There's a lot of movement going on in our solar system. All the planets constantly move at different speeds around the sun. Moons constantly spin around each planet. Every now and then, one of these objects moves into the shadow of another one. We call this an eclipse.
Solar Eclipse
A solar eclipse happens when the moon, on its journey around Earth, happens to pass between the sun and Earth for a moment. A solar eclipse can only happen during a new moon, a phase of the lunar cycle where the moon lies between Earth and the sun. At night, the moon is totally dark during a new moon.
If everything lines up just right, the sun is fully or partially covered by the moon. That means those of us on Earth see a big, round shadow (the moon) sliding over and covering the sunlight we normally see in the daytime sky. The shadow covering the sun is called an umbra.
Total solar eclipses, where the moon perfectly blocks out the entire sun, are rare. Partial solar eclipses, where the moon covers only a part of the sun, are much more common.
Between two and five total or partial solar eclipses happen every year. Whether or not you see a full or partial eclipse depends on where you are on Earth. A full eclipse may be visible to people in the Northern Hemisphere when it is facing the sun, for example. In this situation, people in the Southern Hemisphere will not see the eclipse at all.
Solar eclipses are usually over and done with pretty quickly. Scientists say total solar eclipses, in the very best conditions, can only be seen for about seven minutes. The moon's umbra keeps sliding over until the sun peeks through again.
The moon is just the right size to perfectly cover the sun. The sun is actually many, many, many, times larger than the moon, but the moon is much closer to Earth, so it appears larger.
Lunar Eclipses
Lunar eclipses happen when the moon passes through Earth's shadow. This can only happen when the moon is on the far side of Earth, away from the sun. Think of Earth in the center, with the sun on one side, and the moon on the other. So, lunar eclipses only happen when there is a full moon.
Lunar eclipses are much more common than solar eclipses, and many more people can see them at the same time. In fact, a whole hemisphere can usually see a lunar eclipse happening. Part of why they're easier to see is that they last longer; lunar eclipses last several hours.
Other Eclipses
As other planets pass in front of the sun, they can also cause eclipses. These eclipses are called transits. From Earth, we can only see transits of Mercury and Venus, the two planets closer to the sun than Earth. From the planet Saturn, though, a satellite may view transits of Earth, Mercury, and Jupiter as well.
To view the transit of Mercury or Venus requires a telescope. The sun is so massive that even with powerful telescopes, the planets look like grains of sand moving across a giant beach ball.
