What Causes the Lunar Phases?


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We see it almost every night of our lives. For thousands of years, the greatest philosophers and astronomers alike have watched its face change and wondered why.

Step outside and observe the moon every day for a month and you will notice something fascinating. Over the course of the entire month, the moon will go through an entire cycle of phases—no more, no less.

But why?

The phases of the moon are something I’ve talked about before, but I wanted to spend some time on a few common misconceptions this time around and show you the truth behind the lunar phases.

Misconception #1: The lunar phases are caused by shadows cast on its surface by other objects in the solar system.

Okay…reasonable hypothesis. Why isn’t it true?

Well, let’s take a close look at this idea. How exactly do we cast a shadow?

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There are two essential ingredients to create a shadow. You need an object, and you need a light source.

Think about what a shadow really is. It’s black because it’s the absence of light.

Let’s say we put an object—for instance, a man—in front of a light source. The light source can’t shine through the man; he’s opaque. That means the light can’t reach anything on the other side of the man.

What we get: a man-shaped hole in the lit-up area.

So…our hypothesis is that the lunar phases are caused by the shadows of other objects in the solar system.

Do we have objects? Yes indeed, we have seven other planets, unless you count Pluto.

Do we have a light source? Yes indeed, we have the sun.

So…how come that’s not why the moon has phases?

Well…there’s just one major problem with our hypothesis.

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Take a close look at this model of the solar system. It’s correct, right? All the planets and the sun are in the right order?

Thought so. Okay, ask yourself a question…in what direction will the shadows of these planets be cast?

Confused? Take a look at the dark side of the planets. The night side of any planet exists for the same reason as shadows: the sun’s light can’t reach that side of the planet.

So…any shadow in the solar system will be cast pointing away from the sun. That eliminates five planets and uncounted asteroids, comets, and Kuiper belt objects.

Mars, the asteroid belt, Jupiter, Saturn, Uranus, Neptune, and all the comets and Kuiper belt objects are farther from the sun than the moon, meaning their shadows cannot possibly be cast on the moon.

So what about Mercury, Venus, and Earth? Can’t they cast shadows on the moon?

I suppose technically, their shadows point in the right direction—away from the sun. But now we encounter another problem.

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Here’s where all the inner planets might be at any one time.

The planets don’t whip around the sun. We derive our 365-day year from the time it takes for Earth to go around the sun. The laws of gravity state that the closer in you are, the faster you have to go, but Venus and Mercury don’t go that much faster.

My point is that at any one time in our year, Venus and Mercury might be nowhere near the moon—they could even be on the other side of the sun from us. And their shadows would point away from the sun—and consequently, away from the moon.

What about the Earth, then? I mean, the moon orbits the Earth. Isn’t the Earth always in a position to cast its shadow on the moon?

Yes…but Earth’s shadow is actually very long and narrow. And the moon is actually much farther away from us than you would expect. The Earth’s shadow only manages to land on the moon a couple times a year.

Remember what I said about the moon going through an entire cycle of phases in the space of a month?

Exactly. So the Earth can’t possibly cause the moon’s phases.

Misconception #2: The lunar phases are caused by the moon moving into the sun’s shadow.

I won’t even have to do very much explaining for this one. Consider again our definition of shadows. Does the sun have a shadow?

Remember, in order to cast a shadow, you need both a light source and an object. We have a light source—the sun. But do we have an object?

You might say that yes, we do. The sun is both the light source and the object, so yes, it can cast a shadow.

No. The sun is a light source, but not an object. Remember, a shadow is cast because a light source is blocked by an object. It can’t shine through the object, so an object-shaped hole in the light appears instead.

Every inch of the sun’s surface emits light. Is there a single square inch of the sun that could possibly block that light?

No.

The sun does not have a shadow. So it can’t possibly cast one on the moon.

So…what does cause the lunar phases, then?

You might actually be surprised to hear…the culprit is the sun. But not for the reasons you might think.

How the Moon Really Gets its Phases

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When you look at the moon at any phase besides “full” or “new,” you are looking at both its night and day sides.

Now, the moon doesn’t have a “bright side” and a “dark side.” All areas of the moon’s surface are exposed to both day and night at one time or another. But that’s a misconception I plan to address in another post.

Here’s a diagram of the moon’s phases, for your viewing pleasure:

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Take a good, long look at the phases you see here. Now take a look at this orbital image of the Earth.

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(By the way, before you claim that this isn’t an orbital image because it was taken from the moon…the moon is in orbit. So yes, it’s an orbital image.)

Do you notice that the moon and the Earth don’t actually look that different?

We refer to the phase of the moon that looks like this as a “gibbous” moon, and we say the same of the Earth.

Now, here’s a crescent Earth…also taken from the moon, just for the fun of it.

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Here’s a crescent Earth and crescent moon together for comparison, as well as a gibbous Earth and moon.

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You’re looking at the day and night sides of the Earth and the moon.

So…how come we never notice the Earth’s phases, but we notice the moon’s phases?

Because, quite frankly, we’re not in space. If you were to jump high enough to look down on the Earth from space, you’d see the Earth in a different phase every hour of the day, because Earth’s phases are caused by its day and night.

The same goes for the moon. One month for us is the same as one full orbit of the moon around the Earth, which happens to be the time it takes for the moon to rotate once—one lunar day.

We’re always looking at the same face of the moon, just as you are always looking at the same face of the Earth if you stay at home but keep jumping straight up into space.

But because both the Earth and the moon rotate, every inch of their surfaces is exposed to both day and night at one time or another, and their phases change.

Think about it: if you hovered in space between the sun and the moon (or the Earth, for that matter) and never moved, you would always see the moon in the same phase—full—because you would always see the entire daylight side.

That’s why the moon has phases.

Questions? Or just want to talk?

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