Muggles' Guide to Harry Potter/Magic/Astronomy

Overview
Astronomy instructs students on planets, stars, and other sky objects and their role in the magical world. At Hogwarts, this may be a core subject, required of all Hogwarts students until at least their fifth year; those with sufficiently high O.W.L. marks in the course can then go on to N.E.W.T.-level study in years 6 and 7. We believe that at Hogwarts it is taught by Professor Sinistra.

Extended Description
Astronomy classes take place in the top-most level of the Astronomy Tower, the tallest tower in Hogwarts Castle, and of course are always after dark.

Evidence is that this is a core subject, required from first year onwards. When courses are selected, there is no specific mention of this course, but the first-year book list explicitly calls out the requirement for "a telescope". Additionally, there is later mention that the telescopes are used at night for plotting the positions of the planets, which is clearly astronomy even if the course is not named.

Harry, Ron, and Hermione are seen to take the OWL exam in that course, but as they are disturbed by the firefight surrounding Hagrid's dismissal, Harry and Ron manage only an Acceptable OWL mark in the course, and do not continue at that point. We suspect that Hermione, as usual, has finished her work early and so has already earned her top marks by the time the firefight breaks out.

Analysis
We must be careful in discussion of astronomy to differentiate it not only from the branch of Muggle fortune-telling and Wizarding Divination known as "astrology", but from the study of the motion of spatial objects called "Astronomy" in the Muggle world. It is certain that Wizarding astronomy involves location of the heavenly bodies and determinations of their relative motions, but we expect that the impact of the motions of these bodies also has some magical effect, and the Astronomy course almost certainly is involved with the memorization of these effects and their uses. We note the following two examples of this sort of effect: in Harry Potter and the Deathly Hallows, we see Professor Slughorn collecting something that is "most efficacious if harvested at twilight", as he remarks to Professor Sprout. And in our world, there is a belief that crops will grow better if planted "in the dark of the moon."