Lunar Day 26The week of May 22-28 takes us from lunar Day 26 through Day 3. This week we will highlight Mons Rümker, visible just before dawn on Monday.

Mons Rümker: [NW/E4] Normally this feature is too close to the Moon’s limb to see it very clearly, but the libration on Monday morning is favorable, so take advantage of it. In spite of its name, Mons Rümker is not really a mountain, it’s an extensive complex of domes. The diameter of the formation is over 40 miles.

Mons RümkerAlthough there are about a dozen domes here1, through a telescope it looks like a mound with a bumpy surface. Several of the domes have summit pits. Can you make any of them out?

1 Some sources say 30; how many can you see?


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
Mons Rümker: Extensive Complex of Domes on the #Moon

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