Again we are chasing craters few people pay attention to. In this view we
are looking beyond the western shore to Oceanus Procellarum near the
western limb of the Moon. The two large craters partially filled with dark
basalt are Grimaldi (right) and Riccioli (left). Located at 68 and 74
degrees west longitude, these two craters are foreshortened to our earthly
view and do not look as impressive as they really are.
Both Grimaldi and Riccioli were named after the mid 17th century Italian monks who defined the face of the Moon that we see today. In fact, they are self-named as Riccioli named about 280 of the major features on the Moon we still recognize today. For his lunar work, Riccioli used lunar maps drawn by Grimaldi, who in turn borrowed heavily from the works of Hevelius and van Langren (who were also rewarded with craters named after them).
Grimaldi, at 134 miles wide, is easily seen through a small telescope, but can be mistaken as an extension of nearby Oceanus Procellarum. While Grimaldi's basalt fill shares the same subsurface magma source as Procellarum, it is an independent feature. The crater is only partially filled with basalt and the Grimaldi's actual rim lies some distance from the edge of the lava fill. Riccioli, a floor-fractured crater, is smaller at 88 miles wide and is also partially lava flooded. The rim of the Moon holds delights for those who take the time to search for them.