A noon sight is a sextant observation of the sun’s altitude at local apparent noon, the moment the sun crosses your meridian and reaches its highest point in the sky. It is the oldest and simplest of all celestial observations, and it gives latitude directly with nothing more than a subtraction and an almanac page: no sight-reduction tables, no plotting sheet and, for the latitude alone, no accurate time. A beginner needs to understand that the sun at noon bears exactly north or south, so its altitude depends only on your latitude and the sun’s declination. An experienced navigator adds that the sun ‘hangs’ near its maximum altitude for several minutes, so a noon sight is excellent for latitude but poor for longitude, and that the raw sextant reading must be corrected for index error, dip and refraction before it means anything.
Why Noon Is Special
At any other time of day, a position from the sun means solving the spherical triangle between you, the sun and the pole. At meridian passage that triangle collapses into a straight line. The sun, your zenith and the celestial pole lie on one great circle, so the sun’s zenith distance – 90 degrees minus its corrected altitude – is simply the difference between your latitude and the sun’s declination.
Taking the Sight
Start by predicting local apparent noon. Take 1200, apply the equation of time from the Nautical Almanac, which says how far the real sun is ahead of or behind clock noon on that date, then convert your dead-reckoning longitude to time at four minutes per degree, adding for west longitude and subtracting for east.
Bring the sun’s lower limb down to the horizon and follow it up as it climbs, adjusting the micrometer to keep the limb just kissing the sea, and rock the sextant gently to make sure it is vertical. As noon approaches the sun slows, stops and begins to sink; the highest reading is your sextant altitude. Note the time of the maximum as well as you can, then apply the corrections:
- Index error: the small instrument error you measured against the horizon before the sight.
- Dip: a correction for your height of eye above the sea, from the almanac; the higher you stand, the farther the true horizon lies below the horizontal.
- Altitude correction: a combined figure for refraction, the sun’s semi-diameter and parallax, from the almanac table for the lower limb.
The result is the observed altitude, Ho. Subtract it from 90 degrees to get the zenith distance.
The Arithmetic
Look up the sun’s declination for the Greenwich time of your sight. If you and the sun are on the same side of the equator, latitude equals zenith distance plus declination; if on opposite sides, subtract. Suppose Ho works out to 52°30′ with the sun bearing south and a declination of 20°00′ north: zenith distance is 37°30′, and latitude is 37°30′ plus 20°00′, or 57°30′ north. Longitude from the noon sight is possible but coarse, because each minute of error in timing the maximum is a quarter of a degree of longitude; the classic ocean routine remains a morning sun sight for a position line, a noon sight for latitude and a running fix between them.
Cruising and Racing Perspectives
For the ocean cruiser the noon sight is both a backup and a daily ritual. It needs no electricity, it is nearly impossible to get badly wrong, and the ceremony of shooting the sun and announcing the noon position keeps the crew connected to the passage.
The offshore racer is more likely to meet it as a requirement. Some notices of race and offshore safety regulations still call for a sextant, an almanac and a navigator who can use them, on the sound principle that a fleet a thousand miles from land should not depend entirely on satellites. In practice the racing navigator’s day is ruled by weather models and polars, but the noon sight is the one piece of celestial work quick enough not to distract from sailing the boat. See also sextant, meridian passage, latitude, running fix and dead reckoning.
Frequently Asked Questions about the Noon Sight
Do I need an accurate watch for a noon sight?
For latitude alone, no; you only need to catch the sun’s maximum altitude and know the date well enough to look up declination, which changes slowly. For any estimate of longitude you need time to within seconds, and even then the noon method is rough.
What if it is cloudy at noon?
Then you miss it, which is why the traditional routine also includes morning and afternoon sun sights and star sights at twilight. A single missed noon is no disaster if the dead reckoning is kept up.
Why subtract the altitude from 90 degrees?
Because latitude is measured from the equator and altitude from the horizon, and the two meet through the zenith. Ninety degrees minus altitude is the angular distance from the sun to the point directly overhead, and that zenith distance combines with declination to give latitude.
The noon sight is celestial navigation stripped to its essentials: one instrument, one page of the almanac and a subtraction. Learn it and you will always be able to find your latitude at sea, and you will understand in your hands what the chart’s minutes of latitude actually mean.