Celestial navigation glossary

Hour circle

A half great circle on the celestial sphere from pole to pole through a body: the sky’s meridian. Hour angles are measured between them.

Where the course teaches it

The celestial sphere, in the module Where the Sun is: GHA and declination. Read the Sun's GHA and declination from the almanac for any second of UT.

Ground position (GP): The point on the Earth directly under a body: the body is straight overhead there.
Greenwich hour angle (GHA): The GP’s angle west of Greenwich, 0° to 360°: a longitude measured westward.
Declination (Dec): The GP’s latitude: how far north or south of the equator the body is.
Celestial sphere: The sky drawn as a huge sphere with the Earth at its center; each body is placed where its direction meets it.
Celestial poles: The Earth’s axis extended to the celestial sphere; the sky turns about them once a day.
Celestial equator (equinoctial): The Earth’s equator projected onto the celestial sphere; declination is measured from it.
d correction: The change in declination over the hour, scaled to the minutes past the hour.
Increments and corrections: The almanac’s tables of GHA gained in the minutes and seconds past the hour.