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Cosmic Challenge: Lunar Craters Messier and Messier A

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Phil Harrington's Cosmic Challenge, June 2017

Cosmic Challenge:
Lunar Craters Messier and Messier A


June 2017


Phil Harrington




Best lunar phases (days after New Moon)

Moon: Messier/Messier A

Days 4-8 and 15-16


This month's suggested aperture range:

Giant binoculars and small scopes


3- to 5-inch (75-127mm) telescopes

Summer is in the offing here in the Northern Hemisphere and the longest day of the year is at hand. While many of us enjoy the warmer weather, the dark of night comes late in the evening and leaves all too early the following morning. So while deep-sky observing is limited, we can still enjoy viewing our Moon even if the sky is still bright.  Yes, observing challenges await us on our nearest neighbor in space.

This month, I thought I'd highlight one of my favorite lunar ports-of-call, the unusual pair of craters, Messier and Messier A. The best viewing nights this month are June 10 to 12, and again from June 27 to July 1.

The double craters Messier and Messier A have a story to tell. Named for the famed French comet hunter, these craters are separated from one another by less than 4 miles (6 km). Both are nearly the same diameter and noticeably oval. Messier spans 5 miles by 7 miles (8 km by 11 km), while Messier A is a little larger, 7 miles by 8 miles (11 km by 13 km) across.

Above: Finder chart for this month's Cosmic Challenge.  

Credit: Chart adapted from Cosmic Challenge by Phil Harrington
Click on the chart to open a printable PDF version in a new window

In addition to their similar appearance, both craters also share a common origin. A billion or so years ago, a meteoroid slammed into Mare Fecunditatis (the Sea of Fertility). The meteoroid itself was not necessarily special. What was unique, however, was the narrow angle in which it struck the surface. Tests conducted by Don Gault of NASA's Ames Research Center and John Wedekind of Caltech in 1978 suggest that the impact angle was probably less than 5°. The initial impact created the crater Messier, the western member of the duet. But then, just as a flat stone skips when it strikes a pond at a narrow angle, the meteoroid fractured. A large fragment skipped and was propelled another 4 miles (6 km) downrange, until it struck the surface to create Messier A. An ejecta field from that second impact threw twin rays of bright material further downrange. This unique pair of rays, looking almost like twin comet tails, immediately calls attention to the two craters. The twin parallel rays extend westward some 90 miles (145 km), nearly the full breadth of the mare.

Isn't it wonderfully ironic that a crater with Messier's name attached looks like a comet!

Take a close look at Messier A. Notice how its western rim appears distended and partially upraised, almost as if there is a second rim? Some believe this may be the result of the meteoroid creating Messier A fracturing and ricocheting again either before or upon impact. Nowadays, however, most prefer the theory that a newer impact occurred almost directly on top of an earlier impact, resulting a newer crater almost erasing an older crater.

Although not nearly as obvious as those sweeping westward from Messier A, crater Messier also has a system of ejecta rays. Curiously, the ejecta blanket surrounding Messier fans out to the north and south of the crater itself, perpendicular to the impact. Those same tests conducted in 1978 show that not only is this possible, but is actually very likely during a compound impact. If the lighting is right, the overall appearance of Messier, Messier A, and the craters' respective ejecta fields looks like a parrot in flight, with the fainter rays from Messier posing as the bird's outstretched wings and the twin searchlight-like beacons from Messier A its long tail feathers.

Here are a pair of photos of the pair of craters taken by Paolo Lazzarotti, an exceptional photographer from Italy. His name is likely familiar to those who frequent the Lunar Picture of the Day (LPOD) web site.  And if you enjoy these two perspectives of Messier and Messier A, be sure to visit LPOD’s archives of Paolo’s contributions.

Above: Two perspectives of the unusual impact craters Messier (right) and Messier A (left). The top image was taken on 27 February 2005, when the Moon was 19 days past New.  Notice how the low angle of the setting accentuates the craters' rims, but extinguishes the unique ejecta fields.  The bottom image was taken on 23 April 2007, 6 days after New. With the Sun higher in the craters' sky, the perpendicular ejecta fields become much more evident.

Credit:  Paolo Lazzarotti


The image below was taken from a much closer vantage point, in lunar orbit aboard Apollo 15 in 1971. The steep angle of sunlight at the time the photo was taken offers no shadow relief, but highlights the crater's bright floors, including Messier's strange "coffee bean" appearance.

Above: Close-up image of the unusual impact craters Messier (right) and Messier A (left) taken by Apollo 15 in 1971.

Credit:  NASA/JSC/Arizona State University

Above: As mentioned earlier, Messier A appears to be a double crater itself. This image, taken by the Lunar Reconnaissance Orbiter, shows that effect beautifully. The smaller, older crater appears partial exposed to the lower left of the newer, superimposed crater.

