"Jupiter's biggest moon is Ganymede, not Callisto,... and it's Io that's got the volcanos."
"... I think you must have misheard Professor Sinistra, Europa's covered in ice, not mice"
- Hermione Granger (OP14)
Compulsory subject taught at Hogwarts by Professor Sinistra.
Astronomy is a compulsory subject for the first five years at Hogwarts. They meet at midnight at the top of the Astronomy Tower. Essays for the theory portion involve learning a lot of facts (Ron and Harry's fifth year essay on Jupiter's moons is the best example, OP14). During their practical O.W.L., the fifth years had to fill in a star chart by locating and correctly identifying various celestial bodies then visible in the sky (OP31).
Astronomy Tower (tallest Hogwarts tower)
- star charts
- reference books
none mentioned specifically
- They had to study the night skies through their telescopes every Wednesday at midnight and learn the names of different stars and the movements of the planets. (PS8)
- ... Hermione was testing Ron on Astronomy.
... [Harry] pulled a map of Jupiter toward him and started to learn the names of its moons. (PS15)
- Ron and Hermione ... were ... completing some star charts for Astronomy. ... "You can copy mine, if you like," said Ron, labeling his last star with a flourish and shoving the chart toward Harry. (PA8)
These star charts are mentioned a few times in the books (see also OP26, OP31), although it is not made entirely clear what the objective of this assignment is. Perhaps the students sketch stars and constellations that they see in the sky during one of their midnight observing sessions, and then have to complete these charts later on by identifying these objects in a reference book and labeling them?
- [Harry and Ron] ... turned miserably to Professor Sinistra's equally long and difficult essay about Jupiter's moons. (OP14)
Topics include their physical characteristics (size, surface features). Ron includes a drawing of Io. Hermione fact-checks using reference books.
- "I know that you have learned the names of the planets and their moons in Astronomy," said Firenze's calm voice, "and that you have mapped the stars' progress through the heavens...." (OP27)
- Third Year
- Then came [the] Astronomy [exam] at midnight, up on the tallest tower... (PA16)
No information is given about what was on it, but it was clearly a practical exam.
- Fifth Year
- The [O.W.L.] Astronomy theory exam on Wednesday morning went well enough; Harry was not convinced he had got the names of all of Jupiter's moons right, but was at least confident that none of them was inhabited by mice. They had to wait until evening for their practical Astronomy.... (OP31)
- [O.W.L. Astronomy practical:] Each of them set up his or her telescope and ... proceeded to fill in the blank star chart he or she had been given. ... [T]hey entered the precise positions of the stars and planets they were observing. (OP31)
According to the Oxford dictionary, the word Astronomy stems from the Greek word 'astronomos', meaning 'star-arranging'
Astronomy classes at Hogwarts:
One of the required courses for all Hogwarts students is Astronomy with Professor Sinistra. In this class, they learn the names of stars and constellations, of planets and their moons. They study the motions of the stars and planets through the heavens. They learn how to use telescopes, observe the night skies through them, and make labeled star charts.
They even learn twentieth century knowledge about the solar system, such as the sizes and surface features of Jupiter’s moons (knowledge unknown until the Muggle spacecraft Pioneer 10 and 11, Voyager 1 and 2, and Galileo flew past them and radioed back detailed images).
To aid them in these endeavors, students are required to purchase telescopes (PS5), although Hogwarts has its own supply as well (OP32). Astronomical textbooks do not appear on Hogwarts booklists, but students nonetheless have access to various reference books (such as the ones Hermione used to fact-check the essays on Jupiter’s moons – OP14). Some of these seem to have quite up-to-date information; Ron’s drawing of Jupiter’s volcanic moon Io (OP14) can only have been inspired by a NASA image radioed back via spacecraft (such as this1 one), as no Earth-bound telescope will show Jupiter’s moons as anything more than featureless dots.
Our most detailed look at this class is given in the description of Harry’s O.W.L. Astronomy practical exam (OP31), which is administered at the top of the Astronomy Tower in the middle of the night. Students set up their telescopes on stands, and then fill in blank star charts by sketching what they observe in the sky, taking care to note the “precise positions” of stars, constellations, planets, and the moon.
One may wonder what, exactly, they are doing with the telescopes. All of the objects that they are required to locate and sketch are visible without optical aid – indeed, using a telescope to look at a constellation is inadvisable, as the magnification prevents one from seeing the entire pattern of stars at once. Furthermore, no indication is given that the students are sketching the magnified appearance of the moon or the planets as seen through their telescopes during the exam, even though they would certainly be able to see craters and mountains on the moon, cloud bands on Jupiter, and Saturn’s rings.
Instead, the telescopes seem to be used as surveying instruments, aiding the students in determining the precise positions of celestial objects so that they can enter them on their charts.
[Harry] had quite forgotten Venus’s position – jamming his eye to his telescope, he found it again and was again on the point of entering it on his chart… (OP31)
From the Earth’s point of view, the heavens appear as a large globe surrounding us – the celestial sphere – with the stars “painted” on its inside surface. Astronomers delineate coordinate lines of right ascension and declination on this sphere, in much the same way that the terrestrial globe is mapped by lines of longitude and latitude. If a telescope is equipped with certain dials (setting circles), it can be used to accurately determine the celestial coordinates of the object at which it points. Perhaps this is the use to which the students are putting their telescopes here.
Of course, just as Omnioculars are able to do magical things such as display pop-up labels, present slow-motion views, and provide extreme close-ups and instant replays (GF8), it is possible that the students' telescopes, while looking like Muggle objects, might have magical properties as well. One could imagine the telescopes charmed to show volcanoes on Io, identify celestial objects with labels, display objects' coordinates, and so on. However, as Rowling makes no mention of these things, it seems simpler to me to assume that they are, in fact, Muggle optical devices. (Except for Fred and George's punching telescope, of course!)
Why is astronomy studied by young witches and wizards in training at Hogwarts? In part, the study of the planets' motions must provide a theoretical base for learning astrological divination (as Firenze explicitly states in OP27). This cannot be the whole story, however, for some of the topics studied in Sinistra's class - the physical characteristics of Jupiter's moons, how to use a telescope, locating objects in the sky - have little to do with fortune-telling, which in Trelawney's class is mostly an exercise in "consultation of timetables and calculation of angles" (GF13).
Perhaps an answer lies in the original meaning of the word “wizard” – wise one2. It seems to me that in order to be a wizard, one is expected to have mastered more than just the magical arts; one must also have knowledge of the natural world, through the study of such subjects as botany (Herbology), zoology (Care of Magical Creatures), and chemistry (Potions). Studying the cosmos in Astronomy is another example of this. Indeed, the wizards at the Department of Mysteries have an entire room seemingly devoted to understanding the solar system and its many secrets (OP35).