The Legend of Zelda: Majora's Mask Walkthrough

Majora's Mask p.13

Astral Observatory
There are some stairs here, some pots and... a Scarecrow that's alive... right, well... you'll come back to that later. First, go up the stairs and around to talk to the Professor Shikashi, who likes to stand a little too close for comfort when speaking to you. He rambles a bit, explaining that the Skull Kid has been causing trouble here as well. Are you seeing some sort of ongoing theme? After a moment, he'll ask you if you'd like to look into the telescope. Why yes, thank you, that'd be lovely.

Gaze around if you'd like. If you look to the left, you can actually see a dude off in the grass doing jumping-jacks. Odd. Look towards the Clock Tower and press A to zoom in on the Skull Kid who is atop it. The Skull Kid will glance at the Moon, which will cause it to cry. Jeez, he's mean! The Skull Kid will then insultingly show off his rear.

Press B to stop using the telescope. Professor Shikashi rambles a bit, but he does mumble about something rather important: Skull Kid could only have gotten atop the Clock Tower by using the "clock door" which will only open on the eve of the carnival... Or it could be that he can levitate, as you've already seen....
After you're done talking to him, go outside to find your very own Moon's Tear. Yipee! This will come in handy in just a moment, and it's necessary to get some Pieces of Heart later on in the game, so remember how to get these! Go all the way back through the sewers and back to Clock Town.