When Sirius and his friends arrived at the top of the Astronomy Tower at ten o'clock that night, Professor Sinistra was pacing back and forth among the telescopes, looking more excited than Sirius had ever seen her. "You're in for quite a treat tonight," she told the arriving students. "Take a look at the moon."

Sirius craned his neck upwards and gaped at the giant red-orange disc floating in the darkness above their heads. "Whoa," he breathed. Maybe he'd just never paid much attention before, but he'd never seen the moon looking like that. The rest of the students, he noticed, were having similar reactions.

"Yes," Sinistra said with a nod. "Tonight we are experiencing a total lunar eclipse, the first in quite a while. This kind of eclipse occurs when the sun, Earth and full moon are perfectly aligned, and the shadow of the Earth blocks the sun's light from reaching the moon. Because of its reddish color, the moon under such conditions is often called a 'blood moon.'"

"Ooh, scary," said James, raising his eyebrows at Sirius. "Just in time for Halloween."

Professor Sinistra caught his remark, nodding in James's direction. "Indeed, Mr. Potter. Throughout history, many ancient civilizations have come up with myths and legends about the blood moon. It's often been connected to demon attacks, or evil celestial beings swallowing up the moon and turning it red. However, if any of you end up taking Divination next year, you'll learn that most Seers believe the blood moon is actually a good omen, bestowing luck and fortune upon anyone who falls under its influence."

"Maybe the blood moon will give us enough luck and fortune to figure out what's going on with Remus," Sirius muttered. He was still angry about what had happened with the Whomping Willow earlier, and very, very confused as to what Remus was doing in a secret tunnel underneath it. He'd been wondering about Remus for a while, of course; but it seemed like every time he and the others learned something new about him, the more questions they had.

"I've already set up all your telescopes to focus on the moon," Sinistra said. "Tonight, your assignment is simple: I want a sketch of the blood moon from each of you and a brief written description about its appearance."

Sirius found his telescope nestled between James's and Peter's; closing an eye, he peered up through the eyepiece at the red full moon. Through the telescope, it looked even cooler: its craters were tinted auburn-brown, its smooth surfaces scarlet with a lighter yellowish rim brightening its upper left edge. Sirius wished Remus could've seen it—he loved astronomy, and knew way more about it than Sirius, who had literally been named after a star. Remus seemed to know all the constellations in the sky by heart, and he could always tell his friends without a hint of hesitation what phase the moon was in, almost as if he were personally keeping track of it.

Personally keeping track of it. "Oh, Merlin," Sirius breathed. An idea had struck him, a crazy idea of what possibly could be going on with Remus. It was something that he would've muttered to James as a joke had it not made terrible, perfect sense. All the puzzle pieces that had been swirling around his mind for months came suddenly together in one gigantic, messy explosion. "No."

Sirius toppled backwards, falling back against the stone floor of the tower with his hands not bothering to cushion him. His head was reeling, the stars and the moon spinning through the sky like a top. It couldn't be true—but everything fit together way too well. The scars, the time in the hospital wing, the secret tunnel under the Willow, the blood moon…. The moon….

"Sirius?" James and Peter were crouching over him now, their faces wrinkled up with confusion. "Sirius, are you all right?" James asked him.

Sirius shook his head, feeling the color leaching from his cheeks. "I think I know what's wrong with Remus," he said shakily. "I think…I think he's a werewolf."