The week of May 23-29 takes us from Day 24 to Day 29. This week we will highlight the moon crater Longomontanus, viewable on Tuesday morning just before sunrise.

moon crater Longomontanus[SW/P7; L=22°W] Take a look at Longomontanus and before reading further, decide if something seems slightly “wrong” about the crater …

Longomontanus is an exception to the rule that moon crater age can be determined by their relative sizes. Larger moon craters are almost always older, and when there’s an overlap, smaller moon craters will intrude upon the rims and floors of their larger and older cousins. The east rim of Longo has superimposed itself on top of what is obviously an older but smaller moon crater.


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 Longomontanus: Exception to the Rule About Moon Crater Age

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