Moon Phases: Crash Course Astronomy #4


In this episode of Crash Course Astronomy, Phil takes you through the cause and name of the Moon’s phases.

Transcript Provided by YouTube:

00:03
Besides the Sun, the Moon is the most obvious object in the sky. Bright, silvery, with tantalizing
00:09
features on its face, it’s been the target of imagination, poetry, science,
00:14
and even the occasional rocket.
00:25
If you pay even the most cursory attention to it, you’ll see that it changes every
00:29
day; sometimes it’s up in the day, sometimes at night, and its shape is always changing.
00:34
What causes this behavior?
00:36
The Moon is basically a giant ball of rock 3500 kilometers across hanging in space. Its
00:41
surface is actually pretty dark, with about the same reflectivity as a chalkboard or asphalt.
00:47
However, it looks bright to us because it’s sitting in full sunlight; the Sun illuminates
00:52
it, and it reflects that light down to us here on Earth.
00:54
And because it’s a sphere, and orbiting the Earth, the way we see it lit by the Sun
00:59
changes with time. That’s what causes its phases: geometry.
01:03
The important thing to remember through all this is, because the Moon is a ball and in
01:07
space, half of it is always illuminated by the Sun!
01:11
This is true for the Earth, too, and every spherical object in space; half faces the
01:15
Sun, half faces away. We call the part facing the Sun the daylight or bright side, and the
01:20
half facing away the night or dark side.
01:23
The phase of the Moon refers to what shape the Moon appears to us; how much of it we
01:27
see illuminated from the Earth. The key to all this is this line, dividing the lit day
01:32
side from the unlit night side. We call that line the terminator.
01:36
If you’re facing the moon, with the sun behind you, you’re seeing the half of the
01:40
moon that is fully illuminated by sunlight and it looks full. If you’re off to the
01:44
side you see half of the lit side and half of the dark side and we say the moon is half
01:49
full. If the sun is on the other side of the moon, you’re look at the unlit half, and
01:52
it looks dark. Now, mind you, I haven’t moved anything except our point of view here,
01:56
so at all times the Moon is always half lit, and half dark. Remember that.
02:01
The phase of the Moon we see depends on from what direction the sunlight’s hitting it,
02:05
and the angle we see that from Earth.
02:07
The Moon orbits the Earth roughly once per month. In fact, that’s where the word “month”
02:11
comes from; “month” and “Moon” are cognates, words that have similar etymological
02:15
histories, and in most languages, including English, the two words are very similar. The
02:19
length of time we call the month is derived from the length of time it takes the Moon
02:22
to go through all its phases — 29.5 days.
02:25
So. To describe the phases, let’s start at the beginning: New Moon.
02:30
New Moon happens when the Sun, Moon, and Earth are all more or less in a line. The Moon’s
02:34
orbit is actually tipped a bit to the Earth’s, so sometimes new Moon happens when the Moon
02:38
is “below” the Sun, or “above” it. But at some point in its orbit, at some point
02:42
in the month, it appears to be as close to the Sun as it can.
02:45
What does this look like from Earth?
02:47
The Moon is between the Earth and the Sun, so from our perspective we only see the dark
02:52
half, the unilluminated half, of the Moon. The other side, the far side, of the Moon
02:56
is lit, but we can’t see it. It makes sense then to call this the beginning of the Moon’s
03:00
cycle, hence the term “New Moon”.
03:02
Now think about this for a sec: Because the Moon is near the Sun in the sky, it travels
03:06
across the sky with the Sun. It’s up during the day! You can only see it from the part
03:10
of the Earth that’s lit, which is when it’s daytime. It’s a very common misconception
03:14
that the Moon is only up at night, but it’s up during the day literally just as often.
03:19
At New Moon, the Moon stays near the Sun, so it rises at sunrise, and sets at sunset.
03:24
This makes it extremely difficult to see; it is, after all, sitting next to the brightest
03:28
object in the sky, and only a little bit of it is lit from our perspective.
03:32
But not for long. Because the Moon is orbiting the Earth, after a couple of days it’s moved
03:37
a bit to the east. Now we’re seeing it along a slight angle, and we can see a little bit
03:42
of the illuminated half of the Moon on its side toward the Sun.
03:45
The terminator, the day/night line, appears curved around the Moon, so what we see is
03:50
a thin illuminated crescent Moon. At this point the crescent is still very thin, with
03:55
the horns of the crescent pointing away from the Sun.
03:58
Note that the Moon is still pretty close to the Sun in the sky, just a bit to the east,
04:01
rising maybe an hour or two after sunrise. But this means it’s up all day, and then
04:05
sets after the Sun does. This is the best time to see the crescent Moon, when the Sun
04:10
has already set and the sky starts to get dark. The Moon will be low over the western
04:13
horizon, and it will set soon after the Sun does.
04:16
Let’s wait a couple of days.
04:17
OK, now the Moon has moved a bit more in its orbit around the Earth, and is farther from
04:21
the Sun in the sky. We see a little more of the illuminated part, and the crescent is
04:25
wider. Since it’s getting thicker, we say this is a “waxing crescent” Moon; waxing
04:29
means growing or getting bigger. It’s also well away from the Sun now, so it’s easier
04:33
to spot, even during the day before sunset.
04:35
Seven or so days after new Moon we get to our first milestone: the Moon is now one-quarter
04:41
