The week of October 3rd – 9th takes us from the end of Lunar Day 2 to the beginning of Day 8, a very fruitful time for observing. This week we will highlight the Altai Scarp, visible on Thursday.

The week of Oct. 3rd – Oct. 9th takes us from the end of Lunar Day 2 to the beginning of Day 8, a very fruitful time for observing.
Altai 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 blasted out the Nectaris Basin. Shock waves rapidly expanded through the surrounding terrain and became frozen in place, producing a classic multi-ring basin. (Much like a billiard ball dropped into a tub of thick, viscous mud will leave conspicuous rings of ridges around the point of impact — the advantage here is that the rings remains frozen in place for us to enjoy billions of years later!)

Rupes Altai is a circular mountain range whose highest peaks rise to 13,000 feet.Rupes 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 Thursday 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? Try to locate hints of other ring features surrounding Mare Nectaris.

Thursday is also a great time to revisit the magnificent trio of craters Theophilus, Cyrillus, and Catharina, just above Rupes Altai, which we highlighted on April 11th.


From Monday through Thursday, the Moon dances from Venus through the neighborhoods of Saturn and Mars. This is about your last chance to view Saturn as on Sunday evening at the end of nautical twilight it’s only 15° over the horizon as it plunges towards the Sun. At the same time, Venus is only 2° above the horizon, but it will keep climbing higher towards its greatest elongation from the Sun on Jan. 12th.


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

Andrew Planck

Author and Astronomer Andrew Planck shepherds you to the moon and its mysteries of intrigue and surprise. Learn about the moon’s most fascinating objects, understand how the moon was formed and the names of many of the craters … and why they honor individuals who have changed the course of history.
Andrew Planck

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Rupes Altai: Circular Mountain Range on Moon Whose Highest Peaks Rise to 13,000 Feet
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