Lunar eclipses | The Earth-sun-moon system | Middle school Earth and space science | Khan Academy

Have you ever seen the full moon appear to change from this to this to this all in a couple hours? If so, you've witnessed a lunar eclipse. The word eclipse comes from a Greek word meaning "to leave."

For centuries, people have marveled that a full moon appears to leave during a lunar eclipse. The Incas believed that lunar eclipses happened when a giant jaguar ate the full moon. It turns out lunar eclipses happen when the moon is covered by Earth's shadow.

We can only see the moon because it reflects light from the sun; the moon doesn't generate any light of its own. The sun lights up half of the sphere of the moon. When Earth is between the sun and the moon, we can see the entire lit part of the moon. We call this the full moon.

But sometimes, during a full moon, Earth, the moon, and the sun all line up in such a way that Earth casts a shadow onto the moon. This creates a lunar eclipse. Lunar eclipses can be seen from anywhere on the night side of Earth.

The two most dramatic types of lunar eclipses are total and partial. During a total lunar eclipse, Earth's shadow completely covers the moon. This means that no direct sunlight is reaching the moon. However, the moon is still visible during the eclipse.

Why is this? As sunlight passes through our atmosphere, some of it gets bent slightly, allowing it to reach the moon as indirect sunlight. Most of the light that gets bent is red or orange, meaning that the moon appears reddish-orange during the eclipse.

During a partial lunar eclipse, Earth's shadow only partially covers the moon, so some direct sunlight is still reaching the moon. Lunar eclipses only happen during a full moon since it's the only time in the moon's orbit where Earth is between the sun and the moon.

And you might be wondering, why isn't there a lunar eclipse during every full moon? Well, the moon's orbit around Earth is tilted relative to Earth's orbit around the sun. This means that the moon often passes just above or just below Earth's shadow.

Partial lunar eclipses happen when the sun, the moon, and Earth almost, but not quite, exactly line up so that the moon is only partially covered by Earth's shadow. Total lunar eclipses happen only when the sun, the moon, and Earth all line up precisely.

So if you ever get to see a lunar eclipse, you can thank our planet for blocking the moon's sunlight and letting us enjoy the celestial spectacle.