The Sun in Celestial Navigation

Why the Sun Is the Primary Celestial Body

For the practicing offshore navigator, the Sun offers advantages that no other celestial body can match. It is visible on any clear day, it is unmistakably bright, it requires no identification, and it rises and sets predictably — providing a regular daily schedule of usable observations from dawn to dusk.

Brightness and ease of identification make the Sun the only celestial body suitable for daytime sights. Unlike stars, which require twilight conditions and a visible horizon simultaneously — a narrow window — the Sun can be observed whenever the sky is clear and the horizon is sharp. A beginner can take their first Sun sight without any prior knowledge of star identification or the star finder.

A predictable daily geometry means the Sun's altitude and azimuth change in a regular, calculated way throughout the day. In the morning, the Sun is in the east and rising. At local noon, it reaches its highest point and crosses the meridian. In the afternoon, it is in the west and declining. This geometry creates natural observation opportunities that have defined the rhythm of celestial navigation since the earliest ocean voyages.

The Sun's limitation is that it is a single body. Stars and planets can be observed simultaneously from multiple directions at twilight, giving multiple crossing lines of position and an instant fix. The Sun provides only one line of position at any given moment. To obtain a fix from Sun sights alone, the navigator must take sights at different times and advance earlier LOPs to account for the vessel's movement — the running fix technique. This requires additional care in track-keeping and introduces the vessel's dead reckoning accuracy into the celestial fix.

In practice, the Sun-run-Sun sequence — a morning sight, a noon latitude, and an afternoon sight — is the fundamental daytime navigation schedule for offshore passages. When complemented by a dawn or twilight star fix at the bookends of the day, the Sun provides all the position information a navigator needs for a safe ocean crossing.

Local Apparent Noon

Local Apparent Noon (LAN) is the moment when the Sun reaches its greatest altitude for the day — the instant it transits the observer's meridian and is due north or due south, depending on whether its declination is higher or lower than the observer's latitude. This moment is the foundation of the oldest latitude-finding technique in ocean navigation.

At the instant of LAN, the Sun is on the observer's meridian, meaning the geometry of the sight is simplified enormously. The altitude of the Sun at LAN is directly related to the observer's latitude and the Sun's declination by a simple formula: Latitude = Declination ± (90° − Ho at noon). No sight reduction tables are needed. No LHA calculation is required. The noon latitude is computed with arithmetic alone from the peak altitude and the day's declination from the almanac.

The sign rule follows the relationship between the observer's latitude and the Sun's declination. If the Sun is bearing south at noon (observer is north of the Sun's declination — the most common case in northern latitudes), the formula is: Lat = Dec + (90° − Ho). If the Sun bears north at noon (observer south of the Sun's declination), the formula becomes: Lat = (90° − Ho) − Dec. In both cases, the computation takes less than a minute with a simple calculator.

Predicting the time of LAN is important because the Sun's altitude changes very slowly near noon — the altitude curve flattens out near the peak. If the navigator starts taking sights too late, the Sun has already begun to descend and the peak altitude is missed. LAN time is predicted from the equation of time (printed in the Nautical Almanac) and the vessel's longitude. The navigator begins tracking the Sun's altitude about 20 minutes before predicted LAN and takes sights every two minutes as the altitude approaches its maximum.

Practical technique at LAN is to observe the Sun's altitude carefully as it rises toward its peak, holding the Sun on the horizon and adjusting the sextant to follow it up. When the altitude stops rising and begins to fall — the moment the Sun 'hangs' — that is the peak altitude and the moment of LAN. The navigator notes the final maximum altitude and records it for the noon latitude calculation. The exact time of LAN is less critical than the exact altitude, since the latitude formula requires only the peak altitude.

Combining the noon latitude with a morning Sun line gives a running fix: the morning LOP is advanced along the vessel's track to the time of LAN and crossed with the noon latitude line. This noon fix has been the primary daily position determination method for offshore sailors for centuries.

The Running Fix with Sun Sights

Because the Sun is a single body, obtaining a fix from Sun sights alone requires taking observations at different times and advancing the earlier line of position to the time of the later one. This technique — the running fix — requires accurate dead reckoning of the vessel's movement between sights.

