This curious lunar feature — visible the nights of March 4th and April 2nd — is a delight in small scopes or even binoculars.
Late winter through early spring is the best time of year to be a lunar observer if you live in the Northern Hemisphere. Not only is the weather (slowly) improving, but the evening Moon rides high on the ecliptic where it’s ideally placed for high-magnification viewing. I've discussed the particulars of this circumstance, but suffice it to say, any time the Moon is found in or near Gemini, chances are you’ll get a good look at our nearest celestial neighbor.
And as it happens, that’s where the Moon is located this week, as Sinus Iridum emerges from lunar night.
Sinus Iridum: A Dry Bay
This magnificent formation is better known by its more poetic name, the Bay of Rainbows.
Of course, this 260-kilometer-wide “bay” holds no water. That’s not to say that the Moon lacks water completely, but the Sun-baked flat expanses of the lunar seas, or maria, are the last places you’ll find the precious substance — water is most abundant in permanently shadowed depressions near the lunar poles. Instead, the Bay of Rainbows is filled with basaltic lavas and pockmarked by a handful of tiny craters, the largest of which is 8-kilometer- (5-mile-) wide Laplace A.
Far more interesting for telescope users is the Jura Mountain range, a ring of jumbled peaks that define the Bay itself. How exactly did this neat feature come to be? Its formation makes sense if you picture the Jura Mountains as the northern rim of a huge impact crater that likely formed more than 3.2 billion years ago on the rim of an even bigger and older depression, the Imbrium Basin.
Making Your Bay
So, the sequence of events is this: First the Imbrium Basin was excavated around 3.85 billion years ago. Then, 650 million years later, a large chunk of rock smashed into the the northern rim of the Imbrium depression. Later, the giant basin’s center began to sink as lavas flooded to the surface, causing the southern edge of the Iridum crater to tilt inward, toward Imbrium’s center. Eventually, rising lavas submerged the Iridum crater’s lower, southern wall. As a result, when we look there with our telescopes today, we see the vast expanse of Mare Imbrium and the lovely curve of Bay of Rainbows.
If you train your scope on this region when the terminator sweeps across Sinus Iridum, the easternmost horn of the Bay, named Cape Laplace, first begins to catch the light of the approaching lunar dawn. As the hours pass, more and more of the arc of the Jura Mountain lights up. Eventually, the whole curve of Sinus Iridum up to Cape Heraclides is bathed in bright sunlight, even as most of the flat Bay remains in darkness.This illuminated arc is known as “the golden handle,” and it's something you definitely want to catch when you get the chance.
This week, you have that chance. Look on the night of March 4th (and again on April 2nd/3rd). A night or two later, once Sinus Iridum is completely lit, see if you can spot the subtle wrinkles that delineate Iridum’s submerged southern rim. I’ve seen it in a 3-inch scope. As with so many lunar features, its visibility is less a question of optics than it is of illumination.