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  • Besides the Sun, the Moon is the most obvious object in the sky. Bright, silvery, with tantalizing

  • features on its face, it’s been the target of imagination, poetry, science,

  • and even the occasional rocket.

  • If you pay even the most cursory attention to it, youll see that it changes every

  • day; sometimes it’s up in the day, sometimes at night, and its shape is always changing.

  • What causes this behavior?

  • The Moon is basically a giant ball of rock 3500 kilometers across hanging in space. Its

  • surface is actually pretty dark, with about the same reflectivity as a chalkboard or asphalt.

  • However, it looks bright to us because it’s sitting in full sunlight; the Sun illuminates

  • it, and it reflects that light down to us here on Earth.

  • And because it’s a sphere, and orbiting the Earth, the way we see it lit by the Sun

  • changes with time. That’s what causes its phases: geometry.

  • The important thing to remember through all this is, because the Moon is a ball and in

  • space, half of it is always illuminated by the Sun!

  • This is true for the Earth, too, and every spherical object in space; half faces the

  • Sun, half faces away. We call the part facing the Sun the daylight or bright side, and the

  • half facing away the night or dark side.

  • The phase of the Moon refers to what shape the Moon appears to us; how much of it we

  • see illuminated from the Earth. The key to all this is this line, dividing the lit day

  • side from the unlit night side. We call that line the terminator.

  • If youre facing the moon, with the sun behind you, youre seeing the half of the

  • moon that is fully illuminated by sunlight and it looks full. If youre off to the

  • side you see half of the lit side and half of the dark side and we say the moon is half

  • full. If the sun is on the other side of the moon, youre look at the unlit half, and

  • it looks dark. Now, mind you, I haven’t moved anything except our point of view here,

  • so at all times the Moon is always half lit, and half dark. Remember that.

  • The phase of the Moon we see depends on from what direction the sunlight’s hitting it,

  • and the angle we see that from Earth.

  • The Moon orbits the Earth roughly once per month. In fact, that’s where the wordmonth

  • comes from; “monthandMoonare cognates, words that have similar etymological

  • histories, and in most languages, including English, the two words are very similar. The

  • length of time we call the month is derived from the length of time it takes the Moon

  • to go through all its phases -- 29.5 days.

  • So. To describe the phases, let’s start at the beginning: New Moon.

  • New Moon happens when the Sun, Moon, and Earth are all more or less in a line. The Moon’s

  • orbit is actually tipped a bit to the Earth’s, so sometimes new Moon happens when the Moon

  • isbelowthe Sun, oraboveit. But at some point in its orbit, at some point

  • in the month, it appears to be as close to the Sun as it can.

  • What does this look like from Earth?

  • The Moon is between the Earth and the Sun, so from our perspective we only see the dark

  • half, the unilluminated half, of the Moon. The other side, the far side, of the Moon

  • is lit, but we can’t see it. It makes sense then to call this the beginning of the Moon’s

  • cycle, hence the termNew Moon”.

  • Now think about this for a sec: Because the Moon is near the Sun in the sky, it travels

  • across the sky with the Sun. It’s up during the day! You can only see it from the part

  • of the Earth that’s lit, which is when it’s daytime. It’s a very common misconception

  • that the Moon is only up at night, but it’s up during the day literally just as often.

  • At New Moon, the Moon stays near the Sun, so it rises at sunrise, and sets at sunset.

  • This makes it extremely difficult to see; it is, after all, sitting next to the brightest

  • object in the sky, and only a little bit of it is lit from our perspective.

  • But not for long. Because the Moon is orbiting the Earth, after a couple of days it’s moved

  • a bit to the east. Now were seeing it along a slight angle, and we can see a little bit

  • of the illuminated half of the Moon on its side toward the Sun.

  • The terminator, the day/night line, appears curved around the Moon, so what we see is

  • a thin illuminated crescent Moon. At this point the crescent is still very thin, with

  • the horns of the crescent pointing away from the Sun.

  • Note that the Moon is still pretty close to the Sun in the sky, just a bit to the east,

  • rising maybe an hour or two after sunrise. But this means it’s up all day, and then

  • sets after the Sun does. This is the best time to see the crescent Moon, when the Sun

  • has already set and the sky starts to get dark. The Moon will be low over the western

  • horizon, and it will set soon after the Sun does.

  • Let’s wait a couple of days.

  • OK, now the Moon has moved a bit more in its orbit around the Earth, and is farther from

  • the Sun in the sky. We see a little more of the illuminated part, and the crescent is

  • wider. Since it’s getting thicker, we say this is a “waxing crescentMoon; waxing

  • means growing or getting bigger. It’s also well away from the Sun now, so it’s easier

  • to spot, even during the day before sunset.

  • Seven or so days after new Moon we get to our first milestone: the Moon is now one-quarter

  • of the way around its orbit. It’s 90° away from the Sun in the sky, which means were

  • looking straight down on the terminator, the Moon’s day/night line. It cuts right down

  • the middle of the visible face of the Moon, so it’s half lit, with the sunward side

  • of the Moon visible and the other side dark.

