The Astronomy Tower is the tallest tower at Hogwarts Castle. It is where students study the stars and planets through their telescopes in Astronomy lessons with Professor Aurora Sinistra. This is usually done at midnight where the stars are best seen.  The tower is usually out-of-bounds except for classes. 
After Sirius Black tried to break-in the Gryffindor Tower on the evening of October 31st, 1993, all the staff was to search the Castle for any signs of Black. Argus Filch searched the Astronomy Tower and found nothing. 
The Astronomy practical exam took place here for the O.W.L.s. Under supervision of Professor Tofty, students were required to fill out a star chart. However, Dolores Umbridge, John Dawlish and several other Ministry of Magic officials attacked Rubeus Hagrid during this practical, and the resulting skirmish caused several Aurors and Professor Minerva McGonagall to be severely injured, Fang to become incapacitated, and Hagrid to flee the school. All of this was witnessed by the Fifth years taking the Astronomy O.W.L.s.
The Death Eaters arrived at Hogwarts, most of the Battle of the Astronomy Tower between the opposing sides took place in this tower, and in the corridor below. Death Eaters set off the Dark Mark above this tower; Albus Dumbledore was blasted from the tower's ramparts by a Killing Curse cast by Severus Snape. This was where Dumbledore met his death.
The tower is the tallest in Hogwarts, it lies almost directly above the front doors of the castle. The tower is also surrounded by a parapet and is tall enough to have a perfect view of the starry sky.
Behind the scenes
- The exterior of the Astronomy Tower is featured at The Wizarding World of Harry Potter
- In the PC version of Harry Potter and the Philosopher's Stone, the Hogwarts Library is inside the Tower, on the lower floors.
- The Bloody Baron likes to groan and clank up on the Astronomy Tower according to Nearly Headless Nick.
- In the film adaptation of Harry Potter and the Half-Blood Prince the tower is directly over the Transfiguration Courtyard. This is the reason for the courtyard to have changed shape in both the film and the video game. Unlike the book, the tower isn't the tallest tower at Hogwarts and doesn't have a complete view of the night sky.
- Also, in the films, the witnessing of Hagrid being attacked by Ministry officials by the Fifth years taking the Astronomy O.W.L.s would be impossible, as the Astronomy Tower is located far east of the castle and would not have clear view of Hagrid's Hut.
- Harry Potter and the Philosopher's Stone (First appearance)
- Harry Potter and the Philosopher's Stone (video game) (GBC, GBA and PC versions only)
- Harry Potter and the Chamber of Secrets (video game) (GBC version only)
- Harry Potter and the Order of the Phoenix
- Harry Potter and the Half-Blood Prince
- Harry Potter and the Half-Blood Prince (film)
- Harry Potter and the Half-Blood Prince (video game)
- Harry Potter and the Deathly Hallows (Mentioned only)
- Harry Potter and the Deathly Hallows: Part II
- The Wizarding World of Harry Potter