Moon Phases: Crash Course Astronomy #4

Summary

This script delves into the celestial mechanics behind the Moon's phases, explaining how its spherical shape and orbit around Earth create the ever-changing lunar spectacle we observe. From the New Moon to the Full Moon and back, the script details the progression of lunar phases, their dependence on the angle of sunlight, and the resultant changes in the Moon's visibility and surface features. It also touches on the poetic phenomenon of Earthshine and the comforting predictability of the Moon's monthly cycle, inviting viewers to embrace the cosmos as their celestial neighborhood.

Takeaways

• 🌕 The Moon is a giant rock sphere reflecting sunlight, making it appear bright to us.
• 🌗 The Moon's phases are caused by the geometry of its orbit around Earth and its spherical shape, which always leaves half of it illuminated.
• 📏 The 'terminator' is the line dividing the Moon's lit day side from the unlit night side, key to understanding its phases.
• 🌑 The New Moon occurs when the Moon is between Earth and the Sun, showing only its unilluminated side.
• 🌒 A 'waxing crescent' Moon is visible as a thin crescent with horns pointing away from the Sun, as the Moon moves eastward in its orbit.
• 🌓 The 'first quarter' Moon is when the Moon is 90° away from the Sun, and we see exactly half of its lit side.
• 🌔 The 'waxing gibbous' phase describes the Moon as it becomes more than half full, continuing to grow in illumination.
• 🌕 The 'full Moon' happens when the Moon is opposite the Sun in the sky, and we see its entire lit side.
• 🌖 The 'waning gibbous' phase follows the full Moon, as the illuminated portion begins to shrink.
• 🌗 The 'third quarter' Moon is similar to the first quarter but with the lit and dark sides reversed, occurring when the Moon is 270° from the Sun.
• 🌘 The 'waning crescent' is the thinning phase before the cycle returns to the New Moon, visible close to sunrise and sunset.
• 🌑 The Moon's cycle repeats every 29.5 days, which is the basis for the length of a month.
• 🌌 From the Moon's perspective, the Earth would show phases opposite to those of the Moon as seen from Earth, with 'full Earth' visible during a new Moon from Earth.
• 🌙 The 'Earthshine' effect is caused by the Earth reflecting sunlight onto the Moon's unlit side, making it visible even when not fully lit.

Q & A

Why does the Moon appear to change its shape every day?

The Moon's appearance changes daily due to its spherical shape and its orbit around the Earth. As it orbits, the angle at which sunlight illuminates the Moon and the view from Earth changes, resulting in different visible phases.

What causes the Moon to look bright to us?

The Moon looks bright because it reflects sunlight onto Earth. Despite its surface being quite dark, similar to a chalkboard or asphalt, the Sun illuminates it, and it reflects that light to us.

What is the 'terminator' in the context of the Moon's phases?

The 'terminator' is the line that divides the lit day side of the Moon from the unlit night side. It is the boundary between the areas of the Moon that are illuminated by the Sun and those that are in shadow.

Why is the Moon sometimes visible during the day and sometimes at night?

The Moon is visible during both day and night because its orbit around the Earth is not synchronized with the Earth's rotation. It can be seen at different times depending on its position relative to the Sun and Earth.

What is the significance of the term 'New Moon'?

A 'New Moon' occurs when the Moon is positioned between the Earth and the Sun, making the illuminated side of the Moon face away from Earth, and we see the unilluminated side. It marks the beginning of the lunar cycle.

How often does the Moon orbit the Earth?

The Moon orbits the Earth roughly once every month, which is the basis for the term 'month' and the length of time it takes the Moon to go through all its phases, approximately 29.5 days.

What is the term for the phase of the Moon when it appears half-lit from Earth?

When the Moon appears half-lit from Earth, it is in the 'first quarter' phase. This occurs when the Moon is 90° away from the Sun in the sky, and the terminator line cuts right down the middle of the visible face of the Moon.

