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Moon and Spica

December 29, 2018

The star Spica poses near the Moon at first light tomorrow. It’s the brightest star of the constellation Virgo, and one of the 20 brightest stars in the entire night sky.

When we look at Spica and the Moon, we’re actually seeing the same thing: starlight.

Spica is a ball of hot gas that produces its own light. In fact, Spica is especially hot — tens of thousands of degrees hotter than the Sun. That high temperature makes the star especially bright, and gives it a blue-white color.

The Moon, on the other hand, is a ball of rock. It produces no light of its own. Instead, what we call “moonlight” is actually reflected sunlight — light produced by our own star. The Moon isn’t a very good reflector, though. Its surface is quite dark, so it reflects only about a tenth of the sunlight that strikes it.

As the Moon orbits Earth, the Sun illuminates different parts of it. When the Moon is full, it lights up the entire hemisphere that faces Earth. And when the Moon is new, the Sun lights up the hemisphere that faces away from Earth.

At dawn tomorrow, the Sun will be lighting about a third of the visible hemisphere, so the Moon will be in a “crescent” phase. Over the next few days, the crescent will get smaller and smaller as the Moon moves toward the Sun. The Moon will be new in a week.

For now, look for the Moon near Spica in the early morning sky. They climb into good view by about 2 or 3 a.m., with Spica to the lower right of the Moon.

Script by Damond Benningfield

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