Where Are We? Or, a Brief Guide to Celestial Navigation

Justine Sauvage – Dissolved Gas Analyst

The daily routine of life at sea can be happily offset by our exposure to a diverse set of skills, in the form of scientists, coring experts, and a full crew including mates, engineers, and stewards. One of my favorite things to do while at sea is getting a taste of all the specialties on board that I’m not very familiar with. During some down time or after my shift I like to go for a ‘round’ and see what the ship has to offer. The bridge is a fun place to go explore. First of all it contains some pretty neat navigational and positioning tools (GPS, radar, gyroscope, the ship’s control system, etc.), a global communication and weather report system for the area we are sailing in (think of it as the ship’s news channel, complete with upcoming storms, potential pirate attacks, distress messages, and mermaid sightings), monitors showing our heading, course, and position on nautical maps, a whole bunch of communication and signaling flags (one of them is the Whiskey flag, which means ‘ship in need of medical assistance’ – don’t think too hard about any potential link).

View of the bridge withChief Mate Derek Bergeron and Able-Bodied Seaman Mike Singleton on watch).

A full set of celestial navigation apparatus is also mandatory on the bridge (i.e. the ship’s chronometer, a sextant, and books containing all the information required to perform latitude-longitude position calculations, such as the Nautical Almanac). Lacking any sense of orientation on land, I figured I could possibly compensate for that at sea by learning some celestial navigation skills. The Chief Mate Derek is up for the task, so let’s jump on the opportunity. My training started yesterday and is far from complete. After having gone once through the entire gymnastics of figuring out my position on the earth solely based on celestial navigation tools, I gained even more respect and fascination for the sailors who faced the seas during the pre-GPS centuries, as well as today.

The entire process goes as follows:

Drawing sun lines
The general principle to figure out our position on the surface of the earth consists of drawing some position lines (also known as ‘sun lines’, circular lines of position based on an observer’s distance from a celestial body’s geographic position) at different times of day. For example, we could draw one line at sunrise, local apparent noon (which will give us our latitude) and one in the afternoon. Taking into account the ship’s heading and speed, we can advance our individual lines of position to a common time (i.e. stack them on top of each other) to get our position (the spot where all the sun lines intersect each other).

Time
Firstly, we need a very precise idea of ‘time’, more precisely Greenwich Mean Time. The entire celestial navigation process is extremely sensitive to the smallest uncertainties in time (as well as many other factors). A few minutes timing error could translate to a deviation of many miles from our actual position. The Greenwich atomic clock in the U.K. continuously transmits ‘time information’ (beep-noises) through the radio waves, which we can receive on the bridge if we tune the radio to the right frequency.

Angle between horizon and sun:
The two most important variables in the celestial navigation process are the angle between the horizon and a celestial body (in this case the sun) and the time at which this angle is measured. Measuring the angle is done with a sextant, which I consider the most intriguing object found on the bridge. Using a series of built-in mirrors, we get the sun and horizon in the same frame and then adjust the sextant’s parameters and orientation until the sun appears exactly tangent to the horizon. Once we have everything lined up exactly, we immediately note the time and angle (this sounds relatively straightforward but is far more tricky in practice).

Calculation gymnastics:
Using the angle of the sun and the time at which the angle was taken, we can use the Nautical Almanac to derive the local hour angle of the sun, its declination and the sun’s assumed altitude. These three variables are then used as inputs for another manual (with the fancy name of “Sight Reduction Tables for Marine Navigation”) from which we can finally determine the azimuth line and the altitude intercept, the information required to draw a sun line on our map.
This process will be repeated multiple times a day when the sun is at different angles above the horizon.

Nautical Almanac on the left, Sight Reduction Tables for Marine Navigation on the right.

Putting it all together:
Once we have acquired our three sun lines; we slide the first two along until they intersect the third at a common time. This intersection gives us our hard-earned, estimated position on the earth’s surface.

Blue diagnoal line is the ship’s desired course on November 13th. Pink line is our first sun line, taken at 09:32, green line is the second sun line, taken at 14:15, and the blue line is the final sun line, taken at 15:46. Stacking all the sunlines to intersect at a common time gives us our position.

A little assessment…
Comparing our celestially derived postion to the GPS reading we found that we were off only by about 5 miles…not too bad for a first try!!