"Mars is bright tonight"
-- Bane, Ronan
Harry’s popularity slips when his escapades lose his house 150 points, he overhears another conversation, and they serve detention with Hagrid who takes them to the Forbidden Forest in search of a wounded unicorn, which they find dead. Harry is rescued from Voldemort by a centaur.
Calendar and Dates
The chapter starts in the middle of the night, just after Harry and Hermione have been caught out of bed. After McGonagall punishes them, the time meanders quickly through a couple of weeks, and then spends the focus of the chapter on the night Harry, Hermione, Neville, and Malfoy serve their detention in the forest, which occurs "about a week before exams were due to start." Why the detention is served weeks after it is given is a bit of a mystery.
Interesting facts and notes
This is the first chapter in which we encounter centaurs. From their description in FB, they appear to be based on the Greek legends of centaurs, but that in itself isn't a sufficient description - there are actually two types of centaurs in Greek mythology.
The common sort of centaur in Greek mythology was a violent, rather crude character, very prone to losing control (particularly when alcohol was involved). The other, rarer type of centaur was the type seen here - very learned creatures (the most famous was a noted tutor of various Greek heroes, including the greatest healer of Greek legend). This latter type of centaur is also the type on which the centaurs of Narnia in C.S. Lewis' works appear to have been based.
He couldn't see how they were going to get out of trouble this time
Considering the true story behind this incident, it's quite amazing that they got in much trouble at all. It was Hagrid who should be in trouble, not the least of which because he allowed three first years to get involved and break all sorts of school rules along the way.
Fifty points each...
I'm sorry, but what in the world is McGonagall playing at here? Fifty points each for just being out of bed after hours? What would she do for a truly serious offense, cut off their hands? And she must know that she's personally destroying Gryffindor's chances to win the House Cup. It simply makes no sense at all.
Gryffindors passing the giant hourglasses that recorded the house points
These hourglasses are located in the Entrance Hall, according to book five, so the students would pass them on their way to breakfast. However, in the film they're located along the back wall of the Great Hall, behind the staff table, where no student would pass them at all.
Harry swore to himself not to meddle in things that weren't his business...
Perhaps we see here why McGonagall came down so hard on Harry and the others. The danger is especially acute for Harry, although no one seems to know exactly why, and McGonagall wants to convince him to stay put and not get involved. Chances are Dumbledore has charged Minerva with the specific task of protecting Harry from, well, himself, and she is willing to do whatever it takes to fulfill that duty. Still, a hundred and fifty points...
about a week before exams were due to start...
Exams are typically during the first two weeks of June, so at this point it's the last week of May, well over a month since they were given detention by McGonagall.
"No -- no -- not again, please --"
"All right -- all right --"
This is the only part of the conversation we hear. Harry assumes that this is a conversation between Quirrell and Snape, but we know that he's actually talking to the face on the back of his head. What was Voldemort threatening Quirrell with? Since he's inhabiting Quirrell, it would seem that Voldemort can cause some sort of unpleasantness, possibly pain. And what was he demanding that Quirrell do? Go through the trapdoor and go after the stone, presumably. It's strange, however, that Quirrell was willing to attempt to get the Stone out of Gringotts, in a vault hundreds of miles under London, guarded by dragons, that would trap you more or less forever if you didn't succeed, but was unwilling until now to tackle the relatively simple defences of the chambers under the third floor corridor.
Anti-Dark Force spell
This isn't an actual spell. Ron is just referring to Quirrell's contribution to the defence of the Stone by this generic name. As it turns out, Quirrell's contribution was a Mountain Troll, not a spell at all.
it's dangerous, what we're gonna do tonight
Yes, it is, far too dangerous for a group of eleven-year-olds to attempt (okay, I know, Hermione is twelve at this point in the story). Why did the teachers allow this? Perhaps Hagrid thinks he's doing the kids a favour by "letting" them explore the forest instead of doing more mundane things in the castle, and he's doing this as a way to make up for the fact that it's actually his fault that they're in this predicament in the first place. Almost certainly no one knows what he's actually going to have them do: wander around the forbidden forest in groups of two, unarmed and unable to do much magic at all, looking for something which is evil enough to kill a unicorn.
