How to Watch the Dec. 20 Total Lunar Eclipse

A total lunar eclipse set to take place next Monday night and Tuesday morning (Dec. 20-21) will be well-placed for observers across North America to catch a view.

On the East Coast, it begins half an hour after midnight on Tuesday; on the West Coast, it begins around 9:30 p.m. PST Monday. In all cases, the whole eclipse will be observable before the moon sets in the west just as the sun is rising in the east. Maximum eclipse is at 3:17 a.m. EST/12:17 a.m. PST.

During a total lunar eclipse, the Earth gets between the full moon and the sun, blocking the sun's light from bouncing off the lunar surface. Although lunar eclipses occur fairly frequently, usually at the rate of about two a year, they often hold surprises.

No one knows until the eclipse actually occurs how deep it will be and what color the moon will show. This is determined by weather conditions around the Earth?s rim at the time of the eclipse, as the sun's light is colored and refracted inward by thousands of sunsets and sunrises.

If the air is clear around the rim of the Earth, the eclipse may be quite light; if cloudy, quite dark. Again, depending on those sunrises and sunsets, the moon may appear orange, red, dark brown or slate gray.

This star chart shows where in the sky the upcoming lunar eclipse will appear. And check this NASA lunar eclipse chart to see how visible the eclipse will be from different regions around the world.

Astronomers often try to estimate the magnitude, or brightness, of the eclipsed moon. Because the moon is much larger in apparent diameter than the stars and planets, it's necessary to "shrink" the moon to make comparisons more accurate. One way to do this is to view the moon through binoculars the wrong way around, looking in the objective end. Another way is to view it in a reflecting garden globe.

It is fun to repeat the observations made by early Greek astronomers of the curve of the Earth's shadow on the moon's face, which they used to prove that the Earth was round. Greek astronomers also used the curve of the Earth's shadow to calculate the relative sizes of the Earth and moon.

One thing worth observing is how different the shadow of the Earth falling on the moon looks compared to the regular phases of the moon observed every month during the year. There's a common folk belief that the moon's phases are caused by the shadow of the Earth falling on the moon, rather than by the sun illuminating the spherical globe of the moon from different angles.

The moon's shadow during an eclipse is much less curved than the lunar terminator, and always concave. The Earth's shadow is strongly colored by light refracted through the Earth's atmosphere, while the normal lunar terminator is colorless.

One special object to look for in this eclipse is the open star cluster Messier 35 in the constellation Gemini. This will be just three degrees above the eclipsed moon, half a binocular field away. It's also interesting to see how many faint stars you can see when the moon is fully eclipsed ? you will see far more than are visible against the bright full moon before and after the eclipse.

A lunar eclipse makes for many photo opportunities.

Be sure to bracket your exposures (try different exposures longer and shorter than what your meter says) because a bright moon in a dark sky often fools cameras' exposure meters. If you normally keep a filter on your lens for protection, take it off to photograph the eclipse; otherwise, you may spoil your pictures with a ghost image of the moon. And use a telephoto lens or maximum zoom: The moon always looks larger to the eye than it does on film. Be sure to take some wide-angle shots as the moon gets lower in the sky toward the end of the eclipse.

• Gallery: Photos of the Feb. 2008 Total Lunar Eclipse
• Top 10 Lunar Eclipse Facts
•  Partial Lunar Eclipse Thrills Weekend Skywatchers

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.