Janssen moon craterThe week of Jan. 7-13 takes us from Day 1 to Day 7. This week we will highlight the crater Janssen, viewable on Thursday evening.

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 [SE/N14] (which you may have observed last night and there still may be enough contrast to view it tonight). 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 crater 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 graben1 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!


Monday night marks the 409th anniversary of one of the greatest scientific discoveries every made: On Jan. 7th, 1610, Galileo trained his new telescope on Jupiter and set the world ablaze with his discovery that the planet had four moons, thus providing indisputable proof that the Earth was not the center of all movement. This got him into a great deal of difficulty with the authorities, but the world was forever changed at that moment.

1graben: [German for ditch] an elongated depression between two parallel fault lines. These lines occasionally pull apart with such force that the terrain between them drops.



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 www.skyandtelescope.com 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
409th Anniversary of One of the Greatest Scientific Discoveries Ever Made
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