mare humorum

This dark "sea" is diagonally opposite the Mare Crisium on the lunar disc, near the south western limb.

Unlike Crisium, it is not discrete from the other Maria, and is a little smaller at around 250 miles north-south and 275 miles east-west, correcting for foreshortening. However, at times when Crisium is not visible it makes a useful ready reckoner for judging the distances between lunar features.

Inconstant Moon