Working On The Lunar Chain Gang


Grey clouds skewer the crescent moon around 8:30 Monday night. Photo: Bob King

I caught sight of the thick crescent moon last night just before a mass of grey cloud covered it for good. Our weather tonight looks much more promising for moonwatching although you’ll still need to get out before about 9 p.m. Much later than that and expect to stand on your tippytoes to see it above the treeline.

About two outstretched fists to the left of the moon, Scorpius’ brightest star Antares will become visible by mid-twilight. It’s nodding off in the west these nights as Sagittarius and Capricornus move in to take over the southern sky. Like kings, queens, presidents and prime ministers, every constellation’s dominance is only temporary. Eventually they all move west with the changing seasons and disappear from view.


The moon will be a shiny half-pie Tuesday night in the southwestern sky. You’ll find Antares about two fists to the east. Because the moon is traveling a low arc in the west this time of year, it’s best to observe it before the end of twilight. Created with Stellarium.

Tonight the moon’s terminator, that gently-curved arc that separates the bright, illuminated portion of the moon from the part still in darkness, slices straight across a spectacular trio of craters: Theophilus, Cyrillus and Catherina. The crater rims will glow brilliantly in the early morning sun with bowls still partly steeped in shadow. The terminator is the sunrise line on the waxing moon so anything near it will cast dramatic shadows just as trees and houses do at sunrise here on Earth. The contrast between light and shadow should be strong enough for you to see the crater Theophilus and perhaps the entire trio in a pair of 10 power binoculars. Give it a try and see for yourself. Through a small telescope the chain is one of my top ten coolest spots on the moon.


The striking crater chain of Theophilus, Cyrillus and Catharina will be near the lunar terminator tonight. If you’re clouded out, the next two nights should still provide a fine view. Photo: Bob King

Theophilus is named after an ancient Greek geographer and measures 68 miles across. It fairly fresh as lunar craters go, with a sharp-edged rim and a couple of distinctive central mountain peaks. Theophilus overlaps the much older Cyrillus, which is 61 miles across and named for a 4th century theologian. Scientists determine relative crater age by looking at which craters overlap others (the ones on top are younger) and noting how worn or broken their rims are. Cyrillus’s rim is worn down and much less crisp than Theophilus to the north. The final crater is the chain, Catherina, named for St. Catherine, a Greek theologian and philosopher, is 62 miles across and even more beaten down than Cyrillus.

So we have a neat sequence of craters tonight that show the march of time in the early history of the moon. If you have a telescope, explore the entire moon to see if you can find other craters which hint of their age by appearance and overlap. 


Theophilus up close and personal from the window of the Apollo 16 command module. Can you spot Cyrillus and Catharina above and to the right? Credit: NASA

2 Responses

  1. Rhino

    Observation Report from Dark Skies over the Superior National Forest in Northern Minnesota.

    We were out camping over the weekend and as fate had it we had clear dark skies and just a day or two past new moon.

    We could clearly make out the Andromeda Galaxy and the Double Cluster in Perseus with the naked eye. The Milky Way was high over head at dark. The band was clearly visible with a large gap in the southern sky. The scope brought out thousands maybe millions more of stars.

    I aimed the telescope at M31 Andromeda and I could clearly see it from end to end. Its large bright central core staring me in the face. It is so big though I had to move the scope 3 times to see the entire thing. This at 48x. Its huge… The two companion galaxies were visible as well.

    I took a stab for the first time at M51(Whirlpool Galaxy). I could see a hint of a spiral shape coming from the main section. Each part had a bright white core.

    We also spent some time looking at M27 Dumbbell Nebula and at M57 Nebula.

    Jupiter with the skies so dark was so intense I gave up looking at it. The glare was washing out any detail. My planetary filters arrived yesterday though. So maybe another look in the coming days.

    I struggled to locate M13 Hercules Cluster since its moved into the Western Sky. I need to check out Stellarium to track it down.

    Overall it was an enjoyable trip, it would have been great to have someone with that knew a few more objects. I was able to show some friends the joy of a dark sky. I’m still learning, but my list is growing.

  2. astrobob

    Great report, Rhino! I hope you’re keeping a set of observing notes. One day you’ll look back, read them and see how far you’ve come. Glad you found M13 — nice cluster, isn’t it? I look at it all the time throughout the spring and summer — winter mornings too!

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