Phases of the Moon Introduction
A recent survey of the graduating students from the Harvard School
of Law discovered that 90% of the students could not describe the causes of
the Moon's phases. Ideas about the Moon or Earth shadow abounded, as did a host
of different celestial alignments which were Ptolemaic. While this is a relatively
short page, I consider it to be exceedingly important for the Astronomy student
to study. I will introduce the basic principles, and then move on to the subtleties
that make a study of the lunar phases more interesting and complete.
This page, and the one that follows took a long time to create.
With videos available, I am including a few here now, to give you a general
picture before you do further reading.
Video - 3 min 35 sec video ... very simple with one clear error, but it
covers phases, tides, gravity, surface stuff, etc.
Phases - 2 min 24 sec video ... even more simple with a HUGE error ... can
you find it? Hint (look at the images of the Moon) good at names of phases and
the actual appearance of the phases!
In brief, the phases of the Moon are caused by the position of
the Moon in its orbit around the Earth, and the amount of the sunlit half of
the Moon that we see during that night. The image below depicts the Moon in
its orbit around Earth. For simplicity, we will learn that the Moon requires
28 days to completely orbit Earth. Since the Earth spins completely in 24 hours,
the Moon will move relatively little during those hours relative to its 28 day
orbit. Thus, everyone on Earth will see the same phase during a given night.
At any given time, half of the Moon will be sunlit, and the other half will
be in shadow, as evidenced by the inner circle of moons that represent the position
of the Moon in its orbit. Note too, that half of the Earth is also in sunlight,
and half is in shadow.
diagram has the days listed, so students can see that 28 days are required for
the moon to complete its orbit, and also so students can identify a phase with
a particular day position. Notice too that the Moon orbits in a counterclockwise
direction. Every planet orbits the sun in a counterclockwise direction, and
almost every moon too. From this page, when the Moon is at Conjunction, or Day
0, half of the Moon is sunlit, but humans living on Earth do not see any of
that sunlit half. We only see the half that is completely in shadow. This phase
is called New Moon, and is represented by the outer ring of moon images.
When the Moon is at Opposition (opposite the Sun), 14 days later, half of
the Moon is still in sunlight, but now people on Earth see that entire sunlit
half. This phase is called Full Moon.
People in Russia do not see a New Moon when Americans see a Full Moon. Look
at the image again. When the Moon is at Opposition, the Full Moon is visible
during the night, as evidenced by the shadowing of Earth. When Americans see
a Full Moon, it is during their night. Opposite America, Russia is experiencing
Noon, but the Moon is not up in their sky, since it is opposite of their place
on Earth. Only when the Earth continues to rotate into view of the Moon while
it is at Opposition will the Russians see a Full Moon ... some 12 hours after
we see it.
Also, the Full Moon looks like it is completely in the shadow of the Earth.
The drawing is deceptive for in reality, the Moon is slightly above or below
the Earth-Sun line. The orbit of the Moon is tilted relative to the orbital
plane of the Earth by about 5 degrees, so that it is not in the shadow of the
Earth ... UNLESS the timing of Full Moon and Earth-Sun-Moon lineup on the same
plane occur simultaneously. Then we would experience an Eclipse.
When the Moon is at Day 7 or Day 21, one half of the Moon is still lit by
the Sun, but people on Earth only see one half of that sunlit half. The other
half of the Moon that is in the sky is in shadow. We call these phases First
and Last Quarters respectively. If the right half is lit, then the phase is
First Quarter. When the left half is lit, then the phase is Last Quarter. In
fact, whenever the most right parts of the Moon are lit, it is in a Waxing Phase,
meaning "growing larger" relative to the amount of lit Moon we see.
If the most left parts of the Moon are lit, it is in a Waning Phase, meaning
"diminishing" relative to the amount of lit Moon we see. Crescent
and Gibbous phases are the result of being someplace between Quarter or New
The small image found to your left shows nice photographic images of the phases.
By clicking on the image, you can see a much larger version (may take a long
time to load). As you look at these images, what do you notice?
This long gap here is to give you time to think about my question. I would
normally ask my class students to raise their hands, but it is hard to do unless
you have a webcamera and are standing away from the lens enough for me to see
your hand. Also, I typically ask a question and the kids just sit there. So,
I might patiently wait for them to work up enough courage to actually guess.
What you should notice is that the same features are always present in our view.
This is because the same side of the Moon is always pointed at the Earth.
More, complete, and better explanation about the***
Moon's Phases *** exists where you can find the
"truth" about the length of a lunar cycle, or return to Moon
Introduction, or the Syllabus.
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