PETAVIUS AND THE SOUTHEAST LIMB – JAY ALBERT
Thursday evening, February 11, 2016 was rare. It was clear, haze-free and not windy. The Moon was a thin, waxing crescent, only about 15% lit and still high enough in the western sky for decent viewing. I set up my Celestron NexStar 6" SCT and had a good look at lots of faintly glowing detail in the earthlit part of the Moon. Features like a softly glowing, neon blue Aristarchus crater, Copernicus, Kepler, the Moon's western limb with the dark crater Grimaldi in the middle and the crater Tycho with its ejecta rays extending over a thousand miles across the earthlit moonscape. And lots more. It was magnificent! I had to get a picture of it. I grabbed my Celestron Neximage solar system camera and my wife's laptop and…nothing. Evidently the earthlight was not bright enough to be picked up by the camera, not even after I increased the gain to maximum and lengthened the exposure. Bummer!!
Moving on, I shifted the telescope to the relatively thin sunlit east limb of the Moon and found plenty of interesting sights to see. One of my favorite lunar features, the crater Petavius was well lit and I decided to take a wide angle view of the southeast limb with Petavius near the north end of the field. I set the camera for its high resolution, wide field setting and took a number of 10 second videos. The picture below is a stack of 7 out of 61 frames. Petavius, named for a 17th century French historian, is the large crater near the top (north) with the straight, black line running from its central peak complex to its broad, double-ridged west (right) wall. Petavius is about 110 miles in diameter, just over 2 miles deep and the central peaks are around one mile high. The crater has a convex floor caused by upwelling lava. The black line is a fracture on the floor extending close to 50 miles in length.
The 35 mile wide crater adjoining Petavius to the west is Wrottesley, a crater named for a 19th century English astronomer who catalogued double stars. To the lower right of Petavius are two craters in a vertical line. The upper crater is Snellius, 51.5 miles wide and named for an early 17th century Dutch astronomer and expert in optics. The 46.5 mile wide crater below Snellius is Stevinus. Stevinus was named for Simon Stevin, a Belgian renaissance era mathematician, soldier, optician and engineer…what we might call a real renaissance man. If you look closely, there is a hint of a depression running east-west (left-right) between the latter two craters. The southern part of Snellius is actually superimposed on top of this depression. This depression goes back to the very early history of the Moon and is called the Snellius Valley. It extends from the Nectaris Basin (still well beyond the day-night terminator in this image) roughly 360 miles toward the east limb. It is the longest named valley on the Moon. The Nectaris Basin was formed by a massive impact about 3.9 billion years ago and subsequently flooded with lava. The Snellius Valley is thought to be the remnants of a debris crater chain dating from that impact.
To the east of Stevinus is the 77.5 mile wide crater Furnerius, named for a 17th century French Jesuit mathematics professor. This crater is older and more beat-up than Petavius. The floor of Furnerius is heavily pockmarked with craters, the largest of which is Furnerius B near the northwest wall and shadow-filled in this photo. The walls of Furnerius are heavily eroded, especially in the northwest and southeast, so the crater is relatively shallow. The highest parts of the walls, however, rise over 11,000 feet.
Skipping down a few craters there is a slightly smoother area running along the southeast edge of the Moon. This is the edge of Mare Australis, the Moon's "southern sea". Part of it can be seen to the upper left of the attached craters Steinhall (left) and Watt (right). Each is about 41 miles wide and Watt is at the edge of the terminator in this photo. Watt, as you can guess, is named for the inventor of the steam engine. Mare Australis is even older than Mare Nectaris and has a number of craters and lighter debris on its plains. Mare Australis is around 100,000 square miles and most of it is beyond the Moon's east limb. Unfortunately, the Moon's libration was not favorable for showing much of Mare Australis in this photo. There's a lot to explore here, so try taking a look through a telescope or binoculars next time the Moon is a thin, waxing crescent.