Dead Reckoning

6/11/06 – 6/17/06

C. Zaitz

When I was a teenager I read the entire series of books by C. S. Forester about a 19th century British naval adventurer named Horatio Hornblower. In one story, Horatio had to go for his lieutenant exam and was cramming all the necessary navigation mathematics and trigonometry in his head. Unfortunately, he froze during the examination and was failing. As he stammered out his response, he caught a glimpse of a fire ship- a wooden ship set intentionally on fire to destroy other wooden ships. On instinct he abandoned the exam and valiantly dove into the water, swam to the fiery ship, climbed aboard and steered it to safety, away from the British fleet lying helpless in the harbor. He eventually did make it to lieutenant, and even Admiral, some 10 volumes later.

I have since learned more of the complex navigation about which Horatio was examined. Finding latitude has always been a snap, as long as you can see the North Star, Polaris and have a sextant or angle measuring device handy. It happens that the altitude of Polaris in the sky is equal to your latitude on earth. That is because Polaris lies almost directly over the north pole of the earth. You can prove this to be true with a diagram and a little knowledge of trigonometry. In Dearborn Heights, the height of Polaris in the sky is about 42.3 degrees, and we know that our latitude is 42.3 N. You can get out your sextant tonight and check it out!

However, finding one’s longitude at sea was never an easy feat. To find it you must find the time of your local noon, or when the sun crosses your meridian, and compare it to Greenwich Mean Time. Of course, if you don’t have a watch or a cell phone, neither of which Horatio had, this is difficult. Before the invention of an accurate chronometer in the 1750s, sailors used a technique called Dead Reckoning to find their positions. It was basically a process of extrapolation. If you know how far you’ve gone since your last accurate position, or at least know how fast you’ve traveled and in what direction, you can figure out where you are now or will be in the future. Of course you must correct for wind and waves and human error along the way. Is it any wonder that Columbus’ voyage was a bit hairy?

Nowadays we use satellites in space to accurately find our positions, whether we are out on a boat in Lake Huron or driving from The Heights to Livonia. The Global Positioning System, developed and maintained by the US Department of Defense, uses more than two dozen satellites to send radio signals to anyone with a GPS receiver.

Modern day navigators often use old and new methods to maintain their courses. It’s always handy to have a working knowledge of at least ten or so bright stars in the night sky. All three stars of the summer triangle are considered navigational stars. Planets like Jupiter, though they are bright and easy to find, are not used for navigation, since their position changes noticeably over the course of weeks and months. You can watch Jupiter in Libra all summer, however. His large distance from the sun makes him appear to move slowly in the sky. We can enjoy his bright glow until mid August.

Until next week, my friends, enjoy the view!