The week of October 10-16 takes us from Lunar Day 16 to Day 22. This week we will highlight Mare Crisium, viewable Tuesday evening.

Mare Crisium on the moon resulted from the impact of a large meteor 3.9 billion years agoMare Crisium: [NE/H15] On Day 17 you should be able to observe Crisium’s complexities more successfully than you did on Day 3, particularly the high eastern mountain border. We tend to think of craters as comparatively small objects and lose sight of the fact that large lunar “seas” are the result of projectile impacts also. Basically, if it’s round, it’s a crater. Mare Crisium resulted from the impact of a large meteor 3.9 billion years ago. The event was energetic enough to leave a multi-ring imprint, parts of which we can clearly see on Monday night. Notice the ring features bending around Crisium on its north side. Particularly notice how Mare Anguis on the NE side of Crisium [G16] extends itself toward the west, passes below Cleomedes, and then dissipates at Tisserand. The Crisium basin appears elliptical, with its long axis running from north to south. This is an illusion due to the effects of foreshortening. Crisium actually is elliptical, but, contrary to what your eyes tell you, its long axis runs east to west1.

Once you have become familiar with the details on the surface of Mare Crisium, particularly around Promontorium Agarum [H16], be on the lookout for LTP’s. These have been reported from time to time and manifest themselves as mists that mysteriously appear and obscure some of the surrounding details.


On Wed., the Pleiades are 3° north of the Moon, and on Friday Mars is 4° south of the Moon.

1 If you have the Moon Globe HD app, you can see this for yourself. Do a search for Mare Crisium as it normally appears near the east rim of the Moon. Notice how it is elongated north to south. Now with one finger, rotate Crisium to the center of the Moon and it will develop its true east-west elongation.


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
Mare Crisium Result from Impact of Large Meteor 3.9 Billion Years Ago

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