There's nothin' that lives in the forest that'll hurt yeh if yer with me or Fang
That's not true. About a year from now, Harry and Ron will encounter Aragog and his offspring deep in the forest. Although the boys have Fang with them, the spiders are more than happy to attack and would have eaten them all if the Ford Anglia hadn't intervened. To be fair, Hagrid doesn't really realize this fact about the acromantulas. He is astonished to discover that the giant spiders have only shied away from eating him because of Aragog, and when the gigantic patriarch of the clan is dead, the rest of the spiders attack Hagrid.
Mars is bright tonight
This phrase is repeated a number of times by the centaurs. In actuality, Mars was not very bright during the spring of 1992, but I think we can assume that Rowling didn't check an almanac to determine that as she wrote this chapter. In Harry's world, Mars was bright on that May evening around midnight. Assuming that Mars signifies blood and warfare in Harry's world, we can conclude that the centaurs are seeing the approaching war as Voldemort rises again to power. He won't actually be back in full power for several more years, but the centaurs tend to view events on a grand scale, so a few years will seem very soon to them.
Always the innocent are the first victims
Here Voldemort's first victim is a unicorn, the symbol of purity and innocence. As Voldemort rises again to power and regains some semblance of a body, his next victims are Bertha Jorkins, an old Muggle, and Cedric Diggory. Each of these people is an innocent victim. Quirrell is left to die as well, but it's a little trickier to believe him innocent until we remember that Voldemort twisted his mind and turned him into a pawn.
He had a nasty feeling they were being watched.
Harry has a sixth sense about this sort of thing. If he feels like they're being watched, they probably are, although there are any number of creatures in the forest who could be watching them. Considering how quickly Harry is rescued from Voldemort forty-five minutes later, it's most likely Firenze and possibly other centaurs who are watching him.
Red sparks! The others are in trouble!
This form of signaling is used in book four when someone is in trouble in the maze. It's also used as a signal by the Advance Guard of the Order of the Phoenix as they whisk Harry away from Privet Drive in book five.
"Stay on the path."
The next time Harry enters the forest, following the spiders who are escaping the castle in book two, he vividly remembers this warning and hesitates. In that case, however, there's nothing else to do but follow the spiders off the path. He and Ron learn very quickly the dangers that await someone who leaves the path when they encounter Aragog and his brood of acromantulas. (CS15)
It got to its feet and came swiftly towards Harry
In the film, Quirrell actually flies toward Harry in this scene. According to QA1, however, "no spell yet devised enables wizards to fly unaided in human form," so the film version is not canon.
A centaur was standing over him...
This centaur is Firenze, who five years later will become a teacher at Hogwarts after being expelled from the herd and nearly killed. Firenze is described here as being younger than the other centaurs Harry has encountered, which might account for his being willing to forsake the traditions of the herd here and a few years from now. The name Firenze, incidentally, is the Italian form of the name of the city of Florence and is pronounced "feer-EN-zee", not "fuh-ENZ" as it is in the film.
"Just in case."
Presumably the cloak and this cryptic message are from Dumbledore. Does he expect Harry to go after the Stone? In a way, this is similar to the beginning of book six when Dumbledore asks Harry to carry the cloak with him wherever he goes...just in case.
"It is a monstrous thing, to slay a unicorn. Only one who has nothing to lose, and everything to gain, would commit such a crime. The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something pure and defenseless to save yourself, and you will have a half-life, a cursed life, from the moment the blood touches your lips." [Firenze]
"Good luck, Harry Potter," said Firenze. "The planets have been read wrongly before now, even by centaurs. I hope this is one of those times."