Richard MacKenzie - Collections, Maritime Museum of British Columbia, Discussing the use of an octant

This is what is known as a mariner's octant and it is descendent from the mariners quadrant, which was developed at Sarges by the people who were working with Henry the Navigator. It's a very simple instrument for finding latitude. You have to pretend that it's night, and that we are going to be looking for the North Star because that's right over the pole. So, what I do is I hold this up and the light from the North Star bounces through this mirror [indicates the front of the octant], probably don't need to have this filtered. It goes through here, through here, and through here [pointing at the mirrors on the octant]. I'm going to be on a heaving deck and I'm looking at that, and once my false horizon and my real horizon line up and the star, I move this and I look down here [examining the bottom of the octant] and it gives me how many degrees south I am from the North Star and that gives me my latitude.