Celestial navigation glossary
Horizontal parallax (HP)
The Moon’s parallax at the horizon; sets the second Moon altitude correction.
Where the course teaches it
The Moon, in the module Planets and the Moon. Reduce planet sights at twilight, and (advanced) a Moon sight for a daytime fix with the Sun.
Taught alongside it
- v correction
- Extra GHA for a planet or the Moon moving at a rate other than the table’s, scaled to the minutes.
- Upper and lower limb
- Top and bottom edges of the Sun or Moon: the edge you bring down to the horizon.