Moonwatch
“When the illumination is more oblique, it’s possible to see wrinkle ridges”
Few lunar features have names that translate as beautifully as the Sinus Iridum – the Bay of Rainbows. This is a semicircular protrusion from the northwest shore of the Mare Imbrium, the Sea of Showers. The Sinus Iridum looks like a bay off the larger sea. Its ‘coastline’, represented by the lighter lunar highlands, is prominently defined by the Montes Jura mountain range. Under the favourable light conditions around three days after first quarter, it may be possible to see the peaks of this range illuminated while the majority of the Sinus Iridum remains in darkness. This creates a clair-obscur effect known as the Jewelled Handle, the brightly lit mountains producing a beautiful arc of light that appears suspended above the background darkness. The effect is large enough that it can be seen through a pair of binoculars. The Jura range is partially interrupted along its northern edge by the 38kmwide crater Bianchini.
The ends of the bay are marked by two pointed headlands known as Promontorium Heraclides to the south and Promontorium Laplace to the north. Promontorium Heraclides produces another clair-obscur effect, known as Cassini’s Moon Maiden. To see it, look at the feature around four days after first quarter: if the Maiden’s visible, it will be in a south-up, inverted view. Both headlands cast distinctive pointed shadows across the surrounding surface when the illumination is right. Promontorium Heraclides rises 1.2km above the bay below, but is dwarfed by the Promontorium Laplace to the north, which towers nearly 3km above the surface.
The surface of the bay looks remarkably smooth when the Sun is high. When the illumination is more oblique, however, it’s possible to see a number of wrinkle ridges frozen into the basaltic lava that forms the Sinus Iridum’s surface, giving the appearance of long, rolling waves heading towards the shore.
As you head out from the bay into the Mare Imbrium, you’ll encounter two similar-looking craters, 25km Helicon and 20km Le Verrier. These almost appear as sentries to the bay. Almost on the actual entrance is 9km Laplace A, which should be visible with a 4-inch scope. Spotting features inside the bay is tricky because they’re so small. The two largest craterlets are 4km Heraclides E, near to Promontorium Heraclides and 4km Bianchini G, near to its primary, close to the northern edge.
To the west of the Sinus Iridum, in the lighter highland surface, you’ll find 40km crater Sharp. If you’re an imager or have a large telescope, look at Sharp when the morning terminator is off to the west (approximately four and a half days after first quarter) and see if you can see a curious notch along the edge of the shadow cast by the crater’s eastern rim. The notch is caused by a tiny, 1.6km craterlet, located right on the well-defined edge of Sharp’s eastern rim.