Telescope Reviews: Re: Gassendi and environs 16 May 2008

Sorry about the confusion, Mardi. "Later" was a typo on my part (you may have noticed I also have trouble with left-right, east-west, and many others).

I was going by the time encoded in Larry's filename, which says "16 May 2008 2100hrs". Since I know it's Larry's custom to give local Dunedin time I subtracted 12 hours and came up with 0900 UT. If I'm reading the caption properly (both my screen and my eyes are a bit fuzzy), your own label says "0617 UT". That sounded like about a three hour difference to me, but I didn't check if that was perfectly consistent with the shadow lengths recorded in the two photos. Larry has now posted a corrected time of "2008 hrs local Dunedin time" - 12 hrs = 0808 UT; and you are perhaps now indicating 0615 rather than 0617 (?). So the correct difference is apparently something like 1 hour 53 minutes. For what little it may be worth, the lengths of the shadows in Larry's photo cast by the east wall of Gassendi onto the relatively flat floor are more consistent (relative to the lengths shown in yours) with the assumption that Larry's photo was taken at 0808 UT than at 0900 UT (when one would have expected them to be very slightly shorter than they are).

During the ~2 hr interval from 0615 UT to 0808 UT the sun angle at the center of Gassendi (the height of the Sun in the sky relative to the local horizontal) increases from about +2.25 deg to +3.15 deg. This difference in angles is roughly twice the Sun's apparent diameter of about 0.5 deg. Intuitively one might expect to see noticeable changes in lighting when the Sun rises by about half its own diameter. So yes, a careful observer might well notice changes in one-fourth this time, or about half an hour, perhaps even less in special cases (like a peak just catching the Sun's first rays).

To clarify the differences between the two photos, I've used the free LTVT software to re-map Larry's photo, and also a much higher sun Lunar Orbiter photo, to the same geometry as yours. I've also increased the "gamma" of both Earth-based photos in an effort to bring out the details at the terminator a bit more clearly.

Looking at the region in Mare Humorum around Gassendi L (see yellow arrow in LO photo), one might almost think the terminator had retreated in Larry's later photo relative to yours. But this seems to be an artifact of differences in exposure and processing. As you point out, the shadows in Gassendi has shortened significantly, and new peaks (highlighted by the blue arrows added to Larry's photo) have come into sunlight in the later photo. In particular, the area around Gassendi E has broadened, and a new sections have been added to the feature indicated by the green arrow in the LO photo (which I previously referred to as a "lengthening thin bright line").

The short red line added to Larry's photo indicates the projected direction of the rays illuminating the slope to the east of Gassendi E (and can be applied more generally to both Earth-based photos). In both photos the sunlight along this line is streaming over the west rim of Gassendi and striking the slope near Gassendi E. In your photo it is hitting a point about 50 m above the rim of Gassendi (the remainder of the area around Gassendi E, that sticks up into the sunlight, is higher than this by some amount that cannot be determined from these photos). In Larry's image, the rising Sun has caused the leading edge of sunlit area to move down the slope to a point about 680 m below the rim. A short bit later this progression would be interrupted by the rays from Gassendi's rim striking the dark gap region of the scarp/ridge indicated by the green arrow. The crest of this ridge appears to be about 200-500 m below the crater rim, and a comparison of the two photos shows how it is increasing illuminated as the sun's rays penetrate lower and lower due to the increasing sun angle. Once illuminated, these peaks cast their own shadows to the west.

-- Jim

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