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Moon and Jupiter

December 21, 2016

Three bright objects stair-step up the sky at first light tomorrow: the Moon, the planet Jupiter, and the star Spica. Jupiter looks like a brilliant star below the Moon, with fainter Spica below Jupiter.

Jupiter has more than 60 moons of its own. But from Jupiter itself, you’d be able to see only a few of them. Most are so small, dark, and far away that they’d be invisible from the top of Jupiter’s atmosphere. Four of the moons are big and bright, though, making them easy targets.

The easiest of all is Io, the closest of the four. It’s the same size as our moon, but it’s a little closer to Jupiter than the Moon is to Earth, so it forms a slightly larger target. It’s a more interesting target as well, because its surface is a maelstrom of volcanic activity. About 400 active volcanoes belch out gas and super-hot lava. Some of the volcanic debris reaches hundreds of miles high, so from Jupiter, they’d be visible to the eye alone.

The next moon out is Europa. It’s the smallest of the four, but still big enough to make out details on its surface. The main features are giant tiles of ice floating atop a hidden ocean — features unlike anything on our moon.

The other two big satellites, Ganymede and Callisto, are bigger than the Moon, but they’re far away from Jupiter. Even so, they’re close enough to see as disks, and perhaps to make out a few details — features on big moons in an alien sky.

More about the Moon and Spica tomorrow.


Script by Damond Benningfield


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