The lunar doughnut 01/06/03

by Jane Houston Jones

I enjoy observing interesting shadow features on the moon. They are fleeting and rare. They take a combination of luck, weather, opportunity and inspiration to observe. It didn't take much arm twisting for me to set up my 10-inch f/7.3 reflector on the back deck Monday, January 6th to take a look at the "Lunar Doughnut" first observed by Alan Adler in 2000. Alan is a well-known local astronomer from the San Franciso bay area, and is always alerting our astronomy clubs of interesting observing opportunities such as this.

His annotated images provided just enough detail for me to quickly find this interesting phenomenon. The first craters I spotted were the pair of craters Steinheil and Watt on the south-eastern limb of the moon. In my reflector view south is up so my observations along the terminator were from south to north. These two craters are shown on RUKL 76. North of these two, Janssen was partially visible on the terminator - just its eastern rim. The next in the straight-line trending SW to NE crater lineup was Fabricus, with only a brightly lit central peak and the outline of things to come.

Next to the north, Vallis Rheita formed a long black ropy gash from Rheita "D" to crater Young to crater Rheita, with a shadow on the central peak. RUKL 68 shows this area. Vallis Rheita is the longest valley on the near-side of the moon - 500 km long! A similar gash slightly to the south of Metius and parallel to Vallis Rheita was just another shadow feature I sketched.

The lunar doughnut was easy to see between Rheita and the dark crescent-shaped rim of Neander on the terminator. A similar "cobblestone or hashmark" rippled surface feature filled the lunar space between these two craters and the doughnut, but was the same shade of light gray as the surrounding landscape. Both "doughnut" and "hash" appeared to have a similiar roughness, but the "doughnut" was medium dark grey with a darker center, while the "hash" was lighter. The center does not look like the "hole" in a doughnut exactly. It looks like a bunch of short parallel features, more like fault lines, in shadow. From this northern point at Rheita "E" , an almost straight line forms along the terminator tonight beginning with Crater Janssen to the far south, bisected by Vallis Rheita.

To end the bakers dozen of craters aligned north and south of the doughnut were another set of twin craters Reichenbach and Stevinus. It gave the whole view a harmony and balance I found delightful to observe.

I observed the doughnut for an hour beginning at 5:45 p.m. PDT. My sketch represents the view at 5:50 PST or 01:50 UT 01/07/03. My observing location in San Rafael, California is at 37N 122W, the sky was a sparkling clear, dark and dry. It was great night for doughnuts.

A compilation of three observers images, a Lunar Orbiter annotated image plus my sketch are here: http://mercury.nineplanets.org:8011/astro/donut/