Mars is bright tonight

This quote illustrates the frustratingly vague intuition of the centaurs. Their knowledge of divination is depicted as more authentic than Professor Trelawney's, whose occasional prophecies--of which she seems to have no memory--contrast starkly with her array of confidently-spouted nonsense. While she is occasionally correct (even a stopped clock is right twice a day), her constant predictions of Harry's imminent demise indicate that she can be reckless with her interpretation of signs. Centaurs, on the other hand, seem reluctant to interpret such signs at all, preferring instead to observe them and allow the implications to be considered--a method made almost foolproof through its abundance of caution.

Also, where are the female centaurs? Perhaps we are to believe they are off tending to their young, while the males traipse through the forest in search of miscreants and/or sustenance.

In any case, readers will also appreciate Hagrid's response to this quote, which is spoken first by Ronan, then repeated by Ronan, then spoken independently by Bane. "We've heard," said Hagrid grumpily.

--AK

He is speaking ominously and giving little information, which according to Hagrid, is characteristic of centaurs. 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.