Stardate 11:02:97
In past articles, we have discussed how to tell the time from the positions of celestial objects, but, unless the Moon was visible, we needed to know our position on the Earth in order to obtain a position. This is a neat trick, very helpful, and impressive to non-astronomers.
Here, we will discuss a related subject which has played an important part in history: How to tell your location on the Earth from looking at the sky. As you may have guessed, in order to do that, you need to know the time with some degree of precision.
As trans-Atlantic trade increased between Europe and America in the mid to late 17th century, the need to know one's position on the Earth became greater and greater. It was easy enough to find your latitude, as the north star is always the same number of degrees off the horizon as your latitude.
Why this is can be easily pictured. Imagine yourself at the north pole. Where would the north star be? Answer: directly above your head, or 90 degrees off the horizon. At this point, you are at a latitude of 90 degrees (and most likely, very cold). If you shed your jacket and put on a pair of Bermuda shorts for a trip to the equator, the north star would be exactly at the horizon, or zero degrees above the horizon. Your latitude at this point would also be zero degrees. So, a sailor in the 17th century would have no trouble determining his latitude, as long as he remained within sight of the north star.
The real trick was finding longitude. This proved to be more difficult. The English government offered a prize (in modern currency, about two million dollars) for a solution to the problem. Before this could be done however, stars needed to be plotted accurately, and a clock which would work well on the sea needed to be developed.
The first problem was solved around 1670 by two fellows, Hadley and Godfrey, who, working independently, developed the sextant, which allowed accurate measurements of the arc between two stars.
The second problem was not solved until 1735, when John Harrison developed the first spring driven clock, which could be used on board a ship. The pendulum clocks in use before that were of little use aboard a rocking ship. Now that accurate measurements could be taken of the arc between two stars, and sailors had an accurate time, the rest was child's play.
Here's how it works. Take any celestial event (but not a rising or setting time), for instance, the occultation of Io by Jupiter, or the exact arc between a planet and a star (hence, the sextant). Have a chart with the predicted time for that event coordinated for a given area. In astronomy, most events have their time listed according to Greenwich mean time. This is the time that the event would be seen by someone in Greenwich, England. In case you are wondering, Greenwich was chosen in the 17th century as the basis for all astronomical times because Isaac Newton was working there at the time, and there was (and is) a large observatory there.
If a given event happens at, say, 2 p.m. your time, and it was predicted to happen at 6 p.m. Greenwich time, then you know the difference in time is four hours. You also know your time is behind that in Greenwich. Since the Earth turns at 15 degrees per hour west to east, you now know that you must be 15 times 4, or 60 degrees west (because your clock was behind) of Greenwich. If the predicted time for your chosen event was, again, 6 p.m. Greenwich time, and you observed it at 3:15, you must be 2.75 (there are 2.75 hours between 3:15 and 6) times 15, or 41.25 degrees west of Greenwich.
This is the secret which opened the seas.
Have fun. Try it sometime soon, and see how accurate you can find your latitude and longitude. Can you find your time from the Moon and use that to find your location? Try it and see.
Clear skies, and good viewing.
