Bob Moler's Ephemeris Blog

This is Bob Moler with Ephemeris for Thursday, December 19th. Today the Sun will be up for 8 hours and 48 minutes, setting at 5:04, and it will rise tomorrow at 8:16. The Moon, 3 days before last quarter, will rise at 9:47 this evening.
Callisto is the most distant of the Galilean moons from Jupiter, visible in small telescopes or even binoculars. It has the darkest surface of the four moons, and so appears the dimmest, even though it’s the second largest after Ganymede. The largest crater on Callisto is called Valhalla, and it doesn’t look like a crater at all. There’s many concentric, what look like, frozen ripples surrounding it, and it covers a good percentage of the moon’s surface. It is probably the only moon of Jupiter that we could send a crew to, since it’s the farthest from the intense radiation belts of Jupiter. Made of approximately equal parts of water mostly in the form of ice and rock, Callisto probably has a liquid water ocean underneath its icy crust, like Ganymede and Europa.

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