The week of March 26 – April 1 takes us from Day 10 to Day 16, right after full moon. This week we will highlight Sinus Iridum, visible on Monday and Tuesday nights.Sinus Iridum on the Moon

Sinus Iridum: [NW/E6] Around the end of Day 9 and beginning of Day 10, when the terminator bisects Sinus Iridum (the Bay of Rainbows), located in the northwest sector of Mare Imbrium, you will be able to catch the Jura Mountain Range when parts of it are in full sunlight. This produces an effect sometimes referred to as the “Golden Handle.”

Bay of RainbowsSinus Iridum formed after the Imbrium impact but before the lava flooding that filled the Imbrium basin. This bay is itself an enormous 160-mile crater and is the largest and most spectacular example of subsidence on the Moon. If you have very sharp eyes, without optical aid you should be able to see the “bite” that Iridum takes out of Mare Imbrium. (You may have to wait until tomorrow night.)

shoreline craters would tilt inward and their seaward rims would often sink below "sea level," When the Sun is at a low angle, you can see that Sinus Iridum is covered with wrinkle ridges, resembling many waves lapping toward the shore.

The Jura Mountains, on the rim of Sinus Iridum, curve protectively around the bay and descend, because of the above-mentioned subsidence, to sea level where they terminate in capes at each end: Promontorium Laplace on the northeast, and Promontorium Heraclides on the southwest.

Pay close attention to Promontorium Heraclides. Under the right lighting conditions around Day 10 you can see the profile of a woman’s head with long flowing hair as she gazes wistfully out into the bay. Those who have gotten to know her refer to her fondly as the Moon Maiden. Cassini himself was the first person to describe this feature. She reveals herself best when the seeing is a bit “soft” and the terminator has just cleared the western edge of the Jura Mountains. At this time you will see her gazing across the bay to Promontorium Laplace, her hair streaming out behind her. If the seeing is crisp, try defocusing a little.



It is highly recommended that you get a copy of Sky and Telescope’s Field Map of the Moon, the very finest Moon map available for use at the telescope. It is available for $10.95 at and on Amazon. All features mentioned in this blog will be keyed to the grid on the Field Map and will look like this: Plato: [NW/D9]

Courtesy of Gray Photography of Corpus Christi, Texas
Lunar photos: NASA / USGS / BMDO / LROC / ASU / DLR / LOLA / Moon Globe. Used by permission

Andrew Planck
Features on the Moon: The Bay of Rainbows and the Golden Handle

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