Decide
Question 1/1
What does d at the foot of the Sun column give?
The lesson in plain words
- One opening, three days
- The Nautical Almanac (NP314), produced by HM Nautical Almanac Office and the US Naval Observatory, gives one double page for every three days.
- Left page: GHA of Aries, GHA and Dec of the four planets for every hour of UT, and the SHA and Dec of the 57 selected stars.
- Right page: the Sun and Moon, twilight, sunrise, sunset and moonrise.
- The Sun's columns
- Each hour, 00h to 23h UT, gives GHA and Dec to 0.1′.
- At the foot, SD is the Sun's semi-diameter: half its apparent angular diameter.
- d gives the Sun's mean hourly change in declination. It has no sign.
- With N/S labels, compare the numerical magnitudes at successive hours: add if the magnitude grows, subtract if it shrinks.
- Equation of time and Mer. Pass.
- In the lower corner of the right page: the Sun's equation of time at 00h and 12h, and its Mer. Pass., the local mean time of meridian passage, to the minute.
- When the Sun runs ahead of mean time, as in November, Mer. Pass. is before 1200; when it runs behind, as in February, it is after 1200.
- Reading an entry
- 26 November 2026 at 10h UT: Sun GHA 333°11.2′, Dec S 20°58.5′; foot of the column d 0.5, SD 16.2′; Mer. Pass. 1147.
- At 11h Dec is S 20°59.0′: the Sun is moving south, so add the d correction to the S magnitude.
- v and d
- Between whole hours you add an increment at a standard rate (next part).
- For planets and the Moon the real rate differs from that standard; the hourly excess is v, printed beside them.
- The Sun's GHA values are adjusted so the standard rate works without v.
- d gives the hourly declination change for the Sun, Moon and planets; selected-star declinations have no d correction.
Key data
- Coverage
- Three days per double page, every hour of UT
- Left page
- Aries, Venus, Mars, Jupiter, Saturn; 57 stars
- Right page
- Sun, Moon, twilight, sunrise/sunset, Mer. Pass.
- d
- Mean hourly Dec change; compare N/S magnitudes
- 26 Nov 2026, 10h
- Sun GHA 333°11.2′, Dec S 20°58.5′, d 0.5, SD 16.2′
- Spanish terms
- Almanaque Náutico (ROA): páginas diarias, horario, declinación
Key points
- One opening = three days, every hour of UT.
- Sun: hourly GHA and Dec; SD and d at the foot.
- With N/S labels, add the d correction if the magnitude grows next hour; subtract if it shrinks.
- Mer. Pass. is the LMT of the Sun's meridian passage.
- Planets and the Moon carry v; the Sun does not.
Check what you have read
Question 1/3
How do you decide whether to add or subtract the d correction?
True or false?
Decide each statement before you see the answer.
Mer. Pass. is the UT of noon at your position.
Planets and the Moon carry v; the Sun does not.
Mer. Pass. is the LMT of the Sun's meridian passage.
d is always added because it is printed without a sign.
Practice case & exam keys
Situation
At 10h, Dec is S 20°58.5′; at 11h, S 20°59.0′. A crew member subtracts a 0.1′ correction from the magnitude and writes S 20°58.4′ because south is negative. Is that right?
Show the answer
No. The southward magnitude is increasing, so use S 20°58.6′. With signed numbers, −20°58.5′ − 0.1′ gives the same result. Keep your convention consistent.
Exam keys
- Three days per opening; hourly GHA of Aries, and hourly GHA/Dec of Sun, Moon and planets.
- SD and d at the foot of the Sun column; apply d to the N/S magnitude using its hourly trend.
- Mer. Pass. is LMT of the Sun's meridian passage, to the minute.
- v corrects GHA for planets and Moon; d corrects Dec for Sun, Moon and planets.