The week of May 31 – June 6 takes us from Lunar Day 21 to Day 27. This week we will highlight the Ariadaeus and Hyginus rills.

Ariadaeus: [NE/J11; L=17°E] The Ariadaeus Rille (Rima Ariadaeus) is named after this 7-mi. crater located near its eastern end. Notice that the crater has a slightly smaller companion touching it on its northeast side. Their rims are gently pushing in on each other. Can you tell which one is older?

Ariadaeus and Hyginus rilles on the moonRima Hyginus: [NE/J10; L=8°E] At the west end of the Ariadaeus rille, there is a narrow diagonal shunt that connects Ariadaeus to Rima Hyginus. This new rille parallels Rima Ariadaeus for about 20 miles, then continues west until it encounters the small 6-mile crater Hyginus. At that precise point it changes direction and veers northward toward Mare Vaporum. The fact that Hyginus crater is located precisely at the pivot point is a curiosity. Can this just be coincidence?

Rima Hyginus is 2.5 miles wide and is easily seen in very small telescopes. It is really made up of a line of linked craters which are best seen just northwest of the crater Hyginus. With good optics and steady seeing you might be able to make some of these out even with a three-inch scope. Wood suggests that these are actually rimless collapse pits of internal origin and that the crater Hyginus (also rimless) might be one of their number. Can you see any of the individual craters, or does Rima Hyginus just look like a linear feature?


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
Ariadaeus and Rima Hyginus: Rilles on the Moon

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