I just stumbled across a photo of the transit of the space station, and the
shuttle across the sun.


http://www.astrophoto.fr/iss_atlantis_transit.html


I have two questions, each demonstrating my ignorance...

1) I don't see any sunspots... why not?

2) I see a lot of limb-darkening. Why? If the surface of the sun is
isotropic, isothermal and the atmosphere is at least as hot as the surface,
why the limb darkening?