Why doesn't Lupin become a werewolf the night Harry looks for Peter on the map? The moon is out that night.
2 Answers
There is no indication that there is a full moon on the night that Harry encounters Snape and Lupin, and the very fact that Lupin is human would indicate that it is not. There is definitely moonlight coming in the windows in Harry's bedchamber and Snape makes a snide comment to Lupin about "taking a walk in the moonlight", but the moon could still be bright and visible without being full.
Good question. From my perspective, in the book, Snape mentions that he was bringing Lupin the Wolfsbane potion but ended up following Lupin because he saw him leave the castle. In the movie Sirius asks him if he had taken his potion when they realize its a full moon. So to answer your question it could have been the wolfsbane or it could have been the full moon.
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1Also the Wolfsbane potion allows werewolves to retain some control over themselves when transformed, but does not actually stop the transformation. "As long as I take it in the week, preceding the full moon, I keep my mind when I transform… I’m able to curl up in my office, a harmless wolf, and wait for the moon to wane again." May 7, 2019 at 14:49