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
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,
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?
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- 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?