The Bay of Rainbows lies on the northwest edge of the Sea of Rains.
The Jura mountains form its boundary, and they are high enough to be
illuminated by the rising sun while the floor of the bay yet lies in
darkness. As the terminator progresses westward, the mountains light
up, first at Promentory Laplace, then subsequently at Promentory
Heraclides, then the two sunlit portions join, making a semicircle of
light that rings the bay. This brilliant arc almost suggests the
rainbows for which the feature was named.
Sinus Iridum on the dark side of the moon
(Randy Muller <71172.1234 _at_ compuserve.com>)
[Observing the dark side of the crescent moon ...]
The region north and west of Mare Imbrium, in which Sinus Iridum and
Plato are embedded was very prominent.
Wrinkle Ridges (JRF <freeman _at_ netcom.com>)
At low sun angles, note the long, prominent wrinkle ridges that
cross the mouth of Sinus Iridum and trend southward, past the crater
Caroline Herschel, through Lambert and all the way to the Carpathians
(Rukl chart 20).