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Episode 1: Introduction and the "Dark Side" of the Moon Misconception

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Recap: In this episode, I gave an overview of what the podcast is going to be like as well as my own background in astronomy and public outreach. I then briefly discussed the misconception of a "dark side" to the moon, when there really is no side of the moon in permanent shadow.

Puzzler: Imagine that the moon were a perfect white cube. It still orbits around Earth the same way as it does now with its orbit tidally locked. Would a cuboidal moon show any different phases to Earth, and if it did, what would they be?

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Transcript of the Main Material:

The background information is that Earth orbits the sun in 1 year, or about every 365.25 days. As Earth orbits the sun, it also spins on its axis, once per day, or about every 24 hours. The side facing the sun is in daylight, while the side facing away is in the shadow of Earth, which we call night.

Just as Earth orbits the sun, the moon orbits Earth. But, as it orbits Earth, it's still orbiting the sun and still spinning on its axis. The key to understanding the Moon's orbit is that the moon is in a situation known as "tidal locking," or we say that the moon is "tidally locked." What this means is that its orbital period - its year - is the same length of time as its rotation period, its day.

So while the moon is slowly orbiting around Earth over the course of one month, it is also spinning on its axis just fast enough to compensate, such that the same face is always towards Earth. That's why we never see the far side.

But, we do see the moon go through periods of brightness and darkness, which we call "phases." So as it moves around Earth and spins on its axis, a different side of it will always be presented towards the sun. The sunlit side experiences day - just like the sunlit side of Earth - while the side of the moon in its own shadow experiences night - just like the shadowed side of Earth. When it's a new moon, the entire side that faces us is experiencing night. When we see a full moon, the entire side that faces us is experiencing day. A bit of trivia is that the dividing line between night and day is called the "terminator."

When you stop to think about it, the very fact that we see the moon go through phases should be enough to tell you that there is no permanent "dark side" to the moon, it's a temporary nighttime situation just like on Earth.

In this topic, I also want to go on a bit of a tangent and address the idea that we only see 50% of the moon's surface from Earth. We actually see about 60%. This is because the moon wobbles in its orbit, has different speeds in its orbit, and we can see it from different vantage points on Earth's surface.

For the last point, the different vantage points, imagine looking at the full moon just as its rising in the East at sunset. If you actually want to do this, take a photo. Come back 6 hours later, around midnight, when the moon is at its highest point in the sky. Take another photo. If you do this and compare the two, you will see that there are slight differences to what you can observe because of the vantage point from Earth. It's like if you look at a table top while standing, then kneel down and look at the tabletop again. Your vantage point has changed, so you will see slightly different parts of it.

As for the different speeds, this is because the moon orbits Earth on an ellipse, moving between about 363,000 and 406,000 km from Earth. Kepler was able to figure out about 400 years ago that if an object is on an elliptical orbit, it will move faster when it's closer to one of the focii, and it will move more slowly when its farther away. This is why comets spend very little time close to the sun, and most of their time in the far reaches of the solar system. Getting back to the moon, this means that the moon is going to move faster when its closer to Earth, and it's going to move slower when its farther away. But, it has the same rotation rate. So, we'll be able to see different parts due to the different speed but steady rotation rate.

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