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Observation Report
Carmichael, Thursday, May 31, 2001

Bullialdus "Causeway," Rupes Mercator

by: Matt Tarlach

It's been a pleasant evening for lunar observing, with temperatures in the 80s well after sunset and reasonably steady air, 4/5 around 10PM PDT breaking down to 3/5 when I quit around midnight. I took advantage by viewing with the 80mm f5 refractor, 7mm Orion ortho and 2.8x Klee barlow for a little over 150x. A #11 yellow-green filter was used throughout to combat the little doublet's chromatic aberration.

I began in the Copernicus region, and was treated to a grand view of this magnificent crater. The terracing on the interior of the west wall looked very massive. The double crater Fauth showed well to the South of Copernicus, the western portion of a ridge separating the floors of the two components being visible and more prominent than drawn by Rukl. North of Copernicus, Rima Gay-Lussac was easy to find, though I could not see either of the "cul-de-sacs" that Rukl draws at the ends of this feature.

Cruising south across Mare Cognitum I came to Nubium and the Bullialdus region, one of my favorites on the lunar face. I got a good look at what Rukl calls the "elevated causeway" crossing a sunken valley to the northwest of Bullialdus itself. The valley looks like a smooth and continuous flow of dark mare material, which is spanned by an arch-like structure of bright, rockier-looking matter. I had hoped the illusion of an actual bridge would be more convincing, but it was a pretty sight and something of a puzzler: since the Mare material is generally believed to be the more recent addition to the lunar topography, it logically should overwrite the older, brighter material - the opposite of what seems to have taken place here.

West of the flooded crater Kies I had a good look at the dome Kies Pi, and in steady moments suspected a black craterpit at the summit.

South of Kies Pi the Rupes Mercator was well lit. This feature runs from the northwest to the southeast, and marks the boundary between the dark Mare Nubium and the bright highlands surrounding the crater Mercator. The outer portion of Nubium appears convex here, as shown by the soft feathered shadow of the terminator falling across it. The limit of the highland is quite straight, and bright. I suspected a straight, black trench running near and parallel to the base of the highland; this was easiest to see toward the northwestern end where the highland scarp is somewhat broken up.

South of Mercator the western arm of Rima Hesiodus was seen as a dark line crossing Palus Epidemarium. Where it crossed the highlands near longitudes 25 and 26W, I noted bright patches on the highland promontories. It appeared as if the rille was crossing the brighter terrain, rather than being covered by it. The whole scene made me think of what our roadways look like from high altitude, when cut through hilly country.

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Matt Tarlach
Carmichael, CA

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