The week of September 6-12 takes us from New Moon to Day 7. This week we will highlight the crater Janssen, viewable on Friday and Saturday evenings.

Janssen is a textbook example of how new craters are superimposed on top of older cratersJanssen: [SE/P13; L=40°E] Janssen is located 85 miles southwest of the Rheita Valley (which you may have observed last night). Janssen’s diameter is a robust 120 miles (Copernicus, the “Queen” of all craters, is a “mere” 58 miles in diameter!). Janssen is a textbook example of how new craters are superimposed on top of older craters and, at the same time, are smaller than the older craters that underlie them. Although Janssen is very old and its walls are in ruins, you should pay it a visit. When the Sun is low, it’s rather like visiting the ruins of an old castle; it’s still full of artifacts and reminders of a bygone age. The remains of the walls are still visible and there are mountains, rilles, and well-defined craters on its interior. The largest intruder is the moon crater Fabriciuscrater Fabricius (48 mi.) that occupies the northern portion of the floor. The most impressive rille, Rima Janssen, is visible through very small telescopes. It appears to be a graben that curves conspicuously from the southwest wall of Janssen and attaches to the south wall of Fabricius. It is unusual in that it is a highland rille. Janssen is a rewarding field to play in. Revisit it often!


It is highly recommended that you get a copy of Sky and Telescope’s Field Map of the Moon, the very finest Moon map available for use at the telescope. It is available for $10.95 at and on Amazon. All features mentioned in this blog will be keyed to the grid on the Field Map and will look like this: Plato: [NW/D9]

Courtesy of Gray Photography of Corpus Christi, Texas
Lunar photos: NASA / USGS / BMDO / LROC / ASU / DLR / LOLA / Moon Globe. Used by permission

Andrew Planck
Moon Crater Janssen: New Moon Craters Superimposed on top of Older Moon Craters

Leave a Reply

Your email address will not be published. Required fields are marked *