on the position of the light. Here is how the appearance of the ball changes as the light source is moved around:

Position of light source Appearance of ball Phase
Far behind you Entire visible surface illuminated full
Behind and to the left Right edge in shadow gibbous
Far off to the left Right half in shadow quarter
In front and to the left Most of right side in shadow crescent
Far behind the ball Entire visible surface dark new
In front and to the right Most of left side in shadow crescent
Far off to the right Left half in shadow quarter
Behind and to the right Left edge in shadow gibbous
The third column above gives the names of the various shapes or phases of the Moon; ``full'', ``new'', and ``crescent'' are pretty self-explanatory, and ``gibbous'' just means not quite full. On the other hand, ``quarter'' probably seems counter-intuitive, since half, and not a quarter, of the visible surface is illuminated. The explanation is that a quarter moon is either one-quarter or three-quarters of the way through its orbit.

In reality, of course, the Moon is moving about the Earth, while our ``source of light'', namely the Sun, stays fixed. The odd name astronomers give to a Moon which appears half illuminated reflects this basic fact. But as far as the appearance of the Moon at any given instant is concerned, all that really matters are the relative positions of the Earth (our point of view), the Moon, and the Sun.

With a little experience, you can ``read'' the appearance of the Moon and figure out the Sun's position. For example, if you see a crescent Moon in the east, with its bright side facing down and somewhat to the left, you can deduce that the Sun is below the horizon and somewhat north of the Moon's position. In doing this, you are also learning to see the Sun and Moon as objects in space, rather than light sources attached to the inside of some imaginary celestial sphere.

Earthlight

When you see a crescent Moon in the sky, you may notice that the part in shadow is not completely dark. With binoculars or a telescope, you may even be able to see some details within the shadowed region. This is a beautiful sight, sometimes called ``the old Moon in the new Moon's arms''. Of course, the shadowed side of the Moon would be completely dark if the Sun was the only source of light - so what other source of light does the Moon have? The answer, obvious to anyone standing on the Moon, is the Earth.

The best times to see earthlight are just before and just after a new Moon. At these times the Earth appears nearly full as seen from the Moon and therefore provides the greatest amount of light; also, only a slim crescent of the Moon is sunlit so there's less glare to interfere with our view. If you look closely, however, you may be able to see earthlight at other points in the Moon's cycle. The amount of light reflected by the Earth changes from day to day; clouds reflect more light than open ocean, so earthlight tends to be stronger when storms cover most of the Pacific.

OBSERVING LUNAR MOTION

As the Moon orbits the Earth, its position with respect to the Sun changes from night to night. You can get a good sense of this motion by viewing the Moon at roughly the same time every evening or two between new Moon and full Moon. The point of observing at the same time each evening is to insure that the Sun is in the same position for every observation. That way, the direction of sunlight will be constant, and the Moon's phase is related in a simple way to its position in the sky.

The Moon is new on 09/25/03, 17:09 (09/26/03, 03:09 UT) and full on 10/09/03, 21:28 (10/10/03, 07:28 UT). A series of observations between these dates will give you a good chance to see the Moon move across the sky. (If the weather is too cloudy, we'll try again next cycle.) Observe from a place where you have a fairly clear view to the west (and, if possible, to the east, though this is not as important as the western view). Make your observations between 18:30 (6:30 pm), which is just after sunset, and 19:00 (7:00 pm). If the weather is good, you should be able to spot a slender crescent Moon low in the west on Sep. 26th or 27th; binoculars can help you find the Moon, and it's perfectly safe to use them once the Sun has set.

Once you've found the Moon, note the time in your observing log, sketch the Moon's appearance, and describe its position in the sky. Your description of the Moon's position doesn't have to be very precise; imagine telling a friend where to look for the Moon, and write down what you would say. Go back the next evening and look at the same time; you'll find the Moon much higher in the sky and easier to see. Make observations every evening that you can, and record them in your log. A half-dozen observations spread over two weeks will be enough to show you the Moon's motion; don't worry if you miss a few nights due to bad weather or other commitments.

In addition to describing the Moon's position in your log, you should also plot it on the chart included with this handout. The chart shows a wide-angle view, facing south; west is on the right, while east is on the left. (To visualize this, hold the chart up close at eye level and curve it around your face.) For your first observation, you can assume that the setting Sun is due west; if you stand with the sun on your right, south will be straight ahead, and east will be to your left. Pick out a few distant landmarks (mountains, buildings, or other permanent features), and mark them on your chart to establish reference points for future observations. Sketch the Moon's position and phase on the chart each time you observe, and write the date next to the Moon each time.

IMPORTANT: On Thursday, Oct. 2nd at 18:57 (6:57pm), the Moon will occult a bright star in Sagittarius. This event will give you a chance to see the Moon move along its orbit; another handout will give details. For now, plan to observe a little later on Oct. 2nd, and bring your binoculars!

Moonrise and Moonset

The times that the Moon rises and sets change due to its motion about the Earth. Because the Moon orbits the Earth in the same direction that the Earth spins, the time from one moonrise to the next (or one moonset to the next) is longer than one day - almost 25 hours. For example, on Sep. 25th, the new Moon will rise and set at almost the same times as the Sun. The next day, moonrise and moonset will occur about an hour later, and by Oct. 9th, the full Moon will rise about sunset and set about sunrise. The Moon continues to rise and set later and later through the rest of its cycle; to observe moonrise in late October, you'll have to stay up late, or get up before dawn, or both.

POSITION AND PHASE

As you track the Moon across the sky, you'll see that its phase changes from one evening to the next. In late September the Moon appears as a crescent; by early October it will appear at first quarter, and one week later it will appear nearly full. If you continue observing the Moon for the rest of the month you will see the phase continues to change from full to last quarter and then back to a crescent; the next new Moon is on 10/25/03, 02:51 (10/25/03, 12:51 UT). This cycle of phases occurs every 29.5 days, which is the time it takes the Moon to travel once around the Earth and come back to the same position with respect to the Sun. Cycles are counted from one new Moon to the next; during the first half of the cycle, the Moon is waxing, or getting more full, while during the second half, the Moon is waning, or getting less full.

By now it's probably clear to you that the Moon's phase depends on the angle of the sunlight falling on it. To reinforce this point, however, do a simple experiment. On a clear afternoon in early October, find the Moon in the daytime sky - this should be easy after Oct. 1st or so if you look late in the day. Hold a small ball up in the sunlight next to the Moon, and compare the ball's phase with the phase of the Moon. For best results, use a ball made from some fairly dull, opaque material; if the ball is shiny, it's harder to see phases like the Moon's. Repeat this experiment a few times over the next week or so. Do the Moon and the ball always have the same phase?

WEB RESOURCES

REVIEW QUESTIONS

REPORT: LUNAR MOTION AND PHASES

Make the observations described in the sections on OBSERVING LUNAR MOTION and POSITION AND PHASE, and write a report on your work. This report should include, in order,

  1. a general introduction to the observations,
  2. a description of the observing site and equipment you used,
  3. a summary of your observations, and
  4. the conclusions you have reached.

In more detail, here are several things you should be sure to do in your lab report:


Joshua E. Barnes (barnes@ifa.hawaii.edu)

Last modified: September 16, 2003
http://www.ifa.hawaii.edu/~barnes/ASTR110L_F03/moonphases.html