Sinus Iridum 45.0°N, 32.0°W

Sinus Iridum, the ‘Bay of Rainbows’, is the most striking landmark in the northwestern portion of Mare Imbrium. It is the remnant of a giant crater, bordered on the northwest by the Montes Jura (the Jura Mountains), which are undoubtedly a portion of the original crater wall. On the lunar timescale, Sinus Iridum was formed after the Imbrium impact, but before the various lava flows flooded the Imbrium Basin. Sinus Iridum is about 260 kilometres in diameter, and so may be classified one of the smaller impact basins.

The Jura Mountains end in two capes; in the southwest lies Promontorium Heraclides with a height of 1.7 km, and in the southeast lies Promontorium Laplace, which reaches a height of 2.6 km above the average level of the surrounding surface. At sunrise, Promontorium Laplace casts a triangular shadow, and in larger telescopes a lunar dome becomes visible to the west. Other, difficult to observe, domes line near Promontorium Heraclides. They are, however, only observable under extreme grazing illumination. The lava plain within Sinus Iridum is not broken by any large craters. Large telescopes do, however, reveal a series of craterlets and crater pits on the lava surface.

If the northwestern wall of Sinus Iridum is examined more closely, it will be realized that the height of the wall on the eastern side as far as Cape Laplace is approximately the same height, whereas on the west, towards Cape Heraclides it continuously lowers.