Predicting the time of local noon
1The lesson in plain words
At local noon the Sun crosses your meridian. Latitude needs simple addition or subtraction. Predict noon from Mer. Pass. and your longitude.
Why noon
When the Sun crosses your meridian, it bears due north or south. The celestial triangle's three points—the pole (P), your zenith directly overhead (Z), and the Sun (X)—lie on your meridian. Latitude follows by adding or subtracting, without reduction tables. The noon sight uses the Sun's meridian altitude, its height at passage. On a slow yacht, the highest altitude is a close approximation.
The time
Take Mer. Pass. from the daily page: the LMT of the Sun's meridian passage, to the minute. Add west longitude in time, or subtract east, for the UT of meridian passage at your longitude. Convert to the ship's clock with the zone description (Celestial: time and the almanac; Navigation: Polaris and time).
Worked
26 November 2026, DR at noon 21°10′N 027°45′W. Mer. Pass. 1147. Longitude 27°45′ = 1h 51m, west: 1338 UT. With zone description +2 the ship's clock reads 1138. Dec for 1338 UT: at 13h S 20°59.9′, d 0.5, increasing; +0.3′ for 38 minutes gives S 21°00.2′.
A moving vessel
Use the longitude you expect at noon, not the morning one. Each 15′ of longitude you make westward makes noon 1 minute later; eastward, earlier. Be ready about 10 minutes before the predicted time: that covers a small error in the DR longitude.
Dec at noon
The Sun's declination changes by at most about 1′ an hour, so take Dec for the noon UT with the d correction. A 4-minute error in the time changes Dec by less than 0.1′. Latitude by the noon sight does not depend on accurate time.
2Key data
- Mer. Pass.
- LMT of the Sun's meridian passage, to the minute
- UT of local noon
- Mer. Pass. + W long in time (− E)
- Worked
- 1147 + 1h 51m = 1338 UT; ZT 1138 (zone description +2)
- Moving west
- Each 15′ of longitude west: noon 1 min later
- Dec at noon
- S 21°00.2′ at 1338 UT, 26 Nov 2026
- Spanish terms
- Meridiana, paso del Sol por el meridiano, altura meridiana
3Key points
- 1
At local noon the Sun bears due N or S: latitude by simple arithmetic.
- 2
UT of noon = Mer. Pass. + W long in time (− E).
- 3
Use the DR longitude expected at noon.
- 4
Take Dec for the noon UT; exact time is not critical.
4Frequent mistakes
Wrong: Local noon is at 1200 on the ship's clock.
Right: Work it out from Mer. Pass. and longitude: 1138 ZT in the worked example.
Wrong: Use the declination for 1200 UT.
Right: Use Dec for the UT of your noon, 1338 UT here; in the Atlantic that can be hours from 1200 UT.
5Practice case & exam keys
On 13 May 2026, heading for the Azores, your DR for noon is 37°50′N 035°20′W, zone description +2. The daily page gives Mer. Pass. 1156; Sun Dec at 14h N 18°28.6′, d 0.6, increasing. Your mate plans to start observing at 1230 ZT. What start time and Dec would you choose?
Show the answer
Noon is about 1417 UT, or 1217 ZT. Start around 1207 ZT, about ten minutes beforehand; 1230 is too late. At 1417 UT, the increasing Dec gains about 0.2′, giving N 18°28.8′.
Exam keys
- Noon sight: use the Sun's altitude on your meridian, bearing N or S.
- UT of noon = Mer. Pass. + W long in time (− E); ZT = UT − zone description.
- Use the noon DR longitude; 15′ of longitude = 1 minute.
- Dec for the noon UT with the d correction.
Finish this unit
When you've read it, answer three questions. Get all three right to finish the unit; opening the page alone doesn't count.
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