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Cosmic Challenge: Plato's craterlets

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Cosmic Challenge:
Plato's craterlets



June 2023  


Phil Harrington








This month's suggested aperture range:

6 to 9.25 inch
(15-24cm) telescopes








Best lunar phases

(days after New Moon)

Moon: Plato's craterlets

Walled plain and interior craterlets

Day 8.5 (June 27-28, July 26-27, August 25-26, 2023)

Day 21 (June 10, July 10, August 8, 2023)



One of the great challenges facing even the most devoted lunar observers is trying to see the many small craters that dot the lava-covered floor of the walled plain Plato. Plato itself is a prominent impact scar measuring 62 miles across. It takes no more than 10x binoculars to see Plato once the Sun is up in its sky. In general, the best times to view Plato and its environs are about 1.5 days past First Quarter and again near Last Quarter.


Above: Plato and some of its craterlets. Credit: CN member clavutich.


Unlike typical impact craters, Plato shows no sign of a central peak or a chaotic floor.  Instead, it appears perfectly smooth and, at first blush, featureless.  That's because shortly after the blast had excavated the bowl of Plato, lava welled up to flood its interior, hiding any central remnants of the original impact.  As a result, we see an unusually dark floor that stands in stark contrast against the more lightly shaded Mare Imbrium to its south and Mare Frigorus to its north.


While viewing Plato through his telescope in 1824, German astronomer Franz von Paula Gruithuisen noticed something peculiar.  He spotted a tiny speck on the floor of Plato, the first observation of a Plato craterlet.  While Gruithuisen's place in history was tarnished by a second discovery he made, that of an expansive "lunar city" called Wallwerk north of the crater Schröter, his sighting of the first Plato craterlet prompted others to take a closer look.


Over the next several decades, interest in Plato increased dramatically, as did the number of craterlets.  In a report published in the March 1883 issue of The Observatory (Vol. 6, p. 85-91), British observer A. Stanley Williams described how, over the years 1879 to 1883, he and several other observers had observed and accounted for more than 40 "spots" on Plato's floor, along with several crisscrossing "streaks."


Above: Plato, as sketched by British observer A. Stanley Williams


In a sense, Williams was to Plato what Percival Lowell was to Mars.  Lowell believed that Mars was inhabited by intelligent beings who constructed an elaborate array of crisscrossing canals that spanned the globe of the Red Planet.  Although Williams didn't see canals within Plato, he did believe that the surface of that distant crater was home to a variety of so-called transient lunar phenomena.  He would periodically see streaks come and go across the floor, or perhaps a new crater suddenly pops forth where none had existed before.  While today we understand that what he perceived as changes across the floor of Plato was most likely caused by changes in our own turbulent atmosphere, alternately masking and revealing fine detail just at the edge of visibility, Williams was steadfast in his belief that these phenomena were related to lunar volcanic activity.


Although we shouldn't expect any stray volcanic emissions spewing across Plato, spotting the tenuous detail that so intrigued Williams is a challenge that many enjoy to this day.  Several small craterlets are indeed strewn across Plato's otherwise smooth floor.


First, we need to take care of some bookkeeping.  Since none of the Plato craterlets have official IAU designations, we are left to our own devices to create some.  Although references sort them by letter, that system may be easily confused with the IAU's convention of naming so-called satellite features after a major landmark.  Plato has more than a dozen satellite features associated with it, labeled Plato A, Plato B, and so on.


Above: Finder chart for this month's Cosmic Challenge

Credit: Chart adapted from Cosmic Challenge: The Ultimate Observing List for Amateurs by Phil Harrington.
Click on the chart to open a printable PDF version.



Below: Plato's 16 most prominent craterlets

Craterlet designation

Diameter (km)

Diameter (miles)



















































Instead, let's go with numbers, as Williams and other classic lunar observers did more than a century ago.  Using this system, the most obvious craterlet is denoted on the chart and table above as "Craterlet 1."  When sunlight strikes it just so, its steep walls appear brightly lit in contrast to the dark surrounding floor of Plato.  Telescopes as small as 4 inchers can resolve it as a circular pit.


Four other craterlets, plotted here as 2, 3, and 4, are also visible with difficulty through a 6-inch telescope.  Craterlets 3 and 4 are especially close-set; can you resolve them both?  The best chances to see all four as true depressions occur when the Sun is either rising or setting in their sky, on the nights immediately after the quarter phases.  Closer to Full Moon, the stark lighting turns them into bright, almost starlike points against Plato's floor.


Craterlet 5 is from removed from the rest, embedded in Plato's eastern wall.  Although it's the largest craterlet, the location against Plato's wall tends to mask it more than those centrally located on the floor.  The best chance for spotting it comes during the waxing phases, since it remains hidden by the wall's shadow during the waning phases.


If you have success with the Plato quintet, try your luck with five smaller understudies.  Craterlets 6, 7, 8, 9, and 10 will likely need at least an 8-inch aperture and 200x to be seen, and even then, only with great difficulty.  Yearning for even more?  Try your luck with all 16 listed in the table. How many can you find?


And if all this is not enough, heres an extra challenge. The rim surrounding the Greater Black Lake, as Hevelius called it, is very rugged, which creates some fascinating shadow effects as the Sun rises and sets in its sky every month. One such effect has captured the imagination of observers since it was reported 71 years ago. Its called Plato's Hook.


On April 3, 1952, when the Moons phase was 8 days past New, renowned lunar observers, Patrick Moore and Percy Wilkins, spotted and sketched an unusual hook-shaped shadow on Platos floor. Since then, others have tried to replicate their observations to little or no avail. Where Moore and Wilkins saw a shadow shaped like the dorsal fin of a shark, others record a conical shadow with no evidence of any curvature.


Above: (Left) Lunar Reconnaissance Orbiter image of Plato showing may craters as well as the complex shadow caused by its craggy rim. (Right) Wilkins's rendering of "the hook." Montage posted by John Moore.



CNer John_Moore started a fascinating thread in the Lunar Observing and Imaging three years ago entitled Hooked on Plato - an animated view that offers some great perspective of this elusive shadow feature. If youre interested in pursuing a fun mystery, be sure to visit that thread and watch Johns linked YouTube video on the Hook.


Good luck with this month's challenge! Be sure to post your results in this column's discussion forum. And 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, including Cosmic Challenge: The Ultimate Observing List for Amateurs.  Visit www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2023 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.


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