Wed Jul 20 20:33:46 1994

Visual obs'ns from Cambridge, July 20

Another excellent evening's observing at Cambridge with 30-cm refractor (date 1838, by the way). Main points were as follows.

-- Sites K, L, G paraded across the disk; they are all more spectacular than ever, very large and dark, about as big as the GRS, each with a black spot like a satellite shadow in the north-following corner. Transit times UT:

K, 18.51 (whole spot: rough estimate in daylight);
L, 20.26 (p. edge), 20.56 (black spot), 21.02 (f. edge);
G, 21.45 (whole spot), 21.52 (black spot, +/-5m in bad seeing).

The last value represents L2 = 311, same as two days ago; Chodas prediction was 312. [More longitudes to follow.] [NB: It would help if those who refer to east and west would say whether these are planetary or celestial.] (Does the black spot mark the site of the terminal explosion, which continues to feed black stuff up from below for 1-2 days after the impact?)

-- Limb-brightening north of site K as it approached the p. limb. The STZ along here was bright anyway, but seemed particularly bright at the limb from 20.26 to 20.58 UT (when seeimng worsened). This was the clearest example of a phenomenon I have often suspected over the last 4 days: diffuse light areas adjacent to the dark sites when near the limb, but not so evident when further onto the disk. Has anyone recorded this on images?

-- Possible weakness of the Q impacts. No flashes observed visually (of course), but later there was no dark spot in the expected place for Q2, and only a rather faint, southerly dark area for Q1. Seeing was getting bad at that time but it seemed inferior to site H observed at the same phase two days ago. [Note that the very prominent site following L is G; is there confusion in the recent La Palma messages?] (If Q2 failed, it fits neatly into the pattern that all nuclei which were displaced tailward in the comet have either broken up, disappeared, or impacted without an explosion. Were these nuclei made of some tenuous tacky polymer, subject to solar wind/radiation pressure??)

John Rogers (BAA).