“A seaman always knows his latitude” is an ancient proverb which emphasizes the importance attached to knowledge of the ship's latitude. Hansen’s Ex-Meridian Tables, first published in 1919, have for over a century proven to be essential for mariners to calculate their position on Earth.

The method of finding the latitude of a ship at sea from an observation of the sun is still practiced – even as it became obsolete as technology improved. Historically the only time the ship accurately knew its position in the open ocean was at noon every day, and at other times based on dead reckoning.

To get an accurate latitude this way mariners will measure either the peak upper or lower transit on their sextant. However, if this isn’t possible – either the sun is obscured, or at twilight, you might realize the star or planet you’ve been following is already going back up – the mariner will still be able to get a near miss.

Hansen’s Ex-Meridian Tables are used when a near miss occurs and allows the mariner to still calculate their latitude.