Two noon sights, worked
1The lesson in plain words
One November sight with contrary names off West Africa and one May sight with the same name near the Azores, both from real 2026 almanac values.
Example 1: the data
26 November 2026, DR 21°10′N 027°45′W, bound west. Noon at 1338 UT. Sun lower limb, meridian Hs 47°31.4′, Sun bearing south. IE 1.8′ off the arc, height of eye 2.5 m. Dec at 1338 UT: S 21°00.2′.
Example 1: the answer
Ho = 47°45.8′, as corrected in the sextant unit. ZD = 90° − 47°45.8′ = 42°14.2′, named N because the Sun bears south. Dec S: contrary names, subtract: 42°14.2′ − 21°00.2′ = 21°14.0′N. The DR was 21°10′N: you are 4 M north of it.
Example 2: the data
13 May 2026, DR 37°50′N 035°20′W, bound for the Azores. At meridian passage, the Sun bears south. Corrected altitude Ho is 70°42.8′; declination is N 18°28.8′.
Example 2: the answer
ZD = 90° − 70°42.8′ = 19°17.2′, named N. Dec N: same name, add: 19°17.2′ + 18°28.8′ = 37°46.0′N, 4 M south of the DR. The US Naval Observatory's online almanac computes altitudes of 47°45.8′ and 70°42.8′ at these latitudes, a check on both examples.
Using it
The noon latitude is a position line running east–west. It tells you how far north or south you are, but does not give longitude. Historically, navigators reached an island's latitude, then sailed east or west along it: running down the latitude.
2Key data
- Example 1 data
- 26 Nov 2026: Ho 47°45.8′, Sun S, Dec S 21°00.2′
- Example 1 latitude
- ZD 42°14.2′ N − Dec 21°00.2′ S = 21°14.0′N
- Example 2 data
- 13 May 2026: Ho 70°42.8′, Sun S, Dec N 18°28.8′
- Example 2 latitude
- ZD 19°17.2′ N + Dec 18°28.8′ N = 37°46.0′N
- Position line
- The noon latitude runs east–west
3Key points
- 1
Ex. 1 (contrary): 42°14.2′ N − 21°00.2′ S = 21°14.0′N.
- 2
Ex. 2 (same name): 19°17.2′ N + 18°28.8′ N = 37°46.0′N.
- 3
Always name ZD from the Sun's bearing before combining.
- 4
The noon latitude is an east–west position line.
4Frequent mistakes
Wrong: In November off Africa, add Dec because both latitude and ZD are north.
Right: Compare ZD with Dec: ZD N, Dec S, contrary, so subtract.
Wrong: A noon sight gives a full fix.
Right: It gives latitude only, an east–west position line. For methods of combining position lines, see Celestial: sight reduction.
5Practice case & exam keys
On 26 November 2026 your mate works the noon sight and gets 63°14.4′N. The DR is 21°10′N. Find the mistake and the right answer, using Ho 47°45.8′ and Dec S 21°00.2′.
Show the answer
63°14.4′ is ZD + Dec: the mate added a south Dec to a north ZD. ZD = 42°14.2′, named N because the Sun bore south; Dec is S, contrary, so subtract: 42°14.2′ − 21°00.2′ = 21°14.0′N. That agrees with the DR within 4 M, a further check.
Exam keys
- Contrary names: subtract the smaller magnitude from the larger; keep the larger’s name
- Same names: add the magnitudes and keep their common name.
- Check the result against the DR; a large difference means recheck.
- Running down the latitude: reach the destination's latitude, then sail east or west.
Finish this unit
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