A noon sight is the simplest and most reliable fix in celestial navigation. When the Sun crosses your meridian at local noon it reaches its highest point of the day, and its altitude together with its declination from the Nautical Almanac gives your latitude directly. No sight reduction tables and no exact time are needed. Offshore, it is the classic daily check of your position.
What you need
- A sextant and its index error, checked before the sight.
- Your height of eye above the water, for the dip correction.
- The Nautical Almanac for the day: the time of the Sun's meridian passage and its declination.
- A watch roughly set to UTC and your dead-reckoning longitude, only to know when to be on deck.
Step 1: work out when local noon will be
The daily page of the almanac gives the time of the Sun's meridian passage at Greenwich, for example 12:06. That is local time on any meridian; to turn it into UTC, add your longitude in time if you are west of Greenwich and subtract it if you are east. One degree of longitude is four minutes of time.
Example: meridian passage 12:06, DR longitude 18°30′ W. 18.5° × 4 = 74 minutes, so noon comes at about 13:20 UTC. The sextant calculator converts arc to time for you.
Step 2: take the sight
Be ready 10 to 15 minutes early. Bring the Sun's lower limb down to the horizon and keep it there as the Sun climbs, rocking the sextant to make sure it is vertical. The altitude rises more and more slowly, seems to hang for a minute or two, and then starts to fall. The highest reading is your meridian altitude. You do not need the exact time of it: latitude comes from the altitude alone.
If the boat is moving fast north or south, the Sun's highest point shifts slightly away from the true meridian passage. For a sailing yacht the error is small.
Step 3: correct the sextant altitude
Example: sextant altitude Hs 52°18.4′, index error 2.0′ off the arc, height of eye 3 m, Sun's lower limb, semi-diameter 15.8′.
| Correction | Value | Altitude |
|---|---|---|
| Sextant altitude Hs | 52°18.4′ | |
| Index error (off the arc: add) | +2.0′ | 52°20.4′ |
| Dip, 3 m height of eye | −3.0′ | Ha 52°17.4′ |
| Refraction | −0.8′ | 52°16.6′ |
| Semi-diameter, lower limb | +15.8′ | 52°32.4′ |
| Parallax | +0.1′ | Ho 52°32.5′ |
The sextant altitude correction calculator does these steps for any sight, including the upper limb, the Moon and stars.
Step 4: from altitude to latitude
Subtract the observed altitude from 90° to get the zenith distance: 90° − 52°32.5′ = 37°27.5′. Then take the Sun's declination from the almanac for the time of the sight, here 14°31.6′ N, and combine the two:
- If the Sun was south of you at noon: latitude = zenith distance + declination.
- If the Sun was north of you: latitude = declination − zenith distance.
Count northern declination and latitude as positive and southern as negative. In the example the Sun bore south, so latitude = 37°27.5′ + 14°31.6′ = 51°59.1′ N.
A check in the southern hemisphere: in latitude 30° S with the Sun at declination 20° N, the Sun is north of you and the zenith distance is 50°; 20° − 50° = −30°, that is 30° S.
Getting an accurate result
- Check index error every time. It goes straight into the latitude, minute for minute.
- Use the right height of eye. Dip grows with height: about 1.76′ × √(height in metres), so 3.0′ at 3 m and 4.3′ at 6 m.
- Interpolate the declination. It changes by up to about 1′ an hour near the equinoxes; take it for the UTC of the sight.
- Prefer a sharp horizon. Haze or a swell close aboard spoil a sight more than anything else.
- Be careful when the Sun is very high. Above about 85° its bearing swings quickly and it is hard to find the vertical.
With care, a noon sight from a small boat is usually good to a mile or two, enough to confirm or question the GPS and the logbook.
Frequently asked questions
Do I need the exact time for a noon sight?
No. Latitude comes from the highest altitude and the declination. The time only tells you when to start observing and which hour's declination to use.
Can I get longitude from a noon sight?
Only roughly. The time of the highest altitude gives longitude, but the Sun hangs at the top for minutes, so the time is uncertain. Navigators use timed sights before and after noon, or a running fix, for longitude.
Why is the noon sight still taught?
It needs no sight reduction tables, only the almanac and simple arithmetic, so it is the easiest celestial fix to learn and a dependable backup when electronics fail.