Sirius, James, and Peter ran through the Hogwarts grounds as the night began to darken. They didn't bother to put the Invisibility Cloak back on, because it was still a few hours before curfew, and they wouldn't get in trouble for roaming through the halls. Although James realized after a while that Sirius wasn't leading them into the main castle.
"The Astronomy Tower?" James questioned with surprise and confusion mingled on his face as they reached the bottom of the tower.
"But — why — here?" Peter panted, looking up miserably at the many stairs they had to climb in order to get to the top.
"I'll explain everything in a minute," Sirius said hastily, hurrying up the staircase with James and Peter following behind him.
"You better!" James snapped angrily. "We've been running after you for hours, Black! This better be worth it!"
"Be patient, Potter!" Sirius retorted.
When they finally reached the top of the Astronomy tower, they were all out of breath, panting heavily. The top of the tower was colder than it was down bellow on the ground, specially now that they were in the middle of autumn. A chill struck on them as if the air had lost every particle of warmth it had ever possessed.
Without a single word, Sirius walked toward the Astronomy Tower's wall after he had caught his breath, looking over the many charts that were hung on the wall, looking for a specific one.
James was so close to punching Sirius in the face when instead of giving an answer, the boy started observing star and moon charts, looking over at them with a hungry look in his eyes, as though he was so close to a knowledge he had been searching for all his life.
"Well?" said James impatiently, placing his hands on his hips as he looked at Sirius over his lopsided glasses.
"In a minute," Sirius answered in a mutter, as though he was in a trance. As though his mind couldn't rest until he'd found the answer, and until he had made sure of his theories.
James gritted his teeth together, growing more and more impatient by the second. "Sirius, I'm this close to losing my shi—"
"Here it is! I knew it! I knew it!" Sirius boisterously exclaimed and punched the air in victory, his eyes still roaming the charts which consisted of the lunar calendar.
James huffed out loud and looked at the charts in front of Sirius, trying to see what they were and why they had gotten Sirius so excited. On the far left column of the lunar chart, the dates were written. The top row was split into four, each section showing a different shape of a moon; a full moon, a new moon, a first quarter, and a last quarter moon.
"Well tell us, damn it!" James fumed, running out of patience.
"Come look at this," said Sirius, gesturing Peter and James toward the tower's wall where the charts were hung. "See this?"
"Um... what are we supposed to be looking at?" Peter mused, his eyes fixed on the nonsense on the wall which only Sirius seemed to be understanding.
"Okay, so when was the last time we realized that Remus had gone missing?" Sirius asked them, trying to walk them through it.
"At the end of last year. I remember he went missing in the middle of our finale exams," said Peter. "But I still don't understand what this has to do with Astro—"
"That would mean he had gone missing around the third or fourth week of June," said Sirius. "26th of June, to be exact."
He then took out his wand and tapped the tip of it on the chart, causing a red circle to be drawn around where the date 'June 26th' was written.
"When else did Remus go missing?" Sirius went on.
"Well..." James started thoughtfully. "I remember he went missing a little while before Christmas break last year. Although I didn't pay much attention to it, cause at the time we were convinced that he'd been visiting his ill mother."
"All right. So would you say it happened around 17th of December?" Sirius questioned.
James shrugged. "I suppose so."
Sirius nodded earnestly before drawing a red circle around the 'December 17th' date on the lunar chart with the use of his wand.
"When else?" he said again.
"Does tonight count?" Peter said unsurely.
"Yes, today's November 19th," said Sirius, circling this date as well. He then placed his wand back in his pocket, looking at the two boys gravely. "Peter gave me the idea. I realized that tonight was the third time Peter had mentioned how beautiful the moon looks; all three times were exactly on the nights that Remus had gone missing."
"So?" James frowned.
"Look at the sky," said Sirius, pointing upward at where the moon was illuminating among the hundreds and hundreds of stars, peaking through the clouds. "The moon is full tonight. And it was full on June 26th and December 17th as well."
He pointed at where he had circled the three dates on the chart, and James realized with a jolt that they were all underneath the 'Full Moon' column.
"Oh, Lord... this can't be true..." James said in a breathless whisper, as if speaking it aloud might shatter the reality.
"What? What is it?" Peter asked eagerly, looking between Sirius and James.
"He's right... Sirius is right..." James muttered under his breath, his eyes fixed on the chart with horrified astonishment. "Look at the facts, Peter. Remus keeps missing around once a month, always on the full moon, always looks sick, always comes back with new wounds."
"So?"
James finally tore his eyes away from the chart, pushing up his glasses over his nose with the use of one shaky finger before looking back at Peter, saying,
"Remus is a werewolf."