A good set of landmarks in the Mare Humorum are -- and an interesting
sight in themselve -- are the Castor and Pollux of the moon: Hansteen
and Billy. They are just to the north and west of the Mare Of Rilles,
and are almost exactly the same size, right next to each other. But
Billy is filled in flat with dark mare material, where Hansteen shows as
an old crater with a fractured floor and complex central peaks. It's
interesting to speculate how two such craters, quite probably of similar
age, should look so different while so proximal. Just a little bit of
distance across the great Oceanus Procellarum made all the difference in
their appearances today.
They are a very good jumping off place for spotting the end of
Rima Sirsalis,
as well as a pair of pointers toward the rich Rimae Grimaldi
complex. Billy's dark floor makes it obvious even when the sun angle is
high, and nearby Hansteen makes confirmation of location and direction
very easy.