The Astronomy Tower is the tallest tower at Hogwarts Castle surrounded by a parapet and turret. 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. 
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 Ordinary Wizarding Level.
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 site of the starry sky.
Behind the scenes
- In videogame Harry Potter and the Philosopher's Stone (PC version), 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.
- 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 Prisoner of Azkaban (film) (Mentioned 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)