No sea horizon: the artificial horizon

A sextant measures the altitude of a body above the horizontal. At sea the horizon gives the horizontal (a little below it: that is dip). On land the horizon is hills, trees and buildings, so the navigator makes a level surface instead: an artificial horizon. A shallow tray of liquid levels itself; a mirror can be leveled with a carpenter's level.

Look down into the tray and you see the Sun reflected, as far below the horizontal as the Sun is above it. With the sextant, bring the Sun down onto its own reflection: the angle you measure is twice the altitude.

What changes

• Halve the reading, after applying the index correction.
• No dip. Dip comes from your height above a sea horizon; a level surface has none.
• The rest is the same: the main correction from the almanac, then the almanac, LHA, Pub. 249 and the plot, exactly as at sea.
• Errors are halved too. A 1′ slip in the reading is ½′ of altitude: land sights can be very accurate.
• Limits. Most sextants read to about 120°, so above an altitude of about 60° the body is too high. Very low bodies are hard too: the tray’s rim, trees and walls get in the way.

A bubble sextant (made for aircraft) has its own level built in and reads the altitude directly, with no halving and no dip. Navigators in deserts and explorers on expeditions used both.