No parallax: navigation mark on St Agnes

On the end of the island is St Agnes there is a white pointed column with a black strip down it. If you line it up with the St Agnes lighthouse when you are out at sea, you can sail safely along that line. If two objects don't line up, we say that there is a parallax error. Parallax is used a lot in Physics. The parallax for stars caused by the Earth's motion around the Sun can be used to measure distances to stars. A no parallax method is used to find the position of a virtual image in a mirror.