The week of September 18-24 takes us from Day 27 to Day 10. This week we will highlight the Triesnecker Rilles, an enigma located on the moon in the NE quadrant of the Field Map at J10 and viewable on Thursday and Friday evening.

Triesnecker Rilles on the moonTriesnecker rilles: [NE/J10; L=5°E] What a great place to poke around with your telescope! This is such a complex system of rilles that it looks like a railway switchyard! Before looking at the footnote, which do you think came first, the crater or the rilles1 ?

viewing rilles on the moonThe width of the rilles measures between one-half mile and one mile. The largest rilles can be seen in a three-inch refractor, but the whole system requires larger apertures and good seeing. There are at least nine rilles crisscrossing each other. Make a sketch of how many you can see, then come back later and try to improve on it.

Scientists are not sure how the Triesnecker rilles developed. Their origin still remains a mystery, but the consensus is they are not grabens (as the above Ariadaeus rille is). Enjoy them as one of the Moon’s many enigmas.


On Tuesday, Venus is at its greatest brilliancy, but doesn’t rise until 3:35 a.m.
Friday is the first quarter Moon; Mercury is at its greatest western elongation and sets at 6:20 p.m.
The Fall Equinox is on Saturday.

1 There is a rille approaching Triesnecker’s northeast rim that looks like it has been interrupted by the crater, suggesting that the rilles were there first.


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
Enigma of the Moon – Triesnecker Rilles
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