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.

Background Reading: Stars & Planets, p. 308 to 318 (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 distant source of light will show phases much like the Moon's. Imagine a white ball hanging on a thread in front of you. We will call the side of the ball which faces you the `visible' side, even though part may be in shadow. 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

Visible side 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

Visible side 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 pretty 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 its orbit.

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 experience, you can learn to `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, 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 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 beautiful sight is sometimes called `the old Moon in the new Moon's arms'. 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.

You can see earthlight best 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 our view from Earth. 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.

MOTION AND PHASE

If you follow the Moon across the sky, you'll see that its phase changes from one night to the next. The Moon will be new on 10/03/05 at 0:28 HT (10:28 UT). In early October the Moon appears as a crescent; one week later it's near first quarter, and one week after that it's nearly full. If you continue observing the Moon for the rest of the month you will see it change from full to last quarter and then back to a crescent; the next new Moon is on 11/01/05 at 15:25 HT (11/02/05 at 1:25 UT). This cycle of phases repeats every 29.5 days, which is the average 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.

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 Oct. 3rd, 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, and by Oct. 17th, 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

As the Moon orbits the Earth, the angle of the sunlight falling on it changes from night to night. You can get a good sense of this by viewing the Moon at roughly the same time every few nights for a couple of weeks. The point of observing at the same time each night is to make sure 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 will related in a simple way to its position in the sky.

The best time to make your observations is right after sunset -- say, about 18:45 HT (6:45 pm). If you can observe at this time, you'll get to see the change from a slender crescent Moon to a full Moon. However, some people may not be able to observe at this time on a regular basis. You can also do this project at other times; the main thing is to pick a time of night when you can observe every few nights for a couple of weeks, and stick with that time. Try to make your observations within about 15 minutes of the your chosen time.

For full credit, you should make at least five observations; if bad weather makes this impossible, we'll try again in November.

Depending on your choice of observing time, you will want to start and end your observations on different dates. The table below lists some possible observing times and the dates that go with them. Again, the best time is right after sunset (18:45 HT); that's especially convenient because the start date is a lab night, and we can meet you outside PSB to help with the first observation. If you want to observe at a time not listed in this table, talk to me and I can tell you when you should plan to begin your observations. If you get up early you could even observe just before sunrise (say, 06:00 HT); that would give you a chance to see the Moon change from full to a thin crescent.

Time (HT)

Start date

End date

18:30--19:00

10/05/05

10/17/05

20:30--21:00

10/07/05

10/21/05

22:30--23:00

10/09/05

10/22/05

You need to know compass directions to record your observations. Luckily, most of the Manoa campus is laid out North-South and East-West, so finding compass directions is easy. For example, Correa Road in front of PSB runs East-West; if you stand at the doorway of PSB and face the road you are looking South. If you observe from an off-campus location, check a local map to determine compass directions; if all else fails, remember that the Sun sets in the West! (Actually, at this time of year, it sets about 8, or four finger-widths, toward the South of due West.)

On one side of the worksheet included with this handout is an all-sky chart on which you should sketch the Moon's position and phase each time you observe it. For example, suppose you see the Moon due South about half-way between the horizon and the zenith; you should plot the Moon about half-way between the S compass point and the center of the all-sky chart. Draw a small circle to represent the Moon, and black out the part which appears dark. Write the date of each observation next to the Moon's circle. On the other side of the worksheet are small circles you should use to sketch the Moon each time you observe it. Remember to date each sketch you make. Make a note if you can see earthshine on the dark part of the Moon.

A Moon in your Pocket

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 day in October, find the Moon in the daytime sky - this should be easy after Oct. 10th 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. Sketch the phases of the Moon and the ball on the worksheet. Repeat this experiment on another day. Do the Moon and the ball have the same phase?

REVIEW QUESTIONS

  • Here are four images of the Moon. What is the phase of each image, and where is the Sun located with respect to the Moon?

  • Suppose the Moon was flat like a coin, with one side always facing us. What would the Moon's phases be like?
  • Suppose the Moon and the Sun were actually attached to a celestial sphere, and thus always at equal distance from the Earth. Would the Moon ever appear as a crescent?