Almanac (Nautical)

The nautical almanac is the annual ephemeris a navigator opens to reduce a celestial sight. Its daily pages tabulate, at hourly intervals of GMT (UT), the Greenwich Hour Angle and declination of the Sun, Moon, the four navigational planets, and the GHA of Aries, plus the SHA and declination of 57 selected stars. Increments-and-corrections pages give the minute-and-second interpolation, and the back pages carry altitude-correction tables, the Polaris (a0, a1, a2) tables, and twilight and rise/set data. The almanac supplies GHA and dec; sight-reduction tables or a calculator then yield Hc and Zn.