The standard Sun navigation day begins with a morning Sun line: a sight taken when the Sun is at a convenient altitude in the eastern sky, typically two to four hours after sunrise. This sight produces a Sun LOP running approximately northwest-southeast (in the northern hemisphere with the Sun in the east). On its own, this LOP tells the navigator which line they are on, but not where along it.

At noon, the noon latitude gives the observer's latitude directly (see previous section). By advancing the morning Sun LOP along the vessel's track — the course steered and distance run between the morning sight and LAN — the advanced morning LOP crosses the noon latitude line at the running fix. This running noon fix is the traditional daily position for offshore sailing passages.

The Sumner Line (or Marcq St Hilaire intercept method) underpins modern Sun fix work. The American sea captain Thomas Sumner (1802–1876) discovered in 1837 that a celestial sight defines a circle of equal altitude on the Earth's surface — and that any straight-line segment of that circle can serve as a line of position. This insight, independently developed by Marcq St Hilaire, is the foundation of all modern sight reduction: the LOP is a short segment of the circle of equal altitude, perpendicular to the azimuth.

Advancing a line of position is done geometrically on the chart or plotting sheet. The navigator plots the original LOP, then draws a line parallel to it shifted in the direction of the vessel's track by the distance run between the sights. This shifted line — the advanced LOP — represents where the original LOP would be if the vessel had not moved since the first sight. When a second LOP (or the noon latitude line) is plotted, its intersection with the advanced LOP is the running fix.

For the most accurate Sun running fix, an afternoon Sun sight can be added to the sequence. By afternoon, the Sun has moved significantly westward and its LOP has a very different orientation than the morning line. Crossing the advanced morning LOP with an afternoon LOP (also advanced to a common time) gives a Sun-run-Sun fix with good crossing geometry. This three-sight sequence — morning, noon, afternoon — is the most complete Sun navigation day.

Taking Sun Sights in Practice

Practical Sun sighting technique is developed through repetition. The mechanical skill of bringing the Sun's limb precisely to the horizon, holding it there while rocking the sextant, and recording the time at the exact moment of tangency takes practice — but it is a learnable skill that improves rapidly with use.

Upper limb vs lower limb: The Sun is too large to observe its center directly. Navigators observe either the bottom edge (the lower limb) or the top edge (the upper limb) and apply the semi-diameter correction to find the center altitude. The lower limb is standard for most observations — it is easier to bring to the horizon because you are setting the limb to touch the horizon from above, and the semi-diameter correction is added. The upper limb is used when the lower limb is obscured by clouds or haze near the horizon. When using the upper limb, the semi-diameter is subtracted. The almanac's altitude correction tables are pre-calculated for both cases.

Rocking the sextant is the fundamental technique for finding the exact vertical angle. As the sextant is tilted left and right from the vertical (rocked), the reflected image of the Sun describes an arc. The lowest point of the arc — the nadir of the swing — is where the sextant is perfectly vertical and the altitude is correctly measured. The navigator brings the lower limb to just touch the horizon at the nadir of the rocking motion. This eliminates the error introduced by holding the sextant at a slight tilt.

Dealing with clouds: A thin haze over the Sun's disk requires using a darker shade glass to maintain visibility of the limb. When clouds partially obscure the Sun, take sights in the gaps and average the results. If the Sun passes behind a cloud bank at the moment of LAN, track the altitude carefully before and after — the peak altitude can often be estimated from the symmetry of the altitude curve on either side of the transit.

Multiple sights for averaging: The most reliable way to reduce sighting error is to take four to six sights in rapid succession (within two minutes) and average both the altitudes and the times. Sights that differ by more than 1 to 2 arcminutes from the average should be discarded as outliers — likely caused by a wave obscuring the horizon or an unsteady sextant. The averaged altitude and averaged time are then used for a single sight reduction, producing a more accurate Ho than any individual sight could provide.

Recording the sight: Develop a consistent routine. At the moment the Sun's limb touches the horizon at the nadir of the rocking motion, call out 'mark' (or have a partner read the time), immediately read the sextant arc, and record both time (to the second) and altitude in the sight log. Do not try to read the sextant and check the time simultaneously — the time error will be unacceptably large. A two-person sight team — one on the sextant, one on the watch — is far more accurate than a single navigator attempting both.