Limits and good practice
1The lesson in plain words
The noon sight is simple and robust, but it gives latitude only, needs a clean horizon and becomes awkward when the Sun is nearly overhead.
Very high altitudes
When your latitude is close to the declination, the Sun passes almost overhead, above about 85°. Its altitude then rises and falls quickly near noon and its bearing swings fast, so it is hard to keep the sextant vertical and catch the maximum. Take extra care, or use timed sights instead (Celestial: sight reduction).
Horizon and cloud
Everything rests on a sharp horizon below the Sun. Haze, a big swell or a low eye in breaking seas can give errors of several minutes. If cloud hides noon, use an ex-meridian or intercept sight a few minutes either side.
Latitude only
The noon sight gives latitude, nothing more. Longitude from the time of the maximum is rough: the Sun hangs for minutes, and each minute of time is 15′ of longitude. For a fix, cross the noon latitude with another position line.
Common errors
ZD named the wrong way; the wrong Dec name or day; 90° − Hs instead of Ho; assuming local noon is 1200 on the clock; reading too early because a wave lifted the horizon. Compare with the DR: a large difference is a reason to recheck the working, not to trust it blindly.
At twilight
In the northern hemisphere, Polaris offers a simple latitude check. See Celestial: stars, planets and the Moon for the method.
2Key data
- Above about 85°
- Altitude and bearing change fast: hard to observe
- Gives
- Latitude only
- Longitude from timing
- Rough: 1 min of time = 15′ of longitude
- Check
- Compare with the DR; recheck large differences
- At night
- Latitude by Polaris (stars-planets unit)
3Key points
- 1
Above about 85° the noon sight becomes awkward.
- 2
A sharp horizon matters more than anything else.
- 3
Noon gives latitude only; longitude from timing is rough.
- 4
Check the result against the DR, and recheck big differences.
4Frequent mistakes
Wrong: The time of the maximum gives an accurate longitude.
Right: The Sun hangs for minutes; 1 minute of time is 15′ of longitude. Use timed sights for longitude.
Wrong: A latitude far from the DR must be right because it is a sight.
Right: Recheck the sight’s names, date and corrections first. The DR can also be wrong; a large difference does not tell you which result to trust.
5Practice case & exam keys
Crossing the Atlantic in late June at about 23°N, a crew member wants to rely on the noon sight alone for position. The Sun's Dec is about N 23°26′. What problems do you foresee, and what do you do?
Show the answer
The Sun passes almost overhead, so its bearing swings quickly and the maximum is awkward to observe. Even a good noon sight gives latitude only. Keep the DR current. For alternative observations and how to combine position lines on a moving vessel, see Celestial: sight reduction.
Exam keys
- Above about 85° the meridian altitude is hard to observe.
- Noon latitude only; longitude from timing is rough (1 min = 15′).
- Common errors: ZD name, Dec name or day, Hs instead of Ho.
- Polaris gives latitude at twilight in the northern hemisphere.
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