Title text:
While weather control is typically thought of as a superpower, the unconscious ability of astronomers and astrophotographers to summon clouds is more properly classified as a curse.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3253/
I would think an Astrophotographer needs to know which distant objects in the night sky are going to be illuminated and/or washed out when, so maybe that's what it's referring to? I.e. the joke is not that he knows where the sun is pointing on Earth, but that he knows where in the observable galaxy the light will be best for his needs?