Celestial navigation glossary

d correction

The change in declination over the hour, scaled to the minutes past the hour.

Where the course teaches it

Minutes and seconds past the hour, 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.
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.
Increments and corrections: The almanac’s tables of GHA gained in the minutes and seconds past the hour.