Whitepeak Observatory, Tacoma, WA

Observing rilles in the Gassendi and Aristarchus Areas

This is presented in an observing report format, an example of a typical session with a concentration on observing lunar rilles..

12-02-06. 28F, no surface wind but high, very thin cloud cover with winds aloft. ~2"seeing. Used magnification of 276x most of session: 7.7mm AbbeOrthos, BVer and 1.8 corrector lens with a 5" f/9.3 achromat. Primary features of interest for this session: Rilles.

Started out in the Mersenius area, the western shores of Mare Humorum. Referring to the graphic below (from an earlier lunation), my observations were:

Rima Mersenius II plain as it crosses Mersenius D. Rima Mersenius I plain along entire length to Gassendi G where it fades away. Rima Mersenius III not seen but the Liebig scarp, a subsidence feature of the Maria, was prominent. Moving to Rimae de Gasparis, branch III was plain as it crossed the isolated lava plain, but for some reason I couldn't catch the II branch. I was, however able to spot Rima de Gasparis I running right along the foot of the range, creating a "V" with the III rille. I noticed there was a 'false rille', a light linear albedo feature (i almost mistook for the I branch) running parallel but lying much further out in the plain proper. Rima de Gasparis II continued onto crater de Gasparis' floor, making a plain "X" with de Gasperis IV rille--which was not visible outside the crater floor, though it does continue... Moving to crater Cavendish, was able to clearly make out Rima Cavendish I crossing about 1/2 way the small lava plain towards de Gasparis-- although the II branch was invisible. Rima I appears to run right up the Cavendish's wall; a gash where it did so was clearly visible. The LO confirms a broad, though ill-defined gash at this location. Merseniusdisplayed a short rille trending N_S innthe N part of the floor. Two small craterlets bracketed a small isolated peak in this area as well. Finally, looked at crater Palmeri where the portion of Rima Palmeri that crosses it's floor was *just* discernable through the seeing.

Next I observed Schickard which was a really fine sight this evening. Letter craters A, B and C were plainly resolved as well as many trending smaller to the limits of resolution. An interesting arrangement of craterlets reminded me of the trapezium, lying on the N part of the floor. Not all of these were resolvable however. The nice E-W in-line pair towards crater center from B were plain however.

Aristarchus area was also well placed for observationthis evening. I was interested in seeing if I could make out some of this crater's namesake rille system. Observations refering to the graphic below:

First banding was obvious on the rim of Aristarchus and some interior topography was visible. Vallis Schroteri was plain in breadth and was contiguous all the way through it's bottleneck to Herodotus. As for the Aristarchus rimae, the I branch next to Aristarchus proper was plainly showing. I couldn't make out the III branch, but the IV branch was traceable as a faint white line to Krieger C. I could also follow VI a good distance downhill form Aristarchus C. Branch VII not seen. Branch VIII, the longest and most prominent of the Aristarchus rilles, was a splendid sight. Altogether the "drainage system" aspect of these rilles in relation to the Aristarchus volcanic plateau was plain to see tonight; I think the best view I have had. Out on the plain, Kreiger C was a fine sight, bracketed between Rima Kreiger I and Rima Aristarchus IV.

Finishing of this 3 hour session was Mons Rumker, situated just beyond the terminator, displaying a foreshortened view that made me imagine looking out the window of a far orbiter from space!

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