--- Jeff Crilly <tac@crilly.net> wrote:
> Another question regarding jupiter transits... Is there any
perceived
> black drop effect? (e.g. like you get with venus transiting the
sun).
This might be possible, because I know there is a black drop effect
with Mercury transits of the Sun as well. But undoubtedly it would be
much more difficult to observe due to the smaller objects involved.
> It turns out this is due to limb darkening...
Are you sure? Do you have any reference?
I know there has been a centuries-long question about the true nature
and cause of the black drop effect. (Some early speculation involved
Venus' atmosphere, but that was ruled out with certainty by
observations of the black drop during Mercury transits.) But I
thought it had finally been definitively solved, and my sources say it
is due to a combination of normal refraction within the telescope and
atmospheric seeing. [See Schaefer, Bradley E. "The Transit of Venus
and the Notorious Black Drop Effect", Journal for the History of
Astronomy 32 (2001) 325-36.]
Also, John Westfall has done a digital simulation of the black drop
effect, using a 100-pixel diameter black circle on a white background
at various distances (0 to 5 pixels) away from a black line and then
introducing a "Gaussian Blurring" function to simulate seeing and
diffraction blurring effects (various levels of blurring). The results
show the black drop effect pretty clearly, and there is no limb
darkening in this simulation. This experiment is described in the 2004
book "The Transits of Venus" by William Sheehan and John Westfall
(pages 157-158).
I'd be interested to know if there is an authoritative (recent) source
claiming the limb-darkening cause.
Bob J.
Received on Thu Jul 29 18:16:55 2004