Credit:  NASA/JSC/Arizona State University


Finally, on an historical footnote to you old timers, some of you may recall that Messier A was originally named Pickering, after the Harvard astronomer William H. Pickering. The name was changed to Messier A in 1964 after the International Astronomical Union ruled at the time that Pickering's bizarre ideas about plant life on the Moon, notably in the crater Eratosthenes, made him an inappropriate choice for a crater named in his honor. You will still find a crater named Pickering near the crater Hipparchus, but that is technically for Edward Pickering, William's older brother.  You can read the first of his six-part thesis in this Google Books compilation of Popular Astronomy magazine from 1919.  Three years later, Elizabeth Smith wrote more about the subject in an article entitled Is there life on the moon? (Publication of the Pomona College Astronomical Society, vol. 7, pp.49-51).  

Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's test. Contact me through my web site or post to this month's discussion forum.

Until next month, remember that half of the fun is the thrill of the chase. Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy. Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2017 by Philip S. Harrington. All rights reserved. No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


  • okiestarman56 and The_8_Bit_Zombie like this


Thanx, Phil. Motivated me to haul the APM 100/90 APO Binos out onto the deck, May 31st just after sunset, 1st quarter. I've admired this pair of craters before, but never had researched it. Indeed, most notable recollection is the distinctive, twin, tail-feather ejecta plumes. I love your attached images, the Recon Orbiter showing the interesting flat-looking floor to A and magnificent fine structure. Also jump-started me outa DS chauvinistic mode. Imagine if we Didn't have a moon to admire from so close... and set foot on! Then I swung the binos over to Jupiter + 4 moons, and showed to my sister... as if I owned it !  Tom

    • PhilH likes this
May 31 2017 10:45 PM



I got into lunar observing lately (before that, the moon was an annoyance to me as it meant less DSO observing) and learning to enjoy it in the process.


I plan to inspect this lovely duo and any observation will be enhanced with the new  knowledge I got your your article. Having the history / background of any object makes it more personal!

    • PhilH likes this

A little direction always makes Lunar observing more interesting. This month's Cosmic Challenge is a welcome change from the usual run of DSOs that are out of reach in my not so dark skies.

    • paul hart and PhilH like this

Excellent article!

I like very much to observe it at low power, to give it a more "comet-like" appearance; think always that Charles M. would have liked such dedication

    • PhilH likes this

This is very good timing. I am doing the AL Lunar Program. Your article grants me a new perspective of the lunar craters. Cheers!

    • PhilH likes this

I viewed this tonight in my Celestron 102AZ with a Meade 8.8 mm eyepiece without/with Barlow (75x/150x) with a 13% transmission moon filter. Looks very nice and exactly as Phil describes, although smaller than I expected, especially at 75x. I would expect it to be very difficult to pick up with binoculars in the 10x-20x range. 


Phil, thanks for taking the time to write this up, very interesting crater combination especially with the background information provided in your article. I'm not big on lunar viewing, but it is much more enjoyable and worthwhile when you know what you are looking at.

    • PhilH likes this

Thank you Phil.  Your essay about my favorite lunar feature was refreshing and the formation information is fascinating.  I recall as a young teenager trying to find Messier with a 30mm scope without luck, then when I received a 76mm reflector for Christmas, success!

    • PhilH likes this

Thanks everyone for your encouragement.  Look for more lunar challenges in monthly features to come.

    • nicknacknock, TOMDEY, williew and 1 other like this

A very interesting read, Phil! I managed to image it recently as part of a mosaic.




    • PhilH likes this
Jun 06 2017 04:35 AM
Hi Phil,
On Sunday I took my FS128 and FC76 to a friend's house and spent a goodish 4 hours observing the Moon, Jupiter and Saturn with everything but the kitchen sink. Binoviewers were involved, magnifications close to 300x, smartphone photography through the eyepiece and also some stray beers with snacks.
I spent a good 15 minutes on Messier and Messier A - I found it much more pleasurable to observe with the solid information you provided. Context in observing is like adding salt to food - changes a great dish to an awesome dish grin.gif
Here's a quick one through the FS128 and eyepiece - craters are marked with an arrow.
Messier A


Keep up the great work!

    • PhilH, TOMDEY and ericthemantis like this

Great article and much appreciated.


Odd, almost silly, that Pickering's name would be removed because he thought there was plant/insect life on the Moon. Others with craters named after them thought they saw cities (Gruithuisan) or active volcanos (Herschel) on the Moon or, Schiaparelli  who first drew canals on Mars. Oh well, I regret the pulling of the crater after Patrick Moore as well.  At least both Pickering and Moore studied the Moon!


I hope you will continue your Lunar series. Reiner Gamma would be my choice for one example, and, of course, Lunar Domes!


Thanks again!!

    • PhilH likes this
Jerry Hubbell
Jun 22 2017 09:58 PM

Hi Phil,  It's interesting that we seem to have converged on the same lunar feature. The next issue of the ALPO Lunar Section TLO - Focus On article will be on Messier and Messier A. As the Assistant Coordinator, Lunar Topographical Studies, I am tasked with writing the Focus On articles that are published every other month. Wayne Baily, the Coordinator Lunar Topographical Studies, and I decided on this as a topic a few months ago. Nice coincidence! 


If anyone is interested in submitting images and observations to the ALPO you are welcome to do so. You can find The Lunar Observer at http://moon.scopesan...pes.com/tlo.pdf

    • PhilH likes this
Howard Fink
Jun 27 2017 03:09 PM

Here's a snapshot of the two craters modeled in 3D.


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