  We discuss how the solar occultations of bright sources of energetic gamma
rays can be used to extract non-trivial physical and astrophysical information,
including the angular size of the image when it is significantly smaller than
the experiment's angular resolution. We analyze the EGRET data and discuss
prospects for other instruments. The Fermi Gamma Ray Space Telescope will be
able to constrain the size of a possible halo around 3C~279 from observations
it makes on the 8th of October each year.
