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Moon and Mars

January 2, 2016

This is a great month for early risers who like to check out the sky. Four planets are in view at first light, with several of them huddling close to bright stars. And for the next few days, the waning Moon joins them.

Tomorrow, for example, the Moon forms a tight grouping with a star and a planet. Mars is quite close to the lower left of the Moon, and shines bright orange. Blue-white Spica, the brightest star of the constellation Virgo, stands a little farther to the lower right of the Moon.

As the days roll by, the Moon will slide farther eastward, leaving Mars and Spica behind. It’ll stand lower each day as the first blush of twilight begins to paint the dawn sky.

That change in position is caused by the Moon’s orbital motion around Earth. Right now, it’s moving toward the line between Earth and the Sun, so the angle between Moon, Earth, and Sun changes each day. As it does so, sunlight illuminates less and less of the lunar hemisphere that faces our way, so the Moon will become a thinner crescent.

As that happens, though, the Moon will be looking back at a fuller and fuller Earth. Much more sunlight will reflect off of Earth and onto the Moon. So the nighttime portion of the Moon will glow with reflected earthshine. That’ll give the dark portion of the Moon a ghostly glow, allowing us to see the entire lunar disk — until the Moon disappears in the Sun’s glare late next week.

We’ll talk about Mars and a meteor shower tomorrow.


Script by Damond Benningfield, Copyright 2015

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