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Moon and Planets

July 24, 2012

A compact box of four bright astronomical objects stands in the southwest at nightfall this evening. The Moon anchors the rectangular box. The planet Mars is close to the upper right of the Moon, with the star Spica and the planet Saturn to its upper left, forming the other side of the rectangle.

All four objects are bright enough to see well before the end of twilight — the colorful result of sunlight interacting with the atmosphere.

You’d also see twilight on Mars. In fact, twilight there lasts up to two hours — a bit longer than the typical twilight on Earth. That’s because the Martian atmosphere is filled with dust — tiny grains of sand lofted by the Martian winds.

The dust grains absorb most of the blue wavelengths of sunlight and scatter redder wavelengths. That makes the daytime sky look pink or orange. Much of the sky stays pink as the Sun sets, although the region around the Sun looks blue.

The dust extends many miles above the surface. So well after the Sun sets, its rays are still illuminating dust grains high above the Martian nightside. That prolongs the twilight — keeping the Martian skies aglow with color well after sundown.

Again, look for Mars — which is colored orange by the same dust that colors its sky — quite close to the Moon this evening. By tomorrow evening, the Moon will have left Mars behind, and will stand closer to Spica and Saturn. We’ll have more about that on our next program.


Script by Damond Benningfield, Copyright 2012


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