Um arithmancy is just a fancy way of say arithmetic. I think arithmancy is Latin or something. It wouldn't be very wizardy if it was math. Sent in by Komo.
Arith is the greek root meaning numbers mancy meens magic in greek (necromancy magic doing with death) so arithmancy literily translated would mean magic of numbers. Sent in by Nathen.
Arithmancy is divination by numbers (sometimes wrongly called Arithmomancy). The ancient Greeks examined the numbers and values of letters in each name of two enemies at war. They predicted the combatant having the name of the greater value would be victorious. The Chaldeans also practiced arithmancy. They divided their alphabet into three parts, each part composed of seven letters which they attributed to the seven planets. Through this arithmetic method they made predictions based on the planets. The Platonists and Pythagoreans were also strongly attracted to this form of divination which is similar to certain aspects of the Jewish Kabbalah. Sent in by Will.
Hmmm... So magic of numbers. Yeah... I can imagine Hermione saying "Why is 6 afraid of 7? Because 7, 8, 9." While she's saying that 7 really does eat 9. Nevermind. Sooo interesting....