Lunar eclipse: Why will this one be so big and red?

Lunar eclipse watchers are looking forward to the last lunar eclipse of 2011 Saturday morning. In the West, the eclipse will happen just as the moon is setting, creating quite a show. 

Bullit Marquez/AP
In this June 2011 file photo, the moon exhibits a deep orange glow as the Earth casts its shadow in a total lunar eclipse as seen in Manila, Philippines, before dawn. The last total lunar eclipse of the year is Saturday, Dec. 10, 2011. And there won't be another one for three years. Viewers in the western half of the United States will have the best views Saturday well before dawn, Pacific and Mountain Standard Time. The farther west the better.

The last eclipse of the moon for 2011 takes place this Saturday, Dec. 10. It's a total eclipse. And for people living west of the Rockies, it promises to be a beauty, weather willing.

People in the Western United States won't see the full lunar eclipse; the moon will be setting Saturday morning as the eclipse is occurring. But when it's low on the horizon, it can appear significantly larger than when it's high in the sky.

That means the reddening moon could be eye popping, but more on that apparent size discrepancy later.

For eclipse-watchers in the Western US who have low horizons to the west, or a strategically located mountain pass, the main show starts about 4:45 a.m. Pacific Standard Time, according to data posted by NASA. The moon will be moving into partial eclipse

The eclipse will reach totality at 6:32 a.m. Pacific Standard Time. That means the best places to watch the eclipse from start to finish stretch from Hawaii west into Asia.

These eclipses occur when the Earth blocks direct sunlight from reaching the moon. But at totality, the moon still gives off a red glow, thanks to sunlight that passes through Earth's atmosphere.

The moon's ruddy hue during a lunar eclipse owes its color to the way light scatters in the atmosphere.

Coming from the far side of Earth, as an astronaut on the moon might see it, the sunlight must travel a relatively long distance through the atmosphere before it exits and heads moonward.

During the course of its travels, the light looses its other colors – think the color spectrum, here – to atmospheric molecules and dust. These scatter the shorter wavelengths of light. The more dust, the darker the red left to daub the moon's surface.

The color is created by the same effect that causes the deep reds late in a sunset or early in a sunrise.

As for the apparent size of the moon when it's close to the horizon, chalk it up to appearances.

Some researchers hold that the large size is purely an optical illusion, triggered because the eye is comparing the moon's size to buildings, trees, mountains, or other familiar objects on the horizon. Once the moon rises above the familiar, its true size wins out. The illusion can be demonstrated with circles drawn on a piece of paper.

Others suggest the apparent variation is based on distance, perspective, and our human-vision system. Normally, objects appear to shrink as they move toward the horizon. But the size of the moon does not appear to shrink because for all practical purposes, its distance doesn't change over the course of an evening. So as the moon approaches the horizon, the brain tries to make its unchanging size conform to its perspective-based expectations by making the moon appear to grow as it approaches the horizon.

Whatever the explanation for the apparent change in size, this means that if you want to photograph the eclipse and want the moon to loom large in images, use a telephoto lens. Otherwise the moon that appears to your eyes will shrink mightily by the time you look at the image you've produced.

For some clever lunar photography, check these out.

And if merely watching the eclipse doesn't do it for you, Sky and Telescope magazine has a list of observations you can make that scientists can use. 

If you miss this total eclipse, whether you are in a location to view the whole show or not, be patient. The next full eclipse of the moon comes overnight April 14 and 15, 2014. And it will be visible all through its phases throughout the Americas, according to the folks at Sky and Telescope.

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