Gruithuisen Gamma on the moon Andrew PlanckThe week of September 23-29 takes us from Day 23 to Day 1. This week we will highlight the lunar dome Mons Gruithuisen Gamma, viewable early Tuesday morning.

Gruithuisen Gamma second largest dome on the moon Andrew PlanckGruithuisen Gamma1: [NW/F5; L=41°W] There is a peninsula-like piece of highland terrain that extends southwest of Sinus Iridum. At the end of this peninsula you will find Mons Gruithuisen Gamma. Next to the so-called Valentine Dome, this is the Moon’s second largest dome. If you can manage to spot the summit crater (3,000 feet in diameter, about one-half an arc-second at the Moon’s mean distance) you will have accomplished a major feat! (This would require at least a 10” telescope and a night of exceptionally steady viewing.)


The autumnal equinox this year occurs on Monday, September 23. And is the moment when the Sun is directly over the Earth’s equator. As a result, daytime and nighttime are of approximately equal duration.

1Pronounced /GROOT.hoo.zen/


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
The Moon’s Second Largest Dome: Gruithuisen Gamma

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