Phases of the Moon

The Moon's monthly cycle is due to its orbital motion about the Earth. Tracking the Moon and its phases can help you to see the sky in three dimensions.

Reading: Stars & Planets, p. 308 to 310 (The Moon).


The Moon is a ball of rock lit by the Sun. As it orbits the Earth, different parts of its surface are in sunlight, 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 distant source of light will show phases much like the Moon's. Imagine a white ball hanging on a thread in front of you. Here's how the ball's appearance changes as the light source moves around:

Position of light source Appearance of ball Phase
Far behind the ball Completely dark new
Behind ball, to the right Most of left side in shadow crescent
Far off to the right Left half in shadow quarter
Behind you, to the right Left edge in shadow gibbous
Far behind you Completely illuminated full
Behind you, to the left Right edge in shadow gibbous
Far off to the left Right half in shadow quarter
Behind ball, to the left Most of right side in shadow crescent

The third column above gives the names of the various shapes or phases of the Moon; `new', `crescent', and `full' are self-explanatory, and `gibbous' just means not quite full. On the other hand, `quarter' may seem counter-intuitive, since half, and not a quarter, of the visible side is illuminated; a quarter moon is either one-quarter or three-quarters of the way through the cycle from one new Moon to the next.

In reality, of course, the Moon is moving about the Earth, while our `source of light', namely the Sun, stays fixed. 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 use the appearance of the Moon to 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, a bit further north, and much further away. By doing this, you are also learning to see the Sun and Moon as objects in space, rather than light sources glued to the inside of some imaginary celestial sphere.

Earthshine

When you see a crescent Moon in the sky, you may notice that the part in shadow is not completely dark. If you look closely, you may even be able to see some details within the shadowed region. The shadowed side of the Moon would be completely dark if the Sun was the only source of light; what other source of light does the Moon have? As anyone standing on the Moon could tell you, the other source of light is the Earth.

Earthshine is easiest to see 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. In addition, only a slim crescent of the Moon is sunlit, so there's less glare to interfere with your view of the dark side. If you look closely, you may be able to see earthshine 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 earthshine tends to be stronger when storms cover most of the globe.

MOTION AND PHASE

If you follow the Moon across the sky, you'll see that its phase changes from one night to the next. New Moon occurs on 08-Sep-2010 at 00:30 HST (10:30 UT). A few days later the Moon is visible as a crescent; by next week's lab (14-Sep-2010) it's near first quarter, and one week after that it's nearly full. If you continue observing the Moon you will see it change from full to last quarter and then back to a crescent; the next new Moon is on 07-Oct-2010 at 08:44 HST (18:44 UT). This cycle of phases repeats 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, we say the Moon is waxing, or getting more full, while during the second half, we say the Moon is waning, or getting less full.

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 08-Sep-2010, the new Moon will rise and set at almost the same times as the Sun. The next day, moonrise and moonset will occur almost an hour later. Each day these times will shift by roughly one hour, until on 22-Sep-2010, 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.

OBSERVING LUNAR PHASES

To see the relationship between the Moon's phase and its position relative to the Sun, it helps to make observations at the same time from evening to evening. That way, the Sun is in (roughly) the same position each time you observe, so the direction of sunlight will be constant, and the Moon's phase will related in a simple way to its position in the sky.

A convenient time to observe is around 18:45 (6:45 pm), which is currently just after sunset. It's not necessary to make your observations at exactly the same time each night — any time between 18:30 and 19:00 (6:30 to 7:00 pm) is fine. On Tuesday nights you can observe just before class.

The chart distributed with this handout represents a panoramic view facing South, with East on the left and West on the right; these compass points are marked along the bottom. The grey region is below the horizon (elevation < 0°), while the rest is the sky above. The light grey circles show the Moon's position at 18:45 each September evening that it's visible. The dark circle below the horizon to the West indicates the Sun's position at this time in mid-September.

Each evening that you observe the Moon in September, fill in the circle for that date to represent the Moon's phase. Use a sharp pencil — not a pen — to shade the dark side of the Moon's disk. To represent the shape accurately, begin by sketching the line dividing the light side from the dark (this is known as the terminator); pay attention to the way it crosses the Moon's disk.

The circles on the panoramic chart are too small to sketch the features you can see on the Moon's bright side. You can do that using the larger circles on the other side. Again, use a pencil for these sketches, and begin by drawing the terminator. Fill in the date and time of each sketch, and note if you can see earthshine.

We will repeat this sequence of observations in October and November.

A Moon in your Pocket

After watching the Moon's phase change for a few weeks, 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, do a simple experiment. On a sunny day, find the Moon in the daytime sky. This can be a bit tricky if you're not sure where to look. In general, if the Moon is waxing, you expect it to rise after the Sun and follow it across the sky. For example, on 14-Sep, the Moon will rise about 6 hours after the Sun, and during the afternoon you can expect to find it somewhere to the south-east. If you're not sure if the Moon is even visible, you can alwys use the Your Sky web page to get a current chart of the sky from Honolulu.

Once you've found the Moon, 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. Sketch the phases of the Moon and the ball on the back of the worksheet. Repeat this experiment on another day. Do the Moon and the ball have the same phase?

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Joshua E. Barnes      (barnes at ifa.hawaii.edu)
Updated: 06 September 2010
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