In most fiction I've read or encountered, the change occurs as a simple consequence of the lunar cycle. Though from what I know of Harry Potter, the change that takes place is due to sighting the full moon (as occurs in the third movie, Prisoner of Azkaban) where the gang, plus Lupin, Severus, Sirius and Pettigrew emerge from the Whomping Willow and upon sighting the full moon he begins to change.

Is this the trigger for werewolves in the Potterverse? If not, why did he (Lupin as an example, though for any werewolf in general) not change whilst in the shrieking shack that night?

Edit: This question addresses Lupin's change but I'm after more general information about werewolves.


1 Answer 1


I think sighting the full moon is the main trigger for a werewolf to transform in Potterverse, but I also wonder if it has something to do with the time of day. As you probably know, Lupin transforms at night in chapter 20 of Prisoner of Azkaban. He also references being escorted while still a Hogwarts student to the Shrieking Shack in the evening¹ by Madam Pomfrey, and clearly he hasn't transformed yet or else Madam Pomfrey would have been injured or killed. So I'm going to postulate that not only is the full moon the trigger for Lupin's lycanthropy, but perhaps Lupin's transformation is contingent upon the time of day. Specifically, the full moon at night may be the trigger. Not, for example, the full moon at sunrise or slightly before.

As a side note of sorts, Boggarts can induce a real reaction to the thing a person fears most -- for example, the Boggart!Dementor Harry used to learn the Patronus Charm induced the same reaction -- hearing his mother screaming and pleading, and him passing out -- as the Dementor on the Hogwarts Express did. However, Lupin's Boggart -- the full moon -- did not cause Lupin to transform into a werewolf. Perhaps this means the werewolf transformation cannot take place during the day, for Lupin's Boggart showed itself during a Defense Against the Dark Arts class. (There could be a number of factors as to why Harry reacted but Lupin did not when they faced their respective Boggarts)

As for why Lupin didn't transform into a werewolf while everyone was all in the Shrieking Shack, if indeed the transformation is time contingent maybe it simply wasn't dark yet.

¹1. the latter part of the day and early part of the night. 2. the period from sunset to bedtime: He spent the evenings reading. 3. Chiefly Midland and Southern U.S. the time between noon and sunset, including the afternoon and twilight. DICTIONARY.COM

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    Just to get your astronomy right, the full moon cannot be seen during daytime. It always rises around sunset. (The moon in other parts of the lunar cycle can indeed be seen during daytime.) Apr 24, 2012 at 12:44
  • @HendrikVogt - I'm curious if by this rule, is the full moon required to have disappeared from view before sunrise? :) Apr 24, 2012 at 14:50
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    Same thing, the full moon disappears around sunrise. (The moon is not in the plane defined by Earth's orbit, so it may disappear shortly before or after sunrise.) The reason for this is actually not that difficult: a full moon means that the sun and moon are on opposite sides of the Earth, so the Earth is between, and the moon is fully illuminated by the sun. (Note that "between" can be exactly between - then we have a lunar eclipse, which therefore can only happen during full moon.) Apr 24, 2012 at 17:08

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