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Phases of the moon

A moon phase is the shape of moon's sunlit portion as seen from Earth. There are a total of eight moon phases: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, and waning crescent. These phases repeat every 29.5 days. Created by Khan Academy.

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Video transcript

- [Narrator] Imagine that one day, all of the clocks and computers on Earth broke, and all the calendars disappeared. How would you keep track of how much time had passed? Well, you could look to the Moon. Humans have used the Moon to keep track of time for thousands of years. It isn't a coincidence that the word moon is related to the word month in old English. The Moon is Earth's only natural satellite. A natural satellite is a naturally occurring body that orbits a planet. Some of the planets in our solar system have more than 50 satellites or moons, but the Earth just has one. The Moon does not generate its own light. We can see the Moon from the Earth, because it is partially lit by the light from the sun. In fact, moonlight is just sunlight reflected from the Moon onto Earth. The Moon takes about 27 days to make a full orbit around Earth. And as it does, the lit part of the Moon appears to change shape to us here on Earth. These shapes are called the moon phases, or lunar phases. So why do we see different lunar phases? Well, the 27 days it takes the Moon to orbit the Earth once is the same amount of time it takes the Moon to rotate on its axis once. This means that the same side of the Moon is always facing us. At the same time, the sun always lights up half of the sphere of the Moon. However, the side of the Moon that's facing us isn't always the same part of the Moon that the sun lights up. This causes the Moon to appear to have different shapes, or faces in the sky, depending on the time of the month. Let's take a closer look at the Moon's phases. First, we have the new moon, which happens when the Moon is closest to the sun in its orbit. During this phase, the lit side of the Moon is completely facing away from Earth. So it looks like the Moon has disappeared. In a day or two, we'd be able to see a little sliver of moon in the sky. Over the next few days, the crescent moon will appear to get bigger and bigger. When the Moon appears to get bigger from one day to the next, we say that the Moon is waxing. So this phase of the Moon is called the waxing crescent moon. You might notice that sometimes we can still see the rest of the Moon in the shadow. This is because the Earth reflects sunlight onto the Moon, just like the Moon reflects light onto Earth. Eventually, the Moon appears to change shape so much that it isn't a crescent anymore, but a half circle in the sky. This is called a first quarter moon. There are two ways to think about why this phase is called a quarter moon. Even though it looks like the Moon is half illuminated, the Moon is a sphere. So we can only ever see half of the Moon from Earth. During a quarter moon, the Moon forms a right angle with Earth and the sun. This means that we a half lit portion of the half of the Moon that's always facing us. It's half of a half, so it's a quarter. Also, a first quarter moon occurs when the Moon is a quarter of the way through its new cycle. Next, there's the waxing gibbous moon. The word gibbous comes from the Latin word meaning humpback. Once the Moon is farthest from the sun in its orbit, the full sun lit side of the Moon faces Earth. We call this phase the full moon, but the Moon is not done yet. It's only finished half of the cycle. Next, we have the same phases, but in reverse. As the Moon appears to get smaller and smaller, we say that it's waning. The full Moon appears to shrink, and then we see a waning gibbous. Then there's the third quarter moon. This happens when the Moon is three quarters the way done with its cycle, and the Moon forms another right angle with the sun and the Earth. The Moon appears to shrink even more, and it becomes a waning crescent, and the cycle starts anew with another new moon. Even though the Moon completes an orbit every 27 days, the lunar phases actually repeat about every 29.5 days. This is because the Earth is revolving around the sun while the Moon completes its orbit. So the Moon has to travel a little extra to catch up. The Moon isn't just something beautiful to look at in the sky, or just an easy way to keep track of time. The Moon's gravity controls the tides, which are the rise and fall of water in oceans, lakes, and rivers. Tides allow for unique ecosystems, like tide pools to exist. And we can use the tides to create electricity, and tidal power plants. The Moon also helps keep Earth's access stable. Without it, our planet would wobble more dramatically on its axis over long periods of time, which would change up our weather and our seasons. So even though your calendar is pretty unlikely to spontaneously combust tomorrow, you could still thank the Moon for being such a stabilizing influence, and good companion to our planet.