Dave Mitsky's Corner

Discovering the Walter Lunar Sunset Ray

By Dave Mitsky

Walter lunar crater

Make sure to check out Bill Arnett's website on the Walter Ray

There's always something new to be seen on the moon or so it seems. Because of the changes in illumination as the moon goes through it monthly cycle of phases and the fact that libration* allows access to different parts of the moon a dedicated lunar observer can be pleasantly surprised from time to time by a number of different lunar phenomena.

That is exactly what happened to me on the morning of May 29th, 1997. I had been observing Herschel 400 globular clusters and other deep-sky objects with the 17" f/15 classical Cassegrain until the light of the rising moon obliterated the Milky Way. Before closing up the French Dome I scanned the lunar terminator at 249x (26mm Ploessl) and at 07:10 UT (03:10 a.m. DST) came upon a lunar sunset ray in the crater Walter. I had previously observed the most well known of the lunar sunrise rays in the crater Hesiodus on several occasions and was thus able to recognize just what it was that I was seeing. A break in the Walter's western wall permitted the light of the setting sun to illuminate the western portion of the interior floor and the lower part of the central peak. At 07:42 UT I noticed an interesting "reverse" triangular shadow that was cast from an object of the western wall onto the illuminated crater floor.

Since the subject of lunar rays had come up from time to time on the amateur astronomy newsgroup sci.astro.amateur I made a report about my observation and asked if anyone was aware of a sunset ray in the crater Walter. I received no replies and, other than mentioning my observation at the June, 1997 ASH meeting, let the matter drop.

Over a year later I came across a digital camera astrophotograph of the crater Walter that Iowan Steve Rasmussen had taken on September 13th, 1998 which confirmed my "discovery"! I sent an e-mail message to Steve and suggested that he contact Rob Robinson, a serious lunar observer whose home page contained information on the various known lunar rays. Rob subsequently posted my original s.a.a. report. Akkana Peck also included the report on her excellent lunar home page, the Hitchhiker’s Guide to the Moon.

Akkana also runs an e-mail list known as the Shallow Sky, which is dedicated to observing objects within the solar system. Last November the topic of lunar rays came up and respected amateur astronomer and ALPO member Jeff Medkeff , who wrote the article on averted vision in the June 1998 issue of Stardust, expounded on the matter.

I added to the discussion with my observation of the Walter Sunset Ray and Jeff took it upon himself to research the matter. Apparently the Walter Sunset Ray had not been previously reported. Jeff’s cogent report "Lunar Sunrise/Sunset Rays" goes on to say that "Sunrise and Sunset rays are optical effects caused by light from the sun shining through narrow clefts onto a portion of the lunar floor that is otherwise in darkness." (Jeff's discussion and Rob’s predictions for future occurrences of the Walter Sunset Lunar Ray are linked to the ASH home page under Dave’s Lunar Discovery.)

Lunar sunrise and sunset rays, which are quite distinct from the "rays" of ejected material seen emanating from craters such as Tycho and Copernicus during full moon, have been known for over 100 years. But the subject was largely unknown to modern amateur astronomers until an article on the Hesiodus Sunrise Ray appeared in the July, 1996 issue of Sky & Telescope. Other lunar sunrise rays include the Torricelli Double Shadow and Ray and the Burnham Ray.