Theophilus, Cyrillus, Catharina moon cratersThe week of March 11-17 takes us from Day 5 to Day 11. This week we will highlight the trio of craters Theophilus, Cyrillus, and Catharina, viewable right next to the terminator on Monday evening, and on Tuesday evening.

Theophilus, Cyrillus, Catharina: [SE/L12] This is the most imposing trio of craters on the Moon. They are located just west of Mare Nectaris and have their own distinct personalities.

Theophilus moon craterTheophilus 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. Observe it over the next few nights and 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.

"Sloshed" is such a gentle term. It is frequently used to describe the Theophilus event but reminds us only of the last time we carelessly handled a cup of coffee. Now imagine yourself as an astronaut standing on the rim of Theophilus whose entire floor, an incredible three miles below, is bubbling with hot lava. Suddenly huge sections of the far wall, 60 miles away, detach themselves and slam into the lake, raising a tidal wave of lava so high that it will crash over the cliff you are standing on and flood the plains to the north for another 60 miles. "Slosh" is probably not the first word that would come to mind.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 March 13, 1781, Sir William Herschel discovered Uranus. He also discovered planetary nebulas, so named by Herschel because they resembled the planet Uranus. (See comparative images below)

The Eskimo Nebula, a planetary neb- ula discovered by Herschel in 1789
The Eskimo Nebula, a planetary neb- ula discovered by Herschel in 1789











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
The Most Imposing Trio of Craters on the Moon

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