Learn how the sun, earth, and moon interact with each other to create tides, phases, eclipses, seasons, solar winds, and precession.

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## Interaction of Earth, Moon, ; Sun

The sun, earth, and moon interact in a complex ballet of motions and forces. The moon orbits the earth once per month, and the earth orbits the sun once per year. This happens because all bodies with mass in the universe attract each other. The earth’s pull on the moon keeps the moon in orbit, and the sun’s pull on the earth keeps the earth in orbit.But it doesn’t end there.

Based on the exact positions of the three bodies, you can have phases and eclipses; sea tides on Earth; the seasons of the year; and gradual changes in orbit, rotation, and tilt. In this lesson, we’re going to go through a few of these interactions in the sun-earth-moon system.

## Geometry: Phases ; Eclipses

Some interactions are direct: The sun literally pulls on the earth. But some are more about coincidence or alignment. The sun always lights up exactly half of the earth and exactly half of the moon. The other side of each object is in shadow.

This is for the same reason that the sun might light one half of a building, while the other side is in shadow and dark. But, we don’t always see half of the moon lit, because it depends on our point of view.If the moon is roughly between the sun and the earth, we might see none of the moon lit because the lit half is on the opposite side of the one facing us. This is called the new moon. If the moon is on the far side of the earth, we might see the whole sphere lit. This is called the full moon.

And then there’s everything in between: half moons and crescent moons. These are called the phases of the moon.If the moon happens to be positioned perfectly between the earth and sun from our vantage point on a particular part of the earth, we’ll see something special: a solar eclipse. This is where the moon goes right in front of the disc of the sun, blocking it out.

## Tides & Forces

But then there are direct interactions, where the objects really affect each other and it isn’t just about the way things appear. An example of a direct interaction would be the tides.The tides are the periodic rising and falling of the oceans each day. They’re caused by the gravitational pull of the moon and sun on the oceans of the earth.

As the moon moves across the sky, it pulls the oceans with it. The moon has the biggest effect, because even though it’s small, it’s also much closer than the sun. But, the gigantic sun plays a part, too; about half as strong an effect as that of the moon.

## Sunlight, Seasons, & Solar Winds

The sun gives us the energy that we use to live. It causes plants to grow, which are eaten by animals like us. So when the intensity of sunlight changes, it’s a big deal.

This is why many places on Earth have seasons. The earth’s rotational axis is tilted relative to the sun. When it’s summer in the Northern Hemisphere, that half of the earth is tilted towards the sun, causing the light we receive to be more concentrated; more directly overhead. When it’s winter in the Northern Hemisphere, that half is tilted away from the sun, causing the sun to be lower in the sky and the rays to be less concentrated.The amount of energy we receive from the sun also varies with solar activity.

The sun’s activity increases and decreases on an eleven-year cycle. This affects how hot it is on Earth, but it also affects the solar wind. The solar wind is a wind of charged particles that get blown towards Earth all the time. Thanks to the earth’s magnetic field—the fact that the earth acts like a giant magnet—most of these particles get pushed aside and don’t affect us.

This is a good thing because if they all hit us directly, our electronics would be damaged, and we would be exposed to so much radiation it’s doubtful life could survive on Earth. The few particles that do make it to us create the beautiful aurora borealis.

## Long-Term Interactions

The exact way the earth, moon, and sun are arranged in relation to each other isn’t always the same.

The earth is more like a spinning top that wobbles than a perfect and stable sphere the way we tend to picture it. Over thousands and millions of years, the exact arrangement changes.For example, right now we only see one side of the moon. The moon spins at the same rate as it rotates the earth. This is called tidal locking and tends to happen over time thanks to gravity. Eventually, the earth will become fully tidally locked with the moon, and the same side of the earth will always face the moon, too.Another example is the way in which the earth’s tilt changes and the way the earth wobbles.

Right now, the earth is tilted at 23.5 degrees in its orbit around the sun. But, that changes over time. The tilt angle itself goes up and down by about 2.5 degrees, and the direction we’re tilted changes (like the wobbling of a spinning top) over a period of 26,000 years. This wobble is called precession.

## Lesson Summary

The sun, earth, and moon are held together by gravity, and they interact in lots of ways.

The moon orbits the earth because of the pull of the earth. And the earth orbits the sun because of the pull of the sun. Because of the way they move relative to each other, we see phases and eclipses.

The sun always lights up half of the moon and half of the earth, but we don’t always see the part of the moon that is lit up from our vantage point. This causes phases: new moons, crescent moons, half moons, and full moons. When the moon is directly between our vantage point and the sun, we get a solar eclipse, which is when the moon blocks out the sun.The tides are another interaction in the sun-earth-moon system.

The tides happen because the moon and sun pull on the oceans, causing them to rise and fall each day. The moon has a bigger effect than the sun because it is closer.The angle of the sun as the earth orbits over the course of a year causes the seasons. The angle changes because the earth is tilted.

The sun also interacts with us by a sea of charged particles called the solar wind. These particles would be highly dangerous, but the earth’s magnetic field turns most of them away. What passes through creates the aurora borealis.Last of all, in the long term, gravity causes changes in the sun-earth-moon system.

The earth’s tilt changes, the earth wobbles, and the three bodies affect each other’s rotation rates. This happens over tens of thousands, or even millions, of years.

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