Course / The assumed position and LHA

Part III · The line of position · Page 1 of 4

LHA: the hour angle from your meridian

GHA measures the Sun's ground position west from Greenwich. What matters for your sight is how far west it is from your meridian: the local hour angle, LHA.

Greenwich (London)Sun overheadGHALHAYouN
GHA runs west from Greenwich; LHA runs west from your own meridian. Drag "You" east and west.
GHA (from Greenwich, west)
75°00.0'
Your longitude
W 040°00.0'
LHA = GHA + east, − west
75°00.0' − 40°00.0' (west) = 35°00.0'

LHA is the GHA as seen from your meridian instead of Greenwich's.

LHA = GHA + east longitude, or GHA − west longitude

Keep LHA between 0° and 360°: if the sum is negative, add 360°; if it passes 360°, subtract 360°. A memory aid: "longitude west, GHA best; longitude east, GHA least" (Greenwich's hour angle is bigger when you are west).