Course / Latitude from the noon Sun

Part II · First sights · Page 2 of 3

When is noon by my watch?

The Sun is near the top of its curve for only a few minutes, so you need to know when to be on deck. The almanac gives the time of meridian passage ("Mer. Pass.") at the foot of each daily page: the local mean time (LMT) at which the Sun crosses any meridian. It is within about 16 minutes of 12:00, because the Sun runs a little fast or slow through the year.

  1. Read Mer. Pass. for the date (LMT).
  2. Turn your DR longitude into time (15° = 1 hour, 1° = 4 minutes).
  3. UT of noon = LMT + west longitude, or − east longitude.
  4. Watch (zone) time = UT − zone description. Be on deck 10 or 15 minutes before.

January 14, 2026, DR W 157°50′, zone description +10. Mer. Pass. 1209.

Longitude in time: 157°50′ ÷ 15 = 10h31m20s.

UT of noon = 12h09m00s + 10h31m20s = 22h40m20s

Zone time = UT − 10 h = 12h40m20s, by the watch.

Worked to the second for this longitude the Sun crosses at 22h40m32s UT: whole minutes from the almanac are plenty for getting on deck.

The same steps, with the twilight table in place of Mer. Pass., tell you when to be ready for star sights: that comes in the module on planning a twilight fix.