Course / The assumed position and LHA

Part III · The line of position · Page 2 of 4

LHA on the celestial sphere

Your zenith is the point on the celestial sphere straight above you; the hour circle through it is your celestial meridian. LHA is the angle west along the celestial equator from your meridian to the body's hour circle, just as GHA is from Greenwich's.

SunZenithN poleS poleLHA 56°your meridian
May 21, 15:00 UT. Earth fixed: the sky turns west. Drag to turn the sphere; drag the body to run the clock; drag your zenith to move yourself. Not to scale: the real sphere is so large the Earth would be a point.

Local hour angle

Like GHA, but measured west from YOUR celestial meridian (the hour circle through your zenith): LHA = GHA + east longitude, or − west longitude.

GHA
45°51.0'
Your longitude
10° E
LHA
55°51.0'

Move your longitude east and LHA grows by the same amount; move west and it shrinks. Compare it with GHA: the difference is always your longitude.