The week of October 9-15 takes us from Day 25 to Day 1. This week we will highlight a series of rilles called Rimae Sirsalis, located in the SW quadrant of the Field Map and viewable on Tuesday morning before sunrise.

Rimae Sirsalis on the moonRimae Sirsalis: [SW/L3-4; L=62°W] Just in from the southwest shore of Oceanus Procellarum, you will see two small overlapping moon craters, Sirsalis and Sirsalis A (by now you should be able to tell immediately which crater is older). Neighboring these two moon craters to the east and south is a complex of rilles. Rima Sirsalis is the most conspicuous, but it has lots of neighboring offshoots. How many can you see? With your telescope at its highest usable power, trace Rima Sirsalis through its full length. Along the way, try to discern the many smaller rilles that branch off. These require larger apertures and good seeing, but it’s worth the effort.

Rima Sirsalis is a gift to amateur astronomers because it can be easily seen in the smallest of telescopes. At 240 miles long and two miles wide, it is one of the longest rilles on the Moon, and it reveals itself just before full Moon–a time when most astronomers are putting their telescopes away! Rima Sirsalis is unusual not only because of its great length, but because it cuts straight through a highland region and is rarely persuaded to deviate from its course. On its southern end, the rille begins just east of the crater Darwin, then proceeds north where it “empties” into Oceanus Procellarum. Spend a lot of time here. Draw the network of rilles and keep coming back to improve on what you saw.

The experts haven’t quite decided what kind of rille Sirsalis is. Some say it is a collapsed lava tube (but why would a lava tube, which forms around meandering lava flows, be so straight?) Others say it is a graben, terrain that has fallen between two parallel fault lines that have pulled apart. What do you think?


On Tuesday, Venus is 6° south of the Moon.


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
Rimae Sirsalis – Series of Rilles on the Moon
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