The week of August 21-27 takes us from Day 5 to Day 11. This week we will highlight Rima Ariadaeus on the moon, viewable Tuesday night.

Rima Ariadaeus - Rille on the Moon that is a GrabenRima Ariadaeus: NE/J11; L=14°E] The best place to see rilles on the Moon is in the area just west of Tranquillity. Here you will find a remarkably varied collection–to wit, Ariadaeus, Hyginus (two of the best-known rilles on the Moon) and Triesnecker. We’ll start with Rima Ariadaeus tonight. The other two may still be hidden behind the terminator.

Mare Tranquillitatis on the moonWhen the terminator is close, Rima Ariadaeus is an enjoyable target even for small telescopes. It is a classic example of a graben, an elongated depression between two parallel fault lines where the ground in between has fallen away. If you have at least a 4″ telescope with good optics and steady seeing you will just be able to make out that the fault lines have pulled apart and the ground in between has sunk. (This is a challenge; Ariadaeus is a shade over two arc-seconds wide.) The rille appears to be broken in a few places, indicating that the lunar terrain has shifted since it was created, and there is a shunt on its western end connecting it to Rima Hyginus.


On Thursday, Antares is 1.1° 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
Rima Ariadaeus – Rille on the Moon that is a Graben
Tagged on:                         

Leave a Reply

Your email address will not be published. Required fields are marked *