A day or two after the Moon reaches first quarter, sunlight spreads over a striking region on the Moon known as Sinus Iridum, the “Bay of Rainbows”. This region, a small semicircle on the northwest corner of the larger Sea of Rains, offers much for casual stargazers, including smooth floors of ancient lava, small symmetric craters, and the rugged peaks of the Jura mountain range. If you catch it at the right time, you can see sunrise catching the peaks of these mountains before it fully lights the floor of the Bay of Rainbows… it’s an unforgettable sight.
The Jura mountains lie on the northeast rim of Sinus Iridum (see below). Named after the terrestrial mountain range of the same name in Switzerland and France, this range runs for about 420 km around the Bay of Rainbows and features mountains as high as 6,100 meters.
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Most features on the Moon look best in a telescope when they’re on the “terminator”, the line between night and day. The terminator approaches this region just after first quarter, and the long shadows make for astounding vistas when observed in a small telescope at moderate to high magnification of 150x or more. Sunrise on the Jura mountains is a fleeting event. The peaks take the appearance of a ring of tiny jewels, and you can see changes in illumination over the course of just ten or twenty minutes. You may not be able to see this event each month, of course, since it may occur when the Moon appears in the day time in your area, or may have not yet risen.
The Jura mountains, like all mountains on the Moon were created by impact of comet or asteroids in the early days of the solar system. The impacts formed large craters and maria and churned lava onto the surface. The material sloshed up at the edges of these impact basins to form lunar mountain ranges which have remained, virtually unchanged, in the airless lunar environment for nearly 4 billion years. The impact that created Sinus Iridum and the Jura mountains created a circular impact crater. Half the crater was obliterated a little later when a larger impact created the Sea of Rains, which itself holds several more mountain ranges along its perimeter. All are visible in a small telescope.