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 in the night-time sky to figure out where the Sun is. For example, if you see a crescent Moon in the east with its bright side facing down and somewhat to the left, you can be sure that the Sun is below the horizon, a bit further north, and much further away. When you do 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

As the Moon moves around us, its phase changees from one night to the next. The Moon was new at 17:04 on 28-Aug-2011 HST (03:04 on 29-Aug UT). When the lab met two days later (30-Aug) the Moon was visible as a slender crescent; by next week's lab (06-Sep) it was just past first quarter, and one week after that it was just past 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 at 01:09 on 26-Sep HST (11:09 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 28-Aug-2011, 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 about 50 minutes, until on 11-Sep, 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 September, 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, we would like to track it over several months. This is best done as an out-of-class activity, since the Moon may not be visible during our observing sessions, even on days when it is visible at other times. For maximum credit, you should try to observe as much of the Moon's cycle of phases as you can.

The worksheet provided with this handout is a convenient place to record your observations of the Moon's phase. For this purpose, you don't need a telescope — just the worksheet, a pencil, and a way to tell time. Locate the Moon in the sky and note the date and time (including `am' or `pm' if you're not using 24-hour time). Begin by sketching the line dividing the light side from the dark side (this is known as the terminator); pay attention to the way it crosses the Moon's disk. Be sure to show the direction of the light — if you're sketching a crescent lit from below, don't show it lit from the side! Quickly shade the dark side of the Moon's disk; note if you can see earthshine on the dark side. Finally, if you want to, try to show some of the visible features on the Moon's bright side.

To see the complete cycle of lunar phases, you will need to observe at different times. When the Moon is waxing, you can be pretty sure of seeing it somewhere in the sky just after sunset. However, when the Moon is waning, it may not rise until late at night — a waning crescent won't rise until shortly before dawn. Since most of us find it inconvenient to get up before sunrise, the solution is to observe the Moon during the day. In general, if the Moon is waxing, it will rise after the Sun and follow it across the sky. Conversely, if the Moon is waning, it will rise before the Sun and lead it across the sky.

The table below may help you find the Moon in the daytime sky this semester. For example, from 12-Sep through 19-Sep, the Moon is in a waning gibbous phase, and can be seen in the morning setting in the west. The next week, it is a waning crescent, visible mornings in the east. On 27-Sep the Moon is new and therefore invisible — and in fact it will be quite hard to see on the previous and following days, unless you know exactly when and where to look. After that, the Moon will fill out more and more each day, until 11-Oct, when it is full and only visible at night.

Dates Phase Time Where
12-Sep — 19-Sepwaning gibbousmorningwest
20-Sep — 26-Sepwaning crescentmorningeast
27-Sepnew
28-Sep — 03-Octwaxing crescentafternoonwest
04-Oct — 10-Octwaxing gibbousafternooneast
11-Octfullnight
12-Oct — 18-Octwaning gibbousmorningwest
19-Oct — 25-Octwaning crescentmorningeast
26-Octnew
27-Oct — 02-Novwaxing crescentafternoonwest
03-Nov — 09-Novwaxing gibbousafternooneast
10-Novfullnight
11-Nov — 18-Novwaning gibbousmorningwest
19-Nov — 23-Novwaning crescentmorningeast
24-Novnew
25-Nov — 01-Decwaxing crescentafternoonwest
02-Dec — 09-Decwaxing gibbousafternooneast
10-Decfullnight

A Moon in your Pocket

After watching the Moon's phase change for a few weeks, it will probably be 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. 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 material; if the ball is shiny, the reflected light makes it harder to see phases like the Moon's. Sketch the phases of the Moon and the ball in the spaces provided on 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: 13 September 2011
http://www.ifa.hawaii.edu/~barnes/ast110l_f11/moonphases.html
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