  • Confusingly, this phase is properly namedfirst quarterbecause the Moon is ¼

  • of the way through its cycle, ¼ of the way through its orbit around the Earth, even though

  • it looks half full. So it’s not really the half-full moon -- astronomers preferfirst

  • quarter,” so if you want to sound all astronomery, then you should call it that.

  • But time marches on. The Moon continues on its gravitational dance with Earth, swinging

  • around its orbit. Now more than half full, we say its shape isgibbous”, which means

  • swollen or convex. Since it’s getting wider, this is actually the waxing gibbous phase

  • of the Moon. It rises in the late afternoon, and is up most of the night.

  • Our next big step comes two weeks after new Moon, when it’s moved halfway through its

  • orbit. It is now opposite the Sun in the sky, 180° around. The Earth is between the Moon

  • and Sun, so were looking at the fully-illuminated half of the Moon. This is the full Moon.

  • Because it’s opposite the Sun, it rises at sunset and sets and sunrise; it’s up

  • all night shining down on the Earth.

  • But again, wait a couple of days and things change. When the Moon is full it’s 180°

  • around the sky from the Sun, so as it continues to move around the Earth in a circle the distance

  • between it and the Sun is now starting to decrease, even as it continues on in the same

  • direction. As before, it keeps rising and setting later, but now it rises after sunset,

  • and sets after sunrise. If you get up early in the morning as the Sun is just rising in

  • the east, youll see the nearly-but-not-quite full Moon setting in the west.

  • Not only that, but were about to go through all the phases again, but in reverse order.

  • A few days after full Moon the lit side is shrinking. It’s in the waning, or shrinking,

  • gibbous phase.

  • Then, three weeks or so after new Moon, and a week after full, the Moon is once again

  • half lit, the terminator splitting the Moon’s face in two even halves. This is thethird

  • quarterMoon, because the Moon is ¾ of the way through its cycle. It’s a lot like

  • the first quarter, but the side that was lit is now dark, and vice versa. It’s 270°

  • around the sky from the Sun. It rises at midnight and sets at noon.

  • A few days later and the Moon is a crescent again, getting thinner. It’s now a “waning

  • crescent.” It rises just a couple of hours before sunrise, and sets a couple of hours

  • before sunset.

  • Then, finally, were back where we started. One month after new Moon, the Moon has traveled

  • 360° around the sky, and is once again as close to the Sun as it can get. It’s new

  • Moon, and the cycle starts up again as it has for time immemorial.

  • An interesting thing happens if you move your perspective from the Earth to the Moon. The

  • phases of the Moon we see from Earth depend on the angle of the Moon and Sun in the sky.

  • But on the Moon, the angles are exactly 180° reversed; at new Moon, when the

  • Moon is between the Earth and Sun, the Earth is opposite the Sun as seen from the Moon.

  • It’s full Earth!

  • All the other phases are opposite too, so when we see a full Moon, a Moon-dweller would

  • see a new Earth, and so on.

  • Have you ever looked at the thin crescent Moon and seen the ghostly face of the rest

  • of the unlit side? That’s because it’s not really unlit: the nearly full Earth is

  • reflecting sunlight on the Moon, lighting up the otherwise dark part.

  • The Earth is bigger and more reflective than the Moon, so it’s actually 50 times brighter

  • than a full Moon! This glow is called Earthshine, a term I quite like. Even more poetically,

  • it’s been calledthe old Moon in the new Moon’s arms”, referring to the unlit

  • part surrounded by the crescent new Moon’s horns.

  • That’s lovely, isn’t it?

  • The Moon is one of the most beautiful and most gratifying objects in the sky to observe.

  • It’s different every day! Yet it’s also the same, because we see, more or less, the

  • same half of it, the same face all the time. It’s big and bright, and the features on

  • its surface discernible by eye (and even better with binoculars or a small telescope).

  • As the phases change, inexorably, day after day, the angle of sunlight hitting the surface

  • changes, bringing new things into our view. The motions become comforting, even familiar.

  • It’s a reminder that the Universe may seem strange and complicated and forbidding at

  • first, but over time, as you get outside and experience it, it becomes your neighborhood.

  • Welcome home.

  • Today you learned why the Moon has phases: It’s a sphere, and it orbits the Earth,

  • so the angle at which we see its lit side changes. It goes from new, to waxing crescent,

  • to half full, waxing gibbous, full, waning gibbous, half full, waning crescent, and then

  • the cycle starts all over again. This also affects when it rises and sets, and what we

  • see on the surface.

  • Crash Course is produced in association with PBS Digital Studios. This episode was written

  • by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr.

  • Michelle Thaller. It was co-directed by Nicholas Jenkins and Michael Aranda, and the graphics

  • team is Thought Café.

Besides the Sun, the Moon is the most obvious object in the sky. Bright, silvery, with tantalizing

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Moon Phases: Crash Course Astronomy #4

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    Bruce Wu posted on 2017/05/31
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