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SYFY WIRE Bad Astronomy

The Familiarity of a Crescent Moon

By Phil Plait

I love pictures of the crescent moon. There is something so wonderful about the shape, the elegance, even the geometry of it; knowing the you need a spherical world lit by the Sun from the side and seen from an angle to produce that lovely slender shape. Add to that an artistic shot done in black and white, and you get a photograph of great beauty:

Isnât that nice? I like how the surface features on the moon are just barely visible, only slightly more than hinted at, teasing your brain and your eye. The sharp edge, the fuzzier day-night line (called the terminator) softened a bit by the irregular terrain, the perfectly inky depths of the dark side, the way the tips of the crescent taper to needle-sharp points.

All in all, itâs a remarkably pretty shot of the moon.

Oh, my apologies. Did I not mention that this isnât the Earthâs Moon?

No, itâs actually Enceladus, a moon of Saturn, taken by the Cassini spacecraft nearly a billion miles away. Enceladus is a tiny iceball, less than a third the diameter of our Moon, and it was 530,000 kilometers (330,000 miles) from Cassini when this shot was taken. Even with the probeâs powerful cameras, that was far enough to reduce the surface features of the moon to a blur, taking away our eyesâ ability to distinguish it from a photo of our own natural satellite.

Over the weekend I put up a splendid time-lapse video made from Cassini images of Saturn, its rings, and exotic moons. The jerky, somewhat lurching nature of the video only reinforces how alien those worlds are, so it tickles me to see a photo just days later that so hauntingly reminds us of home. For all the differences between bodies in the solar system, there are still overarching similarities, features and environments that are familiar to us even though vast gulfs of emptiness separate us.

If there is a morality lesson to be learned here, I leave it to you to find.

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