The week of March 11-17 takes us from Day 2 to Day 8. This week we will highlight moon craters Theophilus, Cyrillus, and Catharina viewable on Saturday night.

Theophilus, Cyrillus, Catharina: [Repeated from Day 5–SE/L12; L=24°E] This is the most imposing trio of moon craters on the Moon. They are located just west of Mare Nectaris and have their own distinct personalities.

Theophilus is a spectacular formation with all of the attendant complexities of a Tycho-class crater: terraced walls, a flat floor, and magnificent central mountain peaks. It is 60 miles in diameter, and the drop from the highest mountains on the rim to the floor below is a breathtaking 2.7 miles! Observers have reported that the shape of the central mountain seems to change as the lunation progresses. If you observed this crater last night, see if you can notice any differences in the shape of the central mountain.

Notice how the floor of Theophilus is much smoother than the floors of Cyrillus and Catharina. When the impact occurred that produced Theophilus, much of the material that was excavated shot straight up. When it returned (in the form of molten rocks and mountain-sized boulders) it splashed, spread out, and then resurfaced the floor with a smooth veneer.

There is also impact melt around the crater exterior that can be easily seen with backyard telescopes. Take advantage of this, as there are not many places on the Moon where you can see such a thing. Most of this exterior impact melt occurs to the north of the crater and flows into Sinus Asperitatis. Charles Wood points out that this is because the south rim is higher. Shortly after impact, the terraces collapsed into the lake of molten rock below. This sent tidal waves of hot lava racing toward the opposite side. Because the north rim is lower, the material slammed into the wall, “sloshed” over the rim and pooled on the north side as impact melt.

Can you figure out the comparative ages of the three craters?1


On Wednesday, Jupiter is 3.6° south of the Moon.
On Thursday, the Pleiades are 2° ENE of the Moon.

1 Theophilus is younger than Cyrillus because it intrudes upon the latter’s rim. Catharina seems to be the oldest because there are five craters superimposed on it, and two elongated craters on its northeast rim point back toward Mare Imbrium, suggesting that Catharina is even older than Imbrium!

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
Trio of Moon Craters: Theophilus, Cyrillus, Catharina

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