Chapter 19: The Nautical Almanac
The almanac supplies the position of every navigational body for each hour of UT, plus the tables and corrections needed to interpolate and reduce sights.
Daily pages
Each daily opening covers three days. The left page tabulates GHA and declination hourly for Aries, Venus, Mars, Jupiter and Saturn, plus the SHA and declination of 57 selected stars. The right page gives the sun and moon, with v and d corrections, moon horizontal parallax, times of sunrise, sunset, twilights, moonrise and moonset, and the sun's semi-diameter and equation of time in the daily notes.
Increments and corrections
The yellow Increments and Corrections pages ('Aries/sun/planets/moon' columns) convert minutes and seconds of UT into additional hour angle. The v correction handles the planets' and moon's irregular hour-angle rate; the d correction interpolates declination change. The altitude correction tables at the front give refraction, semi-diameter and parallax for sun, stars, planets and moon, with dip in a side column.
- Enter the increments page by minutes, then read seconds
- Apply v and d with their tabulated signs — d follows the direction declination is changing
- Polaris tables give latitude directly from a Polaris altitude plus a₀, a₁, a₂
Meridian passage
Local apparent noon occurs when the body's LHA is 0°; the almanac's tabulated time of meridian passage, adjusted for longitude, predicts it. A noon latitude is the simplest of all sights: latitude = (90° − Ho) ± declination with the zenith distance named toward the elevated pole.
Formula cards
Latitude by meridian altitude
Lat = (90° − Ho) named N or S away from the body, combined with declination
Zenith distance plus declination, applying like/unlike naming rules.
Key takeaways
- • Daily pages give hourly GHA and dec; the increments pages supply minutes and seconds.
- • v corrects hour angle rate; d corrects declination change.
- • Meridian altitude gives latitude with no need for hour angle at all.
Knowledge check
Immediate feedback with rationales. Answer each question to see the explanation.
Chapter 19 quiz
0 of 3 answered
1.The v correction in the Nautical Almanac accounts for:
2.A meridian-altitude observation of the sun yields directly:
3.Star SHA and declination are tabulated: