The latitude of a ship is discovered by observing the altitude of the sun by means of a sextant. At midday a
captain finds out exactly the angle formed by lines drawn from the point of observation to the horizon and
the sun respectively.Suppose this angle be 30°, he will then be in latitude 90° — 30° =r 6o°. But this is
assuming that the sun is always in the position, in which it stands, at the equinox—overhead at the equator.
But the sun is not always overhead at the equator, so allowance must be made for its apparent change of position The captain of a ship is always provided with an almanac, called “ The Nautical Almanac,” from which he can learn what allowance must be made on any day of the year for the sun’s declination.
If he is sailing in the northern hemisphere he must subtract the number of degrees of declination from the angle between the sun and the horizon when the sun is in north declination (March 21st to September 21st), and must add
them wnen it is in the south declination (September 21st to March 21 st). I f he were sailing in the southern
hemisphere, the calculations would be, naturally, reversed.