![]() Bob King using the ACT-REACT Quickmap / LRO / NASA The table at right identifies each by number. This map of the Moon shows the locations of a dozen key domes or dome fields within easy reach of amateur telescopes. Seeing the summit blow-hole, you really see a dome for what it is: a formerly active volcano back in the Moon's rough and tumble days. Many appear almost smooth, though a few have rougher areas that again show up best in low, slanted sunlight. I get most excited when I can tease out a caldera. Under good light and excellent seeing, domes look like swellings or blisters on the lunar surface. The west side (lunar west) of the Megadome is rugged, while the east appears to have been smoothed over by lavas. No shadows, no domes! This enormous "bubble" or rise called the Gardner Megadome is 38 miles (61 km) across and features a large depression at its center suspected of being a volcanic caldera. ![]() Low light, the kind that produces long shadows from peaks and crater rims, brings out their gently sloping forms and provides the best contrast. Don't bother dome hunting when the Sun rides high in the lunar sky. Most domes are subtle, low contrast features that turn mushy in poor seeing. There are two key requirements for happy dome watching - steady atmospheric seeing and observing the dome near the terminator shortly after lunar sunrise or before sunset. We'll look at the historically most interesting and easiest examples along with a few oddballs tossed in to keep things lively. Over 300 lunar domes are known, with many visible in amateur telescopes with apertures from 3-inches on up. Omega's summit crater, with a diameter of 1.2 miles (2 km), is one of the easier ones to see in a modest telescope. Both rise about 650 feet (200 m) above the surrounding mare. ![]() There's a lot to see near the crater Cauchy: a rill cliff and two good-sized domes, Omega (diameter 6.2 miles / 10 km) and Tau (8.7 miles / 14 km). Slopes are very gentle - only a few degrees at most - making for very easy walking should astronauts ever get the chance to explore one. A typical lunar dome measures between 5 and 7.5 miles (8-12 km) in diameter with a peak or caldera ~900 feet (~300 meters) high. Shield volcanoes can be small like the Icelandic and lunar varieties, or broad and massive like Olympus Mons. They're almost all of low-explosivity, unlike their cousins, the more violent stratovolcanoes that grab the headlines.Īs sheet after sheet after sheet of lava oozes up from beneath the crust, a dome slowly builds up over time into a broad, gently-sloped mound shaped like a warrior's shield with a raised center and lower edge. Similar to shield volcanoes in Iceland, Hawai ʻi (including Mauna Kea on the Big Island), and Olympus Mons on Mars, they form when highly fluid lavas erupt through a central caldera onto the surface. They formed as a result of the Moon's own internal volcanism. Craters, rays, mountain ranges, maria, and basins abound. Many of the Moon's characteristic landscapes were created by impact. Most of us will put deep-sky observing on hold as lunar glare intrudes on dark skies. Instead of capping your scope, why not make the Moon your focus? If you haven't already, it's a good time to get acquainted with one of our satellite's most evocative features: domes. In the coming week, the Moon will wax from a thick crescent to nearly full. In this and the following photos, south is up unless otherwise indicated. One of the best known lunar dome fields is located just north of the crater Hortensius not far from the crater Copernicus. ![]() With this week's waxing Moon, we set off to explore its volcanic past with a look at a dozen intriguing lunar domes.
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