*Spoilers*
So assuming you have access to the mountain peak, you should have gotten a brief dialog saying to check out Horace's house in feasterra. go there, click on the dog bed and you'll find a key. That key opens the plunger in "The Gents" room in the dungeon during night. This will be in the stall closets to the door. Use the plunger in Grants room on the telescopes on the wall. When you get the lens, use it on Ghart's telescope (It's on his balcony) at night and the location to the dragon will be unlocked, at night only.