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Greg, 

I've been trying to think of some object that would employ that circular shape.
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Also I've been thinking that maybe your use of the word "impressionism" was a pun. Is it a seal for making impressions in wax perhaps? For sealing letters? I agree completely with David's analysis that it's most likely the transit of Venus observed at Tahiti in 1769 by Cook et al. (I figure David's 1789 was just a sticky neuron). From the location of Venus on the disk, we could work out the time quite accurately. The very crude form of the vessel, apparently a bark so consistent with HMS Endeavour, suggests something carved on a small scale, which fits with my thought that this is a very small "stamp" of some sort. My guess for the arrow is that it represents due north, and thus the instant when the Sun was transiting the meridian while Venus was "transiting" its face --the transit during the transit. That would make the local time 12:00:00 and the GMT is 21:55:55 or so, on June 3, 1769. The true altitude of the Sun's center, about 50° 00', while the true altitude of Venus is about 11.5' lower.

By the way, I see several faces staring from various odd shapes in that image, but I figure those are mostly cousins of the "Face on Mars" ...imagination filling in faces in face-like shapes. Or are they? I have no guess on the artist!

Frank Reed
Conanicut Island USA, two miles from the suspected final resting place of HMS Endeavour.