of the way around its orbit. It’s 90° away from the Sun in the sky, which means we’re
04:45
looking straight down on the terminator, the Moon’s day/night line. It cuts right down
04:49
the middle of the visible face of the Moon, so it’s half lit, with the sunward side
04:53
of the Moon visible and the other side dark.
04:55
Confusingly, this phase is properly named “first quarter” because the Moon is ¼
05:00
of the way through its cycle, ¼ of the way through its orbit around the Earth, even though
05:05
it looks half full. So it’s not really the half-full moon — astronomers prefer “first
05:10
quarter,” so if you want to sound all astronomery, then you should call it that.
05:14
But time marches on. The Moon continues on its gravitational dance with Earth, swinging
05:18
around its orbit. Now more than half full, we say its shape is “gibbous”, which means
05:23
swollen or convex. Since it’s getting wider, this is actually the waxing gibbous phase
05:28
of the Moon. It rises in the late afternoon, and is up most of the night.
05:32
Our next big step comes two weeks after new Moon, when it’s moved halfway through its
05:36
orbit. It is now opposite the Sun in the sky, 180° around. The Earth is between the Moon
05:41
and Sun, so we’re looking at the fully-illuminated half of the Moon. This is the full Moon.
05:47
Because it’s opposite the Sun, it rises at sunset and sets and sunrise; it’s up
05:52
all night shining down on the Earth.
05:54
But again, wait a couple of days and things change. When the Moon is full it’s 180°
05:58
around the sky from the Sun, so as it continues to move around the Earth in a circle the distance
06:03
between it and the Sun is now starting to decrease, even as it continues on in the same
06:08
direction. As before, it keeps rising and setting later, but now it rises after sunset,
06:13
and sets after sunrise. If you get up early in the morning as the Sun is just rising in
06:17
the east, you’ll see the nearly-but-not-quite full Moon setting in the west.
06:21
Not only that, but we’re about to go through all the phases again, but in reverse order.
06:27
A few days after full Moon the lit side is shrinking. It’s in the waning, or shrinking,
06:32
gibbous phase.
06:33
Then, three weeks or so after new Moon, and a week after full, the Moon is once again
06:38
half lit, the terminator splitting the Moon’s face in two even halves. This is the “third
06:43
quarter” Moon, because the Moon is ¾ of the way through its cycle. It’s a lot like
06:47
the first quarter, but the side that was lit is now dark, and vice versa. It’s 270°
06:52
around the sky from the Sun. It rises at midnight and sets at noon.
06:56
A few days later and the Moon is a crescent again, getting thinner. It’s now a “waning
07:01
crescent.” It rises just a couple of hours before sunrise, and sets a couple of hours
07:06
before sunset.
07:06
Then, finally, we’re back where we started. One month after new Moon, the Moon has traveled
07:11
360° around the sky, and is once again as close to the Sun as it can get. It’s new
07:16
Moon, and the cycle starts up again as it has for time immemorial.
07:20
An interesting thing happens if you move your perspective from the Earth to the Moon. The
07:25
phases of the Moon we see from Earth depend on the angle of the Moon and Sun in the sky.
07:29
But on the Moon, the angles are exactly 180° reversed; at new Moon, when the
07:35
Moon is between the Earth and Sun, the Earth is opposite the Sun as seen from the Moon.
07:40
It’s full Earth!
07:41
All the other phases are opposite too, so when we see a full Moon, a Moon-dweller would
07:46
see a new Earth, and so on.
07:48
Have you ever looked at the thin crescent Moon and seen the ghostly face of the rest
07:53
of the unlit side? That’s because it’s not really unlit: the nearly full Earth is
07:57
reflecting sunlight on the Moon, lighting up the otherwise dark part.
08:01
The Earth is bigger and more reflective than the Moon, so it’s actually 50 times brighter
08:05
than a full Moon! This glow is called Earthshine, a term I quite like. Even more poetically,
08:10
it’s been called “the old Moon in the new Moon’s arms”, referring to the unlit
08:15
part surrounded by the crescent new Moon’s horns.
08:17
That’s lovely, isn’t it?
08:18
The Moon is one of the most beautiful and most gratifying objects in the sky to observe.
08:22
It’s different every day! Yet it’s also the same, because we see, more or less, the
08:27
same half of it, the same face all the time. It’s big and bright, and the features on
08:32
its surface discernible by eye (and even better with binoculars or a small telescope).
08:36
As the phases change, inexorably, day after day, the angle of sunlight hitting the surface
08:42
changes, bringing new things into our view. The motions become comforting, even familiar.
08:47
It’s a reminder that the Universe may seem strange and complicated and forbidding at
08:52
first, but over time, as you get outside and experience it, it becomes your neighborhood.
08:57
Welcome home.
08:58
Today you learned why the Moon has phases: It’s a sphere, and it orbits the Earth,
09:02
so the angle at which we see its lit side changes. It goes from new, to waxing crescent,
09:08
to half full, waxing gibbous, full, waning gibbous, half full, waning crescent, and then
09:14
the cycle starts all over again. This also affects when it rises and sets, and what we
09:18
see on the surface.
09:19
Crash Course is produced in association with PBS Digital Studios. This episode was written
09:24
by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr.
09:29
Michelle Thaller. It was co-directed by Nicholas Jenkins and Michael Aranda, and the graphics
09:33
team is Thought Café.


This post was previously published on YouTube.

Photo credit: Screenshot from video.