The week of May 29 – June 4 takes us from Day 10 to Day 16. This week we will highlight the Hippalus rilles, viewable on Tuesday evening.

Hippalus RillesRimae Hippalus: [SW/M6; L=29°W] On the east shore of Mare Humorum you will find the remnant of the moon crater Hippalus whose southwest rim, because of subsidence, has disappeared into the mare. The area around Hippalus contains the finest examples of arcuate rilles to be found on the Moon. Each one is about 2 miles wide. (Catch these rilles when the terminator is around 40° west which will be the case on Tues. evening.) Notice how some of the rilles plough through mountain ridges and craters, and some are interrupted completely by small craters but continue on the other side. One prominent rille cuts straight through the middle of Hippalus. This gives you a clear indication of the sequence of activity. What features were in place before Humorum filled with lava and subsided? What features appeared after subsidence took place?


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
Moon Crater Hippalus: Finest Examples of Arcuate Rilles

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