Eclipses: Shadows in the sky

What makes the sun and moon go dark during an eclipse?

On Dec. 21, 2010, the moon will pass through the darkest part of the Earth's shadow, and for the first time in 372 years, the spectacle will take place during the winter solstice.

The rare, 72-minute lunar eclipse — when the sun, the Earth and the moon align — will begin in the early morning hours in North America, and should cast an amber glow on snowy landscapes, said NASA.

In general, an eclipse occurs whenever an astronomical body, such as a planet or moon, comes between a source of light, such as a star, and another body, casting a shadow.

On Earth, though, when we talk about eclipses, we're usually talking about one of two types: lunar or solar.

Lunar eclipse

A lunar eclipse occurs when the Earth comes between the sun and the moon, casting the Earth's shadow on the moon. Such an eclipse is generally only noticeable when the moon travels into the dark inner potion of the Earth's shadow, the umbra.

Quick Fact

When astronomers in ancient Greece observed that the shadow of the Earth on the moon during a lunar eclipse was always circular, they realized the Earth was a sphere.

If the moon goes completely into the umbra, it's called a total lunar eclipse. If only part of the moon becomes dark, it's called a partial lunar eclipse.

Even during a total eclipse, the moon doesn't become completely dark. Light from the sun filters and bends through the Earth's atmosphere and can shine on the moon. The amount of light that reaches the moon depends on the amount of clouds and dust in the atmosphere. And because clouds and dust refract blue light from the sun, the moon glows a coppery-red during a lunar eclipse.

Lunar eclipses only occur during a full moon, but because the moon's orbit around the Earth is tilted relative to the Earth's orbit around the sun, we don't see an eclipse at every full moon.

The next time a lunar eclipse will be visible in North America will be April 15, 2014.

Solar eclipse

When the moon moves in front of the sun from our perspective on Earth, a solar eclipse occurs. These are typically more dramatic than lunar eclipses since the daytime sky can go eerily dark for several minutes as the moon blots out the sun's light.

If the moon completely covers the sun, it's called a total solar eclipse, a rare event. However, if a ring of the sun is still visible around the silhouette of the moon, it's called an annular eclipse. This change in the apparent size of the moon relative to the sun occurs because the moon's orbit isn't perfectly circular, so the moon can be closer to the Earth at certain times.

In some cases, an eclipse can be total when observed from some locations and annular from others, because of the curvature of the Earth. In either case, astronomers can use the opportunity to study the sun's corona, the outer layer of the sun's atmosphere, and solar flares.

If only part of the sun is blotted out by the moon, it's called a partial solar eclipse.

It is never safe to look directly at the sun, even during an eclipse, as the light and radiation can harm one's eyesight. Eclipses are usually viewed by projecting an image of the sun onto a piece of paper though either a lens, such as a telescope, or a pinhole in a piece of cardboard.

The next total solar eclipse that will be visible across North America will be on Aug. 21, 2017.