User blog comment:Hermione Chase/"The Blood of Olympus" Expectations/@comment-6064271-20131104034639/@comment-24062146-20131104130855

An astrolabe is a navigation device for a ship. This one is designed to find Calypso's Island. However, the inventor need a crystal from the island to make it work, which he didn't have. Leo took one, so now plans to use the device to find his way back. We get most of this information from the mischief  monkeys, who had stolen it years ago and had it stashed among their stuff.