mare crisium

This distinctive dark "sea", close to the north eastern limb of the Moon, is highly unusual in being separate from the other seas. It appears as a neat oval measuring about 270 miles north-south and 180 miles east-west (actually 350 miles correcting for foreshortening) ie roughly the size of Ireland.

It is ideal for use as a ready reckoner for judging the distances between lunar features.

Inconstant Moon