The week of April 2-8 takes us from Day 17 through Day 22, the beginning of the last quarter. This week we will highlight the craters Endymion and Geminus, viewable around midnight on Monday.

Endymion and GeminusEndymion: [NE/D15] This is an older crater which somewhat resembles Plato [Day 8; NW/D9] in that it has a smooth, dark-chocolate floor and three-mile-high walls which cast lovely shadow spires on the flood plain below when the Sun is low. And there is an extra treat: 15 miles south-west of Endymion (about 13 arc-seconds) you might be able to spot a beautiful little concentric crater.1


Roughly 200 miles north of Mare Crisium you will find the moderately complex 55-mile crater Geminus.Geminus: [NE/F15] Roughly 200 miles north of Mare Crisium you will find the moderately complex 55-mile crater Geminus. It has terraced walls and small central peaks. If you are new to lunar observing, this will give you an indication of what to start looking for in the following days as craters become increasingly more complex.


Just before dawn on Monday morning, Mars and Saturn are a little over 1° apart in Sagittarius and will both fit in the field of view of a low power eyepiece.

1 Here are several craters on the Moon that have inner rings that curiously make them look a bit like donuts, they seem so perfectly cut out, and are called concentric craters. For a time it was thought that these were craters that had received dead-center impacts from smaller meteorites, but that theory has now been called into doubt. In 1978 Charles Wood catalogued 51 such craters and found two important common denominators: All had diameters of between 1.2 and 12 miles, and most of them occurred near the edges of lunar seas. “The inner donuts,” he concluded, “cannot be impacts that just happened to be centered on preexisting craters.” (Alas, another bubble burst!) Hesiodus A [SW/M8] and Marth [SW/N7] are the Moon’s best examples of concentric craters. (Is it just a coincidence they are both located on the southern shore of Mare Nubium?)



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
Concentric #MoonCraters and Endymion and Geminus
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