A sextant is a tool that measures the angle of a star or planet to the horizon. It uses two mirrors, one adjustable and another adjacent to a panel of glass in which you see the horizon, to put a star or planet next to the horizon. A small telescope serves a dual purpose of putting your eye in the correct position and enlarging whatever you planet or star you are using. The book noted that sighting planets was generally easier, so we started with Venus and Jupiter, two planets that we see every night. After a little practice, Jack and I soon had a technique down.
One of us would sit with a headlamp and a clock waiting for the other, with the sextant, to say when he had the sight. When the sextant user announced that had a sight, the person with the clock would immediately write down the time, since it is very important for the calculation of location. We would then use the light to read the sextant's angle.
On our first night taking readings, we each did three sights of Venus and another three of Jupiter. Once all twelve data points had been entered, we hit calculate. Then we added our GPS position and compared the two. The iPad, which was where we have an app to do the calculations, could compare the two positions much faster than we could. Expecting to be around 100 miles off, I was surprised to find our calculated position was 40 miles off from our actual location, which is close enough to navigate by at sea. On the fourth night, I challenged myself and did a star, Miaplacidus. We got a record of three miles off.
Part of what makes this so fun is the fact that I have recently read the Horatio Hornblower books, by C.S. Forrester. In the books, there are a few interesting parts where Hornblower, a midshipman on a British frigate in the1800s, describes his role in navigating the ship. I was intrigued by the thought of navigating by the stars alone and was excited to learn how to do it myself. It is even more fun because Jack was just as interested in learning how to use the sextant as I was and working with him to record the sights is a lot easier than doing it alone.
So far Jack and I have only done planets and stars. In the future, we plan to do the moon and sun. I also look forward to getting the tables so I can do the calculations by hand. Maybe sometime we will try a short passage navigating only with the sextant, but at the moment I think I need more practice. - Porter
Editors Note: As per the boys celestial fix at 2030 UTC on March 14, 2015 we were located at 18º14'S 19º30W.
It is very impressive that you learned celestial navigation. My mother also knew it. I think she used it to navigate in the Newport to Bermuda race some time in the late 1960's or early 1970's. aided by whatever other technology was available then. It is a rare skill to have these days. Congratulations on learning it.
ReplyDeleteKathy Angell