What Is Meridian Passage?
Every day, the Sun crosses your local meridian — the imaginary line running from the south pole through your position to the north pole. At that instant, the Sun reaches its highest altitude in the sky. This event is called meridian passage, or local apparent noon (LAN). The altitude you measure at this moment gives your latitude directly, without sight reduction tables, without an assumed position, and with only the simplest arithmetic.
Meridian passage has been used by mariners for over 500 years. It was the primary method of latitude determination in the Age of Exploration, and it remains the single most useful celestial observation for an offshore sailor today. If you learn only one celestial technique, make it this one.
Why Meridian Passage Is Special
Most celestial observations require sight reduction — entering tables with an assumed position, looking up computed altitude and azimuth, computing an intercept, and plotting a line of position. Meridian passage bypasses all of that. At the moment the Sun crosses your meridian, the azimuth is exactly 000° or 180° (due north or due south), and the geometry simplifies to a single equation:
Latitude = Declination ± (90° - Ho)
Where:
- Declination is the Sun's north-south position on the celestial sphere (from the Nautical Almanac)
- Ho is your observed altitude (corrected for index error, dip, and refraction)
- 90° - Ho is the zenith distance — how far the Sun is from directly overhead
The sign in the formula depends on whether you are on the same side of the equator as the Sun or the opposite side. We will work through the rules below.
Predicting the Time of Meridian Passage
You need to be on deck, sextant in hand, before the Sun reaches its peak. The Nautical Almanac provides the time of meridian passage at Greenwich (the Mer. Pass. column on the daily page). For June 21, this might read 12h 02m.
This time is in UT (Universal Time) at the Greenwich meridian. To find the time at your longitude, you convert your longitude to time:
Time correction = Longitude / 15 (in hours)
If you are at 063° W, the correction is 063 / 15 = 4 hours 12 minutes. Since you are west of Greenwich, you add this to the UT of Greenwich meridian passage:
LAN at your position = 12h 02m + 4h 12m = 16h 14m UT
Arrive on deck 10 to 15 minutes early. The Sun's altitude changes slowly near its peak, and you want to be settled, sextant adjusted, and tracking the Sun before it reaches maximum altitude.
Taking the Observation
The technique for a noon sight is different from a timed sight. You are not trying to record the exact time of an observation. Instead, you are tracking the Sun's altitude as it rises to its peak and then begins to descend.
The maximum altitude method:
- Begin measuring the Sun's altitude 10–15 minutes before predicted LAN.
- As the Sun rises, keep adjusting the micrometer drum to maintain the Sun's lower limb tangent to the horizon.
- At some point the Sun will appear to "hang" — its altitude stops increasing. Continue watching.
- When the Sun begins to descend (you have to start turning the drum down to keep the limb on the horizon), stop. Read the sextant.
- The reading at maximum altitude is your Hs (sextant altitude).
The beauty of this method is that you do not need an exact time. The altitude at maximum is the same whether you catch it at the precise second of meridian passage or 30 seconds on either side. The Sun moves so slowly in altitude near the peak that timing errors of a minute produce altitude errors of only a few tenths of an arc minute.
Correcting Hs to Ho
Apply the standard corrections:
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Index error. If your sextant reads +2.3' on the arc when the horizon is aligned, subtract 2.3'. If it reads -1.5' off the arc, add 1.5'.
-
Dip. Dip corrects for your height of eye above the water. From the Dip table in the Nautical Almanac: height of eye 3 meters gives dip = -3.1'.
-
Altitude correction (Sun lower limb). From the Sun correction table in the Nautical Almanac. For an apparent altitude around 70°, this is approximately +15.9' (which includes refraction and semi-diameter).
