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. --- Jim Cobb <james@cobbzilla.net> wrote:

My understanding is that the black drop effect is atmospheric. Venus has a dense atmosphere, so the effect is pronounced for a transit of Venus. Mercury has very sparse atmosphere, so the refraction by gases does not occur. At least, this is what I've read.

I have no solar filters (except for naked eye). But I do have a sunspotter ( http://www.starlab.com/ltiprod.html ), which I will be using to view the event. It's not as fancy as a scope with solar filter; certainly not as fancy as a scope with an alpha filter. But, several people can observe simultaneously.

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