Decide
A body's GHA is 200°10′. What is the longitude of its geographical position?
The lesson in plain words
- Declination
- Declination (Dec) is a body's angle north or south of the celestial equator, measured along its hour circle, 0° to 90° N or S.
- It is the sky's version of latitude.
- The Sun's declination changes slowly, by at most about 1′ an hour; a star's hardly changes in a year.
- Greenwich hour angle
- Greenwich hour angle (GHA) is the angle at the pole measured westward from the Greenwich meridian to the body's hour circle, 0° to 360°.
- It is like longitude, but always counted west.
- The Sun's GHA increases by about 15°00′ an hour.
- Aries's increases by 15°02.5′.
- Local hour angle
- Local hour angle (LHA) is the same angle measured westward from your meridian: LHA = GHA + east longitude, or − west longitude, adding or removing 360° to keep it from 0° to 360°.
- LHA 0° means the body is on your meridian.
- Under 180°, it is west of you; over 180°, east of you.
- Sidereal hour angle
- Stars keep almost fixed places against each other.
- So the almanac gives each star's sidereal hour angle (SHA), measured westward from Aries, and its Dec, and tabulates only GHA Aries hour by hour.
- Then GHA star = GHA Aries + SHA, less 360° if the sum passes 360°.
- The geographical position
- The point on Earth directly beneath a body is its geographical position (GP).
- Its latitude equals the declination.
- Its longitude is the GHA, west, when GHA is 180° or less; otherwise 360° − GHA, east.
- At 1300 UT on 26 November 2026 the Sun's GHA is 18°10.6′ and its Dec S 20°59.9′: its GP is 20°59.9′S 018°10.6′W, in the South Atlantic.
Key data
- Declination
- 0–90° N or S of the celestial equator
- GHA
- 0–360°, measured westward from Greenwich
- LHA
- GHA + E long − W long; keep within 0–360°
- GHA of a star
- GHA Aries + SHA (− 360° if over)
- GP
- Lat = Dec; long = GHA (W) or 360° − GHA (E)
- Spanish terms
- Declinación, horario en Greenwich, horario del lugar, ángulo sidéreo
Key points
- Dec is like latitude; GHA is like longitude but always measured west, 0–360°.
- LHA = GHA + E long − W long, kept between 0° and 360°.
- LHA under 180°: body west of you. Over 180°: east of you.
- GHA star = GHA Aries + SHA.
- The GP is the point beneath the body: lat = Dec, long from GHA.
Check what you have read
GHA 15°20′, your longitude 040°00′W. What is the LHA?
True or false?
Decide each statement before you see the answer.
Add west longitude to GHA to get LHA.
LHA = GHA + E long − W long, kept between 0° and 360°.
LHA under 180°: body west of you. Over 180°: east of you.
GHA is measured east or west, up to 180°, like longitude.
Each star's GHA is tabulated hourly in the almanac.
GHA star = GHA Aries + SHA.
Practice case & exam keys
Situation
At 1300 UT on 26 November 2026 the Sun's GHA is 18°10.6′, Dec S 20°59.9′. You are at 21°10′N 027°45′W. Find the Sun's GP and LHA, and say whether the Sun has crossed your meridian yet.
Show the answer
GP: latitude = Dec, 20°59.9′S. GHA is under 180°, so longitude 018°10.6′W. LHA = 18°10.6′ − 27°45.0′ = −9°34.4′, plus 360° = 350°25.6′. Over 180°, so the Sun is still east of you: your noon is still to come, about 38 minutes later.
Exam keys
- Dec: 0–90° N/S of the equinoctial. GHA: 0–360° westward from Greenwich.
- LHA = GHA + E long − W long, wrapped to 0–360°; under 180° the body is west.
- GHA star = GHA Aries + SHA.
- GP latitude = Dec; GP longitude = GHA west, or 360° − GHA east.