Example:
| Step | Value |
|---|---|
| Hs (sextant reading) | 70° 32.4' |
| Index error | -2.3' |
| Dip (3m) | -3.1' |
| Apparent altitude | 70° 27.0' |
| Sun LL correction | +15.9' |
| Ho | 70° 42.9' |
Worked Example: Latitude in the North Atlantic
Date: June 21 DR position: 37° N, 063° W (en route Halifax to Bermuda) Ho at meridian passage: 70° 42.9' Sun declination at LAN: N 23° 26.1' (from the Nautical Almanac, interpolated for the exact UT of LAN)
Step 1: Zenith Distance
Zenith distance = 90° - Ho = 90° - 70° 42.9' = 19° 17.1'
Step 2: Determine the Formula
You are in the Northern Hemisphere (N). The Sun's declination is North (N 23° 26.1'). The Sun is south of your zenith (because 23° 26' declination is south of 37° latitude). Therefore:
Latitude = Declination + Zenith Distance
Wait — let us be precise about the rule. When the Sun is between you and the equator (Sun bears due south in the Northern Hemisphere), the latitude equals the declination plus the zenith distance. When the Sun is on the far side (Sun bears due north because its declination exceeds your latitude), the latitude equals the declination minus the zenith distance.
In our case, the Sun's declination (23° 26') is less than our latitude (approximately 37°), so the Sun is between us and the equator, bearing due south:
Latitude = Dec + ZD = 23° 26.1' + 19° 17.1' = 42° 43.2' N
Step 3: Assess the Result
Our DR latitude was 37° N. The computed latitude is 42° 43' N. That is a difference of nearly 6 degrees — clearly the DR was only approximate for this example. (In practice on a passage, your DR would be much closer, and the noon sight serves as a precise update.)
The Four Cases
The sign rule for meridian passage depends on two things: which hemisphere you are in, and which side of you the Sun is.
| Your hemisphere | Sun's declination | Sun bears | Formula |
|---|---|---|---|
| N | N, but less than Lat | South (180°) | Lat = Dec + ZD |
| N | N, greater than Lat | North (000°) | Lat = Dec - ZD |
| N | S | South (180°) | Lat = ZD - Dec |
| S | S, but less than Lat (abs) | North (000°) | Lat = Dec + ZD (both negative) |
The safest approach: draw a quick sketch. Put the equator on a line, mark the Sun's GP at the declination, mark your approximate position, and see which side of the Sun you are. The zenith distance always goes between you and the Sun's GP; the question is merely which direction.
Why Noon Sights Work in Rough Weather
Most celestial observations require a crisp, level horizon. In heavy seas, the horizon pitches and rolls, making it difficult to bring a star or planet to the exact contact point. The noon sight is more forgiving because:
-
You are tracking a slow change. The Sun's altitude changes by only a few arc minutes per minute of time near the peak. You have time to wait for the boat to steady.
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You take the maximum reading. Even if you miss the exact peak by a minute, the error in altitude is negligible — less than 0.3' in most cases.
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The Sun is bright and easy to find. Unlike twilight stars, there is no difficulty identifying the body or finding it in the sextant.
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No precise timing is needed. In rough weather, pressing a stopwatch at the exact instant of observation is difficult. The noon sight does not require it.
For these reasons, the noon sight is often the only reliable celestial observation available in severe weather — precisely when you need it most.
Historical Significance
Before the invention of the marine chronometer in the 18th century, longitude determination at sea was effectively impossible. But latitude? Latitude had been available since antiquity. Every mariner who could measure the Sun's height at noon and consult a declination table could find their latitude. Columbus did it. Magellan did it. Cook did it.
The technique of "running down the latitude" — sailing to the latitude of your destination and then turning east or west until you reach it — was the standard ocean navigation method for centuries. It was crude, it was slow, and it added hundreds of miles to every passage. But it worked, because the noon sight is reliable.
When you take a noon sight on your passage to Bermuda, you are using the same method, the same geometry, and the same Sun that every ocean navigator has used for 500 years. The only difference is that you also know what time it is.
Practice
The noon sight is the first exercise in every Sailcasted passage. Each simulated day gives you a meridian passage observation with almanac data, correction tables, and a latitude to compute. By the third day, the procedure is automatic.