September 09 – Discovery of Jupiter’s Moon, Amalthea (1892)

Amalthea, named after the foster-mother of Zeus, is Jupiter’s third moon (counting outwards), and the largest of the inner satellites. It was the last to be discovered by an astronomer standing staring up a telescope rather than taking photographs to peruse later. The astronomer was Edward Emerson Barnard (of Barnard’s Star fame) and he discovered Amalthea on September 9th 1892 with the 36 inch refractor at Lick Observatory in California.

Amalthea is roughly ellipsoidal in shape (a bit like a rugby ball), about 250 km long and 140 km wide, and orbits Jupiter with the long axis always pointing towards the planet (in the same was as our own Moon, known as tidal locking). It is mostly reddish in colour, but patches of green have been seen. The surface is widely cratered.

Amalthea from Voyager 1 (image credit: NASA)
Amalthea from Voyager 1 (image credit: NASA)

Amalthea would be a fantastic place from which to view Jupiter. The giant planet occupies 46° in the sky, or about a quarter of it. You would have to careful not to jump up and down with the excitement, though, as the escape velocity of 0.06km/s means that if you did jump up, the “and down” part wouldn’t happen.

The mythology side of today’s post is a little confused, because there are varying versions of Amalthea’s story. She doesn’t even assume human form in all of them, being instead a goat. Some stories have Zeus, after Amalthea’s death, using her skin as a protective aegis. So I’m hoping she was a goat, and not a nurse. That would be particularly unpleasant, even by Zeus’ standards.

Asteroid 56 Melete discovered, 1857.
Asteroid 61 Danaë discovered, 1860.

Asteroid 189 Phthia discovered, 1878.
Asteroid297 Caecilia discovered, 1890.
Asteroid 298 Baptistina discovered, 1890.

2 thoughts on “September 09 – Discovery of Jupiter’s Moon, Amalthea (1892)

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