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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.
A ball illuminated only by a 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 direction of the light. The table below describes the appearance of the ball as we move the light source 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 |
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.
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.
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 the Moon's motion by looking for it at the same time every evening for a few weeks. You should do this in mid-February, 2005; the Moon is new on 02/08/05 and full on 02/23/05, so a series of observations during the second and third week of February will give you a good chance to see the Moon move across the sky. (If this cycle is too cloudy, we'll try again in March or April.) 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). Begin looking around 18:30 (6:30 pm), which is just about sunset. If the weather is good, you should be able to spot a slender crescent Moon low in the West by Feb 10. Binoculars can help you find the Moon, and it's safe to use them after 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. Come back next day and look at the same time; you'll find the Moon considerably higher in the sky and easier to see. Keep on making observations every night you can, and record them in your log. A half-dozen observations spread over the two weeks between new Moon and full Moon 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 full-sky view, with the zenith at the center and the horizon at the edge. For your first observation, you can assume that the setting Sun is at azimuth 255, 15° south of west. 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 and time next to the Moon each time.
The changing times that the Moon rises and sets are due to its motion about the Earth. Because the Moon moves across the sky in a direction opposite the apparent rotation of the celestial sphere, the average time from one moonrise to the next is almost 25 hours (and likewise for the time from one moonset to the next). For example, on Feb. 8th the Moon will rise and set at almost the same times as the Sun, while on Feb. 23 it will rise about sunset and set about sunrise. The Moon will continue to rise and set later and later through the rest of February; if you want to observe moonrise in late February, for example, you'll have to stay up late, or get up before dawn, or both.
As you track the Moon across the sky, you'll notice that its phase changes from one evening to the next. In late January the Moon appears as a crescent; by the end of the first week it will appear at first quarter, and by the end of the second week 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 Mar. 9th. This cycle of phases occurs every 29.5 days, which is the time it takes the Moon to make one trip across the sky and come back to the same position with respect to the Sun. We normally count cycles from one new Moon to the next; during the first half of the cycle, the Moon is waxing (getting more full), while during the second half, the Moon is waning (getting less full).
By now it will probably be obvious that the Moon's phase is determined by the angle of the sunlight striking its surface. To reinforce this point, however, you should do a simple experiment. On a clear day in late January, locate the Moon in the daytime sky - this should be pretty easy after Feb. 12th or so if you look late in the afternoon. Hold a small ball up in sunlight next to the Moon, and compare its phase with the phase of the Moon. For best results, use a ball made from some fairly dull, opaque material; if the ball has a shiny surface, you'll get a bright spot which will make it harder to see phases. Repeat this experiment a few times over the next week or so. Do the Moon and the ball always have the same phase?
This chart shows a wide-angle view looking due south. Compass points are marked along the bottom; the top of the chart is 75° above the horizon.
Animation showing the Moon as seen from the Earth from 12/31/03, 14:00 to 5/31/04, 08:00 (01/01/04, 00:00 UT to 5/31/03, 18:00 UT). Note that the Moon's size changes as its distance varies, and that the Moon "nods" slightly. Generated using NASA's Solar System Simulator.
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Make the observations described in the sections on LUNAR MOTION and MOTION AND PHASES, and write a report on your work. This report should include, in order,
In more detail, here are several things you should be sure to do in your lab report:
Last modified: February 9, 2005
http://www.ifa.hawaii.edu/~mickey/ASTR110L_S05/lunarmotion.html