What is the term 'gibbous' used to describe in the context of the Moon's phases?

The term 'gibbous' is used to describe a phase of the Moon that is more than half-lit but not yet full. It indicates a swollen or convex shape, and when the illuminated portion is increasing, it is referred to as the 'waxing gibbous' phase.

Why does the full Moon rise at sunset and set at sunrise?

The full Moon rises at sunset and sets at sunrise because it is positioned opposite the Sun in the sky when it is full. This means the Earth is between the Moon and the Sun, allowing us to see the fully illuminated half of the Moon.

What is 'Earthshine' and how does it affect the appearance of the crescent Moon?

Earthshine is the glow seen on the otherwise dark part of the Moon, caused by the Earth reflecting sunlight onto the Moon. This effect makes the thin crescent Moon appear to have a ghostly face, as the nearly full Earth illuminates the unlit portion of the Moon.

How does the perspective of the Moon's phases differ when viewed from the Moon?

When viewed from the Moon, the phases are exactly 180° reversed compared to when viewed from Earth. For example, during a new Moon on Earth, the Earth as seen from the Moon would be full, due to the Earth being opposite the Sun as viewed from the Moon.

Outlines

00:00

🌕 Understanding the Moon's Phases

This paragraph explains the Moon's visibility and its phases as a result of its spherical shape and orbit around the Earth. The Moon, being a giant rock with a dark surface, appears bright due to sunlight reflection. Its phases are determined by the angle of sunlight hitting it, which changes as it orbits. The 'terminator' line divides the lit and unlit sides, and the Moon's phases are named based on this division. The paragraph also clarifies that the Moon is always half lit and half dark, regardless of our perspective, and that the phases we see depend on the Moon's position relative to the Sun as observed from Earth.

05:01

🌗 The Lunar Cycle and Earth's Perspective

This paragraph delves into the specifics of the Moon's phases from the perspective of Earth. It begins with the New Moon, when the Moon is between the Earth and Sun, making it difficult to see due to its position during the day. As the Moon orbits, we see a waxing crescent, then a waxing gibbous Moon, leading to the full Moon when the Moon is opposite the Sun. The paragraph describes the subsequent phases: the waning gibbous, third quarter, waning crescent, and back to the New Moon, completing the lunar cycle. It also discusses the timing of these phases in relation to the Moon's position in the sky and the resulting visibility from Earth. Additionally, it touches on the concept of Earthshine, where the Earth illuminates the dark side of the Moon, and concludes with a poetic reflection on the Moon's beauty and its role in the cosmos.

Keywords

💡Moon

The Moon is Earth's natural satellite and the subject of the video script. Defined as a giant ball of rock approximately 3500 kilometers across, it is the most obvious object in the sky after the Sun. The Moon's phases and its orbit around Earth are central to the video's theme, illustrating the changing appearance of the Moon as seen from Earth due to its spherical shape and orbital motion.

💡Phases

Phases refer to the different shapes of the Moon visible from Earth, which result from the varying angles at which sunlight illuminates the Moon's surface as it orbits Earth. The script explains the sequence of lunar phases, from New Moon to Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Third Quarter, Waning Crescent, and back to New Moon, emphasizing the Moon's consistent cycle.

💡Orbit

Orbit is the path that the Moon follows around Earth, taking roughly a month to complete a full cycle. The Moon's orbit is integral to the explanation of lunar phases, as the script describes how the Moon's position relative to the Sun and Earth changes, affecting the portion of the Moon's surface that is illuminated and visible from Earth.

💡Terminator

The terminator is the line dividing the lit day side from the unlit night side of the Moon. It is a crucial concept in the script as it defines the boundary between the visible illuminated portion of the Moon and the portion that is in shadow. The position of the terminator changes as the Moon orbits Earth, creating the various phases.

💡Reflectivity

Reflectivity is the measure of how much light is reflected by a surface. The Moon's surface has a reflectivity similar to that of a chalkboard or asphalt, which is why it appears bright to us despite being dark in itself. The script uses this concept to explain why the Moon is visible and how its brightness is a result of sunlight reflection.

