On Oct 15, 2006, at 8:54 PM, Chuck Hards <chuckhards@yahoo.com> wrote:
I haven't read anything on the black drop in years-obviously it's an optical illusion, but I had thought it was more related to telescope optics or earth's atmosphere. I've seen it with Mercury, but I've never seen a Venus transit to compare it to.
How big is the image with your sunspotter? Mercury may be hard to spot if the projected solar disk is under about four inches in diameter.
Good point, Chuck. The solar image is about four inches with the scope. With Mercury's apparent diameter of 10 arcseconds, that works out to be .02 inches. Pretty small,but I think visible. I would not expect to be able to see any black drop effect at such a size. Venus' disk during transit is considerably larger (58 arcseconds during the 2004 transit) because it is a larger planet, and because it is so much closer to earth at transit. It is visible naked eye (with proper protection). I happened to be in Boston on that date and saw it using eclipse glasses. Jim Cobb james@cobbzilla.net