Decide
Put the steps in order: Sun's GHA, Dec and LHA for a sight.
Tap the step that comes first, then the next one.
The lesson in plain words
- The tinted pages
- The Increments and Corrections tables, on tinted pages near the back, have one table for each minute, 0m to 59m.
- Each lists, second by second, the GHA increment for the Sun and planets (15° an hour exactly), Aries (15°02.46′ an hour) and the Moon (14°19.0′ an hour).
- Beside them, the v or d corrections give the extra for a given v or d at that minute.
- Worked: the time
- 26 November 2026, morning, DR 21°05′N 027°20′W.
- Sun lower limb observed at deck watch 10-14-38.
- The watch was checked against GNSS: 12 s fast.
- UT = 10h 14m 38s − 12 s = 10h 14m 26s, still 26 November.
- With ZD +2 the ship's clock read about 0814.
- Worked: GHA
- GHA at 10h from the daily page: 333°11.2′.
- Increment for 14m 26s, from the 14m table, Sun and planets column: 3°36.5′.
- Check: 14 min × 15′ = 3°30.0′, plus 26 s × 15″ = 6.5′.
- GHA = 333°11.2′ + 3°36.5′ = 336°47.7′.
- Worked: Dec
- Dec at 10h: S 20°58.5′; at 11h S 20°59.0′, so the S magnitude is increasing: add. d = 0.5.
- The 14m table's v or d column gives 0.1′ for d 0.5. Dec = S 20°58.6′.
- Enter 26 November 2026, 10:14:26 UT in USNO's celestial-navigation service: GHA 336°47.6′, Dec S 20°58.6′.
- The GHA differs by only 0.1′.
- Worked: LHA
- LHA = GHA − west longitude = 336°47.7′ − 27°20.0′ = 309°27.7′.
- It is over 180°, so the Sun is east of you, as it should be in the morning.
- DR latitude, Dec and LHA are the inputs to sight reduction (Celestial: sight reduction).
Step by step
Sun's GHA, Dec and LHA for a sight
- Correct the watch time to UT and check the date. The almanac is in UT; a wrong hour or day ruins the sight.
- Take GHA and Dec for the whole hour from the daily page. The page gives them only on the hour.
- Add the increment for the minutes and seconds, Sun column. The tinted table for that minute gives it second by second.
- Apply the d correction to Dec, with the sign from the trend. Dec changes up to about 1′ an hour.
- Find LHA: add east longitude, subtract west, keep within 0–360°. Sight reduction needs the angle from your own meridian.
Key data
- Sun and planet increment
- 15° an hour: 15′ a minute, 15″ a second
- Aries increment
- 15°02.46′ an hour
- Moon increment
- 14°19.0′ an hour, plus v
Key points
- Whole hour from the daily page, minutes and seconds from the increments.
- Use the column for your body: Sun and planets, Aries or Moon.
- Apply d from the v or d column, sign from the trend.
- LHA over 180° means the body is east of you: morning for the Sun.
Check what you have read
What is the Sun's GHA increment for 22m 40s?
True or false?
Decide each statement before you see the answer.
Apply the d correction for the whole hour.
LHA over 180° means the body is east of you: morning for the Sun.
Use the column for your body: Sun and planets, Aries or Moon.
Use the Aries column for the Sun's increment.
Practice case & exam keys
Situation
Same day, 26 November 2026, a sight at 13h 38m 00s UT, DR longitude 027°45.0′W. Daily page at 13h: GHA 18°10.6′, Dec S 20°59.9′, d 0.5, S magnitude increasing. A crew member gets a negative LHA and changes to adding west longitude. Is that justified? Give the corrected results.
Show the answer
Increment 38m 00s = 9°30.0′, so GHA = 18°10.6′ + 9°30.0′ = 27°40.6′. d correction at 38m for d 0.5: 0.3′, added: Dec S 21°00.2′. LHA = 27°40.6′ − 27°45.0′ = −0°04.4′, plus 360° = 359°55.6′. Reject that change: subtract west longitude, then add 360° to wrap the negative result. The Sun is just east of your meridian, approximately local noon.
Exam keys
- GHA = hourly GHA + increment (Sun/planets 15°/h; Aries 15°02.46′/h; Moon 14°19.0′/h + v).
- Dec = hourly Dec ± d correction for the minutes; sign from the trend.
- LHA = GHA + E long − W long, within 0–360°.