Celestial demands the navigator head offshore for a few hundred miles and, from the deck of a rolling sailboat, aim a bronze contraption at the sun, without blinding himself, while measuring its angle off the horizon. Then, using the precise time in Greenwich, England, that he shot the sight, the navigator is asked to log the reading. A mistake of only four seconds will cost him a mile of accuracy.
Regarding celestial, what bogs people down is the theory. Most books on the subject are incredibly dense – technical and uninspiring, they don’t exactly make for pleasant reading. In reality, the theory is simple – once you get a reasonable understanding of your spatial relationship with the earth and your surroundings, celestial theory becomes intriguingly intuitive.
For now, we’ll focus on the primary component of celestial navigation, namely the physical act of taking a sight. As with a gun, practice begets accuracy – it’s no coincidence that sailors, before the age of GPS, referred to ‘shooting’ the sun or stars. Just as someone competing in a winter biathlon must contend with visibility, wind, snow and fatigue while aiming a rifle at a tiny target, the sailor has to contend with wind, waves, clouds, a pitching deck and similar fatigue.
A sextant, in simplest terms, is a device used to measure angles. Steven Callahan famously navigated across the Atlantic in a life raft using a crude sextant he cobbled together from two pencils and a piece of string. A modern sextant is a precision instrument, often cast in bronze, which measures angles to the nearest minute in terms of arc distance. We refer to the angle of any celestial body as its altitude.
You need only two things to get a good sight – a celestial body and a clear horizon. Start with the sun – it’s the biggest celestial body out there, and when it’s up, there is always a visible horizon. It’s easier to first estimate the height of your chosen celestial body and then pre-set the sextant for that altitude. Thanks to biology, most people’s extended fist will measure roughly ten degrees, regardless of the size of their hand – bigger hands mean longer arms, and so from your eye their size appears the same. Using that as a guide, estimate the altitude of the celestial body and pre-set the sextant’s index arm.
With the sextant pre-set, aim the scope at the horizon in the direction of the sun – be sure to put some shade on if you’re shooting the sun. What you’ll see is the horizon itself, through the scope, and (hopefully) the sun as reflected through the index mirror. Now, simply adjust the micrometer drum (the small wheel on the index arm that is essentially the ‘fine’ adjustment on the angle you’re measuring) to move the sun up or down. The object of the game is to get the bottom edge (lower limb) of the sun to ‘kiss’ the horizon. By slightly rocking the sextant back and forth on its vertical axis as you adjust the micrometer drum, the sun will appear to swing like a pendulum. It’s at the bottom of this arc that you want the sun to ‘kiss’ the horizon. Have a partner record the time in GMT, to the nearest second, the instant you say mark, stop your adjustment and read the angle off of the index arm, being careful to interpret the minutes correctly from the micrometer drum. Record the time and the altitude. The prudent navigator will take a series of sights (usually five) and use their average for the actual calculations.
It’s easiest to shoot the sun in the mid-morning or mid-afternoon, when the sun is about 40 to 50 degrees above the horizon. Any higher, and the angles get very large and cumbersome, any smaller and the refraction from the atmosphere will interfere with your accuracy. Once comfortable with the sun, try a trick Bernard Moitessier used on stars – go through the same process of estimating altitude, but before aiming the sextant, remove the scope. Peer through its bracket, and, with both eyes open, it will be much easier to locate your star. Where the sun appears as a large disk, the stars appear as small pricks of light, and are nearly impossible to find in the limited field of view of the scope.
Next issue we’ll follow up with what to do with your sight data and how to actually plot a position from it (easier than you think). The elegance and simplicity of the theory may surprise you.
Don’t miss the full Series on Celestial Navigation.
- Celestial Navigation Part 1: Introduction to Celestial Navigation
- Celestial Navigation Part 2: Predicting the Sun’s Geographic Position
- Celestial Navigation Part 3: Tips and Tricks on Sight Reduction
What is your experience with Celestial Navigation?
Of course comments or questions to the author are welcome below.