Two bright planets accompany the Moon across the sky tonight. Brilliant Jupiter, the solar system’s largest planet, is close to the upper right of the Moon as they climb into good view around 10:30 or 11. And fainter Saturn — the second-largest planet — is about the same distance to the left of the Moon.

Today, scientists can tell you the distance of both planets to within a tiny margin of error. Jupiter, for example, is about 386 million miles away right now, with Saturn about 840 million miles.

But such big numbers are unwieldy. So when talking about distances inside star systems, astronomers typically use the astronomical unit, or AU. That’s the average distance between Earth and the Sun — about 93 million miles. So Jupiter is a little less than 4.2 AU away, with Saturn at nine AU.

The distance from Earth to the Sun isn’t constant, though. Over the course of a year, it varies by about one and a half percent in either direction. Yesterday, for example, we were farthest from the Sun for the entire year: 1.017 AU.

Today, we’re moving back toward the Sun. We’ll be closest to our star on January 2nd, during the dead of winter in the northern hemisphere. The change in distance does little to change Earth’s overall temperature, though. The air and oceans distribute heat around the entire planet — no matter how far we are from the Sun.

Tomorrow: slithering along with the serpent.

Script by Damond Benningfield

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