Mons Gruithuisen is a lunar dome (a shield volcano) that can be observed on the western edge of Mare Imbrium. It lies north of the crater Gruithuisen and you can observe it with a 50mm refractor.
The Gruithuisen Domes
The ongoing fire-related regional haze and rainy weather has created very poor conditions for observing. However, I took advantage of a brief break in the clouds, during which I imaged the Gruithuisen area. The IAU selected the name Gruithuisen after the German astronomer Franz von Gruithuisen.
I did not plan to look at that area, but instead, stumbled upon it while slewing the scope searching for the crater Aristarchus. The rounded domes around Gruithuisen stood out so starkly at the boundary between the flat mare and the cratered highlands that I had to pause for a closer look.
Mons Gruithuisen Gamma and Delta have circular bases and are roughly the same size. And furthermore, you can also see a 900m wide caldera (volcanic crater) on Gamma – observable evidence of the Moon’s volcanic past!
Lunar Domes
Lunar domes are shield volcanoes, named for their resemblance to a warrior’s shield lying on the ground. On Earth, shield volcanoes are among the largest of volcanic features.
Low viscosity lava erupting from vents and cooling over a long period of time form wide, circular features. They are difficult to observe due to their subtle slopes. Hence, the best time to observe them are when shadows are long i.e. when they are near the terminator.