Celestial Navigation

Celestial navigation treats the sky as a sphere of infinite radius concentric with Earth. Every coordinate used in sight reduction is a projection of terrestrial coordinates onto that sphere.

The marine sextant measures the angle between a body and the visible horizon. Converting that sextant altitude to a true observed altitude is a disciplined sequence of corrections.

Comparing a computed true bearing of a celestial body with the observed compass or gyro bearing gives compass error at sea, far from any landmark.

A four-second timing error displaces a sight by one nautical mile of longitude. Celestial navigation is fundamentally an exercise in precise time.

The almanac supplies the position of every navigational body for each hour of UT, plus the tables and corrections needed to interpolate and reduce sights.

The intercept method compares the observed altitude with the altitude computed for an assumed position. The difference, plotted along the azimuth, is a celestial line of position.