The week of October 2-8 takes us from Day 19 to Day 25. This week we will highlight a scarp on the moon called Rupes Altai, visible after 10:00pm on Tuesday.

scarp called Rupes AltaiAltai Scarp (Rupes Altai): [SE/M12] Viewable even through small telescopes, the Altai Scarp is a spectacular example of how shock waves from a major impact can compress the surrounding terrain into a series of outwardly expanding rings. Nearly 3.9 billion years ago, a large asteroid or comet slammed into the Moon and dug out the Nectaris Basin. Shock waves rapidly expanded through the surrounding terrain and became frozen in place, producing a classic multi-ring basin.

Altai Scarp - Rupes Altai - on the moonRupes Altai is a beautiful and conspicuous fragment of the original rings. In truth, it is a circular mountain range whose highest peaks rise to 13,000 feet! But you must catch the scarp under an early morning Sun, it’s majesty fades quickly. View it on Saturday or Sunday night when the face of the scarp is fully illuminated, then revisit the area around Day 19 or 20 at lunar sunset. At this time the scarp sends long shadows over the terrain to the east. Which day shows the scarp to its best advantage?

As daylight moves across the region, try to locate hints of other ring features surrounding Mare Nectaris.



Last quarter Moon is on Friday.


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
Rupes Altai: Scarp on the Moon
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