Crater Petavius

Jay Albert

The large crater Petavius is one of the most impressive and interesting craters on our Moon. Its appearance changes as the Sun rises and sets during the course of each lunation (orbit around the Earth). Petavius was named after Denis Petau, a French historian and theologian who worked in the first half of the 17th century. The crater is 110 miles in diameter and about 2.1 miles deep. The central peaks rising from the convex floor (pushed up by sub-surface magma) are over one mile high. The walls of Petavius are unusually wide relative to the crater's diameter with a double rim on the east and west sides. The crater appears oval rather than round as seen from Earth because it is foreshortened due to its closeness to the Moon's east (sunrise) limb. The east wall rises about 11,000 feet. The precise age of the Petavius impact is uncertain but the crater is considered to have formed during the Moon's Imbrium geologic age, or around 3.2 to 3.8 billion years ago. One of the most prominent features on Petavius is the approximately 50 mile-long, straight fracture running from the central peaks to the crater's southwest wall.

I've tried to image Petavius a few times over the years. The first was this sketch from 14 years ago.

This drawing was made with a #2 pencil as the Sun's angle in the lunar sky was decreasing relative to the crater. The lunar terminator can be seen to the east of the crater while the late afternoon sunlight from the west illuminated the outer west (right) wall and the central peak, casting a long shadow intruding on the inner east wall. The shadow from the west wall fills much of the crater's interior and covers most of the rilles on the floor.
The next view below was made February 13, 2013 at a time when it was morning at Petavius and the Sun was still rising there. Note that the outer east (left) wall was sunlit along with the east face of the central peak. The central peak cast a shadow to its west and the long Petavius rille was filled with black shadow. Petavius' west wall shadow filled the adjoining 35 mile diameter crater Wrottesley. I used my Celestron NexStar 6" Schmidt-Cassegrain telescope ("SCT") and Celestron Neximage 5 Solar System Camera to capture this image. There were 499 frames in the video file, but I did not note the number of best frames I used to stack and process this image. The sky was mostly clear that night with 4th magnitude transparency. Unfortunately, it was very windy with the seeing (atmospheric stability) a miserable 3 out of 10, making focusing extremely tough. The Moon's altitude was still low in the east.  In addition, this image was made before Sam Storch corrected my scope's collimation…all these factors contributing to a rather fuzzy result.

The image below was taken at 1:05am, August 12, 2014 with my Celestron 11" SCT and the same camera as the above image. It is a stack of 10 out of 383 frames. Both images were processed in Registax 6 and Photoshop Elements 9. Conditions for the image below were somewhat better than for the February 2013 image. The sky was hazy with transparency only 2nd magnitude, but the Moon was 15⁰ higher in the sky, there was no wind and seeing was a so-so 5 out of 10. This time, it was afternoon at Petavius and the Sun was still rather high. The long Petavius rille was fully sunlit, as were the interior east wall and west face of the central peaks. Smaller rilles can be seen east and north of the central peaks. Crater Wrottesley was almost fully sunlit with only a little shadow at the base of its west wall.

Jay Albert, 8/16/2014