Moonwatch – Sinus Iridum

If you observe Sinus Iridum at precisely the right time, you'll see the famous Jewelled Handle

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Sinus Iridum, or the Bay of Rainbows, is one of the loveliest features of the Moon: the Bay of Rainbows. It leads off the vast Mare Imbrium and was shown and named by the famous lunar observer Giovanni Riccioli in his map drawn in 1651. In those days, of course, it was generally believed that the dark areas really were seas, and that the Moon could well be a world suited to life.

Sinus Iridum is called a bay, but it is really a crater whose seaward wall has been virtually destroyed; only a few very low, disconnected fragments can be traced. Elsewhere the mountainous ‘wall’ is continuous and fairly high, though the outermost edge is disturbed for a short distance by the prominent crater Bianchini. The continuous section is bounded by two capes, Promontorium Heraclides and Promontorium Laplace.

The floor of Iridum slopes downward from the Mare Imbrium, so that at the far side it is about 200ft (61 metres) lower than the level of the Mare. There is nothing quite like this anywhere else on the Moon, but the sequence of events seems to be fairly straightforward. The Mare itself was formed during the lunar epoch named after it – the Imbrium – which ended over three thousand million years ago, so that it post-dates the Great Bombardment. The Iridum impact followed before the great lava-floods, which accounts for the inundation of the seaward wall.

I have said that Sinus Iridum is one of the loveliest features of the Moon. So it is, provided you catch it at the right moment. As the Sun rises over it, the mountainous border is illuminated first, and the tops of the peaks catch the sunlight while the floor below is still in darkness. The result is that the wall seems to stand out beyond the terminator, giving the impression that it is completely detached from the main body of the Moon. Lunar observers refer to this as the ‘Jewelled Handle’.

This occurs once in every lunation (lunar cycle), well before full Moon, but it does not last for long, and as the sunshine creeps onto the lower-lying floor the ‘handle’ effect vanishes. It is fascinating to follow the changes as the Sun’s altitude increases; even a small telescope will show them well. The floor itself is very smooth, and there is only one reasonably well-marked craterlet, E.

Sinus Iridum can always be identified whenever it is sunlit; note the two well-marked craters Helicon and Le Verrier on Mare Imbrium to the north, each between 10 and 15 miles (16 to 24km) in diameter. Except at the time of sunrise, the area seems ordinary enough – but at the coming lunation, make sure that you do not forget to watch for the glory of the Jewelled Handle.

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This article first appeared in the February, 2010 issue of Sky at Night magazine