Lunar Eclipse 101 | National Geographic

(bright music) - [Narrator] A lunar eclipse happens when the earth blocks some or all of the sun's direct light from reaching the moon. This cosmic event only takes place during a full moon, which happens once every 29 and a half days, or the length of one full orbit of the moon around the earth.

So why don't we have an eclipse every month? The moon's orbit is tilted a few degrees in relation to the earth, so the earth, moon, and sun don't always align. When the earth does eclipse the sun, it casts two types of shadows on the moon: a larger shadow, known as the penumbra, and a smaller, darker shadow, known as the umbra.

There are three types of lunar eclipses. The first is a total lunar eclipse, when the sun, moon, and earth are in perfect alignment and the moon falls within the earth's umbral shadow. Total lunar eclipses are the most striking of the three types because they turn the moon a sunset red.

While shorter, blue wavelengths of light are scattered outward by the earth's atmosphere, longer, red wavelengths are refracted, or bent, inward toward the moon, making it appear red. The brightness of the moon's red glow depends upon how much dust and clouds are in the earth's atmosphere. Following volcanic activity, ash can block out enough light to render the moon a darker red, or even near black.

A partial lunar eclipse, the second type, occurs when the earth, moon, and sun don't perfectly align, so only part of the moon passes into earth's umbra. Earth's shadow appears very dark on the side of the moon facing earth. Last, a penumbral lunar eclipse occurs when the moon passes through earth's penumbral shadow. The event is so subtle that most people don't even notice.

The moon will appear just slightly darker than normal. Lunar eclipses occur up to three times a year and can be observed from the entire nighttime half of the earth. Unlike during a solar eclipse, it's safe to look at the moon with the naked eye during a lunar eclipse.

It is only because of the distances of the sun and moon from the earth that we are able to witness total lunar eclipses. As the moon inches away from the earth each year, one day, billions of years from now, the moon will be too far away to fall completely within earth's umbral shadow.

Until then, we can occasionally enjoy seeing our own planet's shadow cast upon the largest object in the night sky.