Earlier this morning I let the dogs out to do their business and stepped out onto my deck to have a look around. Although it was a bit cloudy out I noticed that the Moon was hanging brightly about 8 degrees above my roof line. Dawn was just starting to break and I figured it would be a good time for this pseudo-mariner to get some practice sights in with the sextant. The moon is entering its last quarter here in Georgia and there was still enough of the orb available for a good upper or lower limb shot.
I grabbed my old Astra IIIB sextant, screwed on the artificial bubble horizon and spent about 10 minutes practicing ‘pulling down the sight’, focusing more on technique than accuracy. With a bubble horizon you have a lot of room for error because the horizon indicator (the bubble) is so large when viewed through the sight tube. Don’t worry – around 0720 EST the Moon was hanging at about 40 degrees 4.8 minutes, right where it should be. The clockwork heavens are still ticking along just fine.
|Astra IIIB Sextant
As I was fiddling with the sextant the winds started pushing the low clouds around and the Moon began darting in and out of view, sometimes partially obscured, sometimes fully obscured. This made for an interesting practice session as I was forced to time the approach and departure of the heavier cloud patches and practice pulling down the sight quickly before the Moon became too indistinct for a good shot. This is a common problem in celestial navigation – the navigator is at the mercy of the weather. That’s why so much emphasis was placed on grabbing a celestial shot whenever the heavens and the weather cooperated. It is also why so much emphasis was placed on accurate dead reckoning – estimating your current location based on distance and direction traveled from your last known location. Since you were never sure when you’d be able to get your next celestial fix an accurate running estimate of your position was absolutely crucial.
I was reminded of the particular problem celestial navigation posed for our submarine crews in WWII. More than any other arm of the Navy, the Submarine Service operated far into enemy waters in search of victims, and they traveled alone. Accurate navigation was absolutely essential and the navigators assigned to our submarines were some of the best the Navy produced.
WWII submarines were extremely vulnerable when caught in the wrong combination of circumstances. Our subs like the Gato-class boats were really highly modified surface ships that could spend limited amounts of time under water on battery power.
|US Gato-class submarine
The lower spaces of these subs were filled with giant lead acid batteries that allowed the boat to remain submerged for up to 48 hours and maneuver slowly (9 knots). Eventually, however, the sub had to surface to charge her batteries, refill her air tanks and get a navigational fix. For a boat operating alone in enemy waters this was a hazardous activity. A submarine was never more vulnerable than when on the surface with low batteries. It was common practice for the subs to surface in the dark of night and make a high speed dash to a new hunting area while replenishing her batteries. The problem is that the middle of the night is generally a lousy time for a celestial fix. Sure, the skies are filled with stars and planets, but the horizon is difficult to distinguish. The best time for a fix is at nautical twilight, when the sun is 6 – 12 degrees below the horizon. At this time the nautical horizon is still distinct and key navigational stars and planets are visible in the darkening sky. But there’s also enough light left to be spotted by an enemy aircraft or nearby surface ship.
This led to a unique ‘navigator’s dance’ on American submarines. At twilight the Captain would bring the boat to periscope depth to check for enemy ships and aircraft and to check weather conditions. If the skies were clear of enemy and clouds he’d give the heads up to the navigator, who was usually the boat’s executive officer. The navigator would have already checked his navigational tables and picked one or more likely celestial objects to try to use for a fix. This could be a planet or bright star or, if he was really lucky the Moon was already up and far enough above the horizon to provide a good fix. The navigator would often wear goggles with red lenses to get his eyes adapted to dark conditions.
The Captain would give the command to surface the boat and once the conning tower was clear of the water the hatch would be opened and the watch personnel would scramble up with binoculars, climb the periscope shears and scan the skies and the horizon for any signs of the enemy. Once the all-clear was given the navigator would come up with the sextant hanging from his neck by a lanyard. He would take a series of quick shots on the available celestial bodies and call the sextant readings down to the navigation team in the control room. The navigation team would note the time of the observations against the boat’s chronometers and begin the process of using the sight readings to establish a line of position. A quick shot on Polaris gave the navigator an accurate and easily determined latitude, but the shots on the stars and planets to determine longitude took a bit more number crunching. Things like the height of the navigator above the surface of the water, the time difference from GMT, the uncorrected error built into the sextant and other factors all had to be calculated. This process was called ‘sight reduction’. It was (and still is) straight forward but somewhat tedious math.
In the end the navigation team (usually consisting of the executive officer, an enlisted navigator known as a quartermaster and another pair of trained eyes, often those of the Captain) would come up with intersecting lines of position, one for latitude and one for longitude, that provided the boat’s true position at the time the sights were taken.
Here’s an interesting description of the process taken from the book ‘The Underwater War 1939 – 1945‘ by Richard Compton-Hall:
Away from land every opportunity for taking sun, moon, planet and star sights had to be snatched. Sight-taking with a sextant was treated as an evolution; if surfacing primarily for that purpose it was combined when possible with ditching (trash) — which made matters no easier for the navigator competing in the conning tower and on the crowded bridge with a hustling (trash) party, the lookouts and the sea itself. The smallest drop of water on the sextant mirror made sight-taking impossible and the instrument had to be wrapped tenderly in a towel when not actually bringing the observed body down on to the lurching, irregular horizon which, with so low a height-of-eye, made the task doubly difficult. The ‘exec’ was primarily responsible for navigation in American boats (assisted by excellent quartermasters) but German commanders relied upon the equivalent of a specially trained warrant officer to take sights. Most British captains thought sight-taking far too important to entrust to Vasco (the navigator) and did the sextant work themselves; but they were quite happy to delegate the long and boring working-out of the sights when they were taken! It could easily take an hour to plod through the spherical trigonometry (which actually amounted to no more than straight forward arithmetic) before arriving at a solution which almost invariably produced a large cocked hat; this led to thinly veiled hints from Vasco to the effect that the captain was incapable of reading sextant angles, and to more direct accusations from the captain that the navigator was incapable of simple addition and subtraction. Some boats carried rapid reduction tables derived from air navigation manuals which greatly shortened the time required to produce a fix: but the Royal Navy and most other services clung doggedly to Inman’s Nautical Tables with their long columns of five-figure logarithms.
Today we are spoiled. Want to know where you are on the face of the earth to within a few hundred feet? Just turn on your smartphone or GPS receiver. Within seconds you’ll get a position fix that is far more accurate than any experienced navigator could have calculated using celestial navigation.
Yet I believe it is important we continue to practice the old techniques. First, it is great mental exercise. To be a good celestial navigator you need to be at least proficient in basic astronomy and mathematics. You need to know how to evaluate and calculate error. You need to be a good problem solver. Celestial navigation is like golf – it takes just a few months to learn but a lifetime to master. It sure beats playing another round of World of Warcraft.
Next, celestial navigation gives one a greater appreciation for the technology we have available today, and that appreciation and the resulting awareness of the GPS system’s capabilities and limitations will make you a better navigator overall.
And last, the celestial navigation techniques and tools we use today are exactly the same as those used by history’s great explorers and navigators – Capt. James Cook, Lewis and Clark, Robert Peary, Roald Amundsen, Earnest Shackleton, Robert Scott, Capt. William Bligh (yes that Capt. Bligh) and many others. Anyone interested in the history of exploration can make a direct and relevant connection to their heroes and better appreciate their achievements by dabbling in celestial navigation.
So that’s today’s report from the deck of the SS Northing & Easting. I’ll keep the spyglass and blunderbuss handy in case the pirates try to board.