UNIT 1: Celestial Navigation Definitions
Sidereal Hour Angle (SHA)
angular distance west of the vernal equinox. we use this to determine the position of stars and planets
Zenith
the point on the celestial sphere vertically above a given position or observer
Celestial Sphere
Imaginary sphere with Earth at the center of the radius
LHA (Sun)
LHA Sun= GHA Sun +/- Longitude West(-)/East(+)
Four Motions of the Earth
1. Rotation 2. Revolution 3. Precession 4. Nutation
LHA (Star)
=GHA Star +/- Longitude West(-)/East(+)
Parallel of Declination
A circle passing through the celestial object and parallel to the celestial equator.
Hour Circle
A great circle on the celestial sphere passing through the celestial poles. A line from the pole, through the celestial object, intersecting the celestial equator at right angles.
Meridian Angle
If LHA< 180 degrees then t=LHA West If LHA>180 degrees then t=360-LHA East
90 degree +/- Declination
If declination and latitude same name PD=90-Declination If declination and latitude contrary name PD=90+Declination
Celestial Equator or Equinoctial
Intersection of the extended plane of the equator and Celestial Sphere
Circumpolar
Never set below the horizon
Nadir
Point on celestial sphere below given position or observer
Declination
The arc of an hour circle between the celestial equator and the celestial body. Measured northward or southward from the celestial equator through 90 degrees.
Diurnal Circle
The circular path that a celestial body traces out as it appears to move across the sky during an entire day.
Local Hour Angle (LHA)
Angular distance west of the observers celestial meridian
Hour Angle
Angular distance measured westward from reference point
Greenwhich Hour Angle (GHA)
Angular distance west of Greenwhich celestial meridian
Polar Distance
Angular distance from the celestial pole, the arc of an hour circle between a celestial pole, usually the elevated pole, and point on the celestial sphere, measured from the celestial pole through 180 degrees