💡New Moon

New Moon marks the beginning of the lunar cycle when the Moon is positioned between the Earth and the Sun, making its illuminated side invisible from Earth. The script describes this phase as the point where the Moon's cycle starts anew, highlighting the alignment of the Sun, Moon, and Earth.

💡First Quarter

First Quarter refers to the phase when the Moon has completed one-quarter of its orbit around Earth, and we see exactly half of the Moon illuminated. The script clarifies that despite appearing half-full, it is termed 'first quarter' due to its position in the lunar cycle, not the amount of illumination.

💡Full Moon

Full Moon is the phase when the Moon is opposite the Sun in the sky, and its entire face is illuminated as viewed from Earth. The script explains that during a Full Moon, the Moon rises at sunset and sets at sunrise, allowing for the entire night to observe the fully lit Moon.

💡Waning

Waning describes the period when the illuminated portion of the Moon appears to decrease, moving from Full Moon towards New Moon. The script uses 'waning' to describe the phases where the visible illuminated portion of the Moon is getting smaller, such as the Waning Gibbous and Waning Crescent.

💡Earthshine

Earthshine is the glow seen on the unlit portion of the Moon, caused by sunlight reflected from Earth. The script poetically describes Earthshine as the 'old Moon in the new Moon's arms,' illustrating the concept of how Earth's reflection lights up the dark side of the Moon, making it visible even when not directly illuminated by the Sun.

💡Crescent

Crescent refers to the thin, curved shape of the Moon during the early and late stages of its cycle, when only a small portion of its surface is illuminated. The script mentions both Waxing Crescent and Waning Crescent, highlighting the thin sliver of light visible during these phases and how the crescent's visibility changes throughout the day.

Highlights

The Moon is a giant ball of rock 3500 km across, reflecting sunlight to Earth.

The Moon's phases are caused by geometry, as half of it is always illuminated by the Sun.

The 'terminator' line divides the Moon's lit day side from the unlit night side.

The full Moon appears when the Moon is fully illuminated from Earth's perspective.

The Moon orbits the Earth roughly once per month, giving us the term 'month'.

New Moon occurs when the Moon is between Earth and Sun, appearing unilluminated from Earth.

Crescent Moon is visible as a thin illuminated arc when the Moon is slightly east of the Sun.

First quarter Moon is when the Moon is 90° away from the Sun, half lit and half dark.

Waxing and waning phases describe the Moon's growth and shrinking as seen from Earth.

Gibbous Moon refers to the more than half-lit phase, either waxing or waning.

Full Moon occurs when the Moon is opposite the Sun, fully illuminated for Earth observers.

The Moon's phases repeat in reverse order after full Moon as it continues its orbit.

Third quarter Moon is when the Moon is 270° from the Sun, with the lit side reversed from first quarter.

Waning crescent Moon is visible as it gets thinner, leading back to New Moon.

The Moon's cycle from New Moon to full and back takes 29.5 days, defining the lunar month.

From the Moon's perspective, Earth phases are reversed, with 'full Earth' during new Moon.

Earthshine illuminates the unlit part of the Moon, visible as a glow on the crescent.

Observing the Moon's phases provides a comforting reminder of the Universe's familiarity.

Transcripts

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, you’ll 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 you’re facing the moon, with the sun behind you, you’re seeing the half of the

moon that is fully illuminated by sunlight and it looks full. If you’re 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, you’re 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 word “month”

comes from; “month” and “Moon” are 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

is “below” the Sun, or “above” it. 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 term “New 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 we’re 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 crescent” Moon; 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 we’re

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 named “first quarter” because 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 prefer “first

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 is “gibbous”, 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 we’re 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, you’ll see the nearly-but-not-quite full Moon setting in the west.

Not only that, but we’re 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 the “third

quarter” Moon, 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, we’re 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 called “the 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.