| Navigation |
| Ah, Navigation, the stars, the sun, the shiny brass tools, complex mathematical calculations, playing with timing glasses, throwing logs, counting knots, pricking charts, squinting at the moons of jupiter. Everything you think of when you think of tall ships isn't it? |
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| Most Chesapeake privateers could use the time honoured practice of dead reckoning, since they mainly sailed within the limits of the Chesapeake Bay and along the coastline. But any man worth his salt knows that the sea is a fickle mistress, but the stars are constant, and you should always know how to find out where you are, or you'll never know where you are going! |
| DEAD RECKONING |
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| If you know where you were yesterday, and what direction you have been sailing in, and how the tides have affected your course, and the winds, and how fast you have been going, you can reckon where you should approximately be now. By using the log to check your speed each hour, and with accurate tidal information and a good compass, you should be able to do well enough at finding a continent. |
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| Now, if you are in the middle of the ocean, are running out of fresh water, and the crew is dropping from scurvy because they haven't had any fruit or sauerkraut in weeks and the closest Island should be nearby, being 60 miles off in your position means you might miss the island, and then, die. We need a better form of navigation. |
| NOON OBSERVATION On clear days, just before the hour of noon, the Ship's Master, his Mates, and the Midshipmen each measured the angle of the sun at it's highest point above the horizon with their sextants or Quadrants, and by this measurement calculated the Latitude position of the ship, and from these calculations and use of either an accurate Chronometer, or the Lunar observations taken as well, and consulting the Almanacs, corrected the chronometers aboard, if correction was needed after the readings were reported to the Captain. |
| Any text you should find on the subject will thoroughly prove to you that it is almost impossible to explain the use of the quadrant in so brief a space as this. It suffices for this page to say that a sextant is a handheld instrument of optics, used to measure the angle between a celestial object and the horizon, and by comparing the difference between the angle at your position, and the known angles in Greenwich, England on the same date and time, you can determine your Latitude and Longitude. Like the tieing of complicated knots, it becomes easier with practice. |
| LATITUDE |
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| NOVA ( PBS) has provided more background on finding latitude by the stars, and other ancient methods. |
| LONGITUDE |
| Calculating Longitude by Lunar Observation. The Nautical Almanac and Astrological Ephemeris was not finally finally published until 1766. Possessors of this volume could then use lunar sights and precalculated tables in the Ephemeris to plot a position, bypassing the time consuming mathematical calculations. It was not until 1803 that Thomas Earnshaw found a way to mass produce an affordable, accurate, sturdy time piece In the 1700s, one could buy a sextant and a set of lunar distance tables for 20 pounds. Thus, Lunar observation remained the popular form of finding longitude until the 1780s. Lunar tables continued to be printed each year in this work until 1907, and the Almanac itself continues to be printed to this day. |
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| Four India Company captains of the 1700s testified to the Royal Observatory that they found the use of lunar measurement quiet accurate, and "They always managed to calculate their longitude in a matter of a mere four hours." One of the problems with the calculations is that the moon seems to wobble on an erratic course through the month, and it seems to speed up and slow down along the way. These make it more difficult to first plot, then mathematically reproduce it's motions. Only after these variables had been taken into account, can the moon be used as a useful point of reference. |
| Also, for about six days each month the moon is so close to the sun as to be hard to make an accurate observation. For 13 days each month, the moon is opposite the sun, and the distance is too great to measure. One has to take a measurement referencing the moon to a star, and then reference the times of the night observations using a watch. The problem with this is however, that time pieces in the 1700's were horribly inaccurate especially on a rolling ship, in a corrosive, salt water environment. It was judged a sufficient method, when one also compared this clock measurement in relation to past days observations and the dead reckoning method, to estimate a position until a true reading could be attained. |
| Calculating Longitude By Chronometer ( Time Piece ) |
| The difference between Greenwich mean time and local time will give the longitude East or west. |
| 1 hour = 15 degrees of longitude |
| 4 minutes = 1 degree of longitude |
| There are 360 degrees of longitude, and 24 hours in a day. 360/24 = 15 deg./hour |
| 15 degrees / 60 minutes = 1/4 degree per minute, or 1 degree every 4 minutes* |
| By using this knowledge and your latitude as determined by the observation of your Quadrant, you can ascertain your position. |
| Examples; |
| If your observation of local noon is 2 hours 16 minutes after noon in England, then you are at 34 degrees West Longitude. |
| (15x2)+(16/4) = 34 |
| Ifyour observation of local noon is 1 hours 12 minutes before noon inEngland, then you are at 18 degrees East longitude. |
| 15 + (12/4) = 18 |
| * of course, the farther north or south one goes away from the equator, the closer together the lines of longitude become, and the more inaccurate this method becomes. |
| The trick is to have an accurate time piece. Today, you could use any two cheap wrist watches. Until 1759 however, when John Harrison finally produced one able to withstand the rigors of shipboard conditions, this method was wonderful in theory, but useless in practical application. Harrison's clocks, though effective, were too expensive and time consuming to mass produce inexpensively, though his revolutionary designs provided all the solutions for the eventual development of a practical time piece. |
| Even Captain Cook stated in his log of the Resolution's voyage(1775), "that indeed our error (in longitude) can never be great, so long as we have a good a guide as {the} watch." |
| The problem with these watches, however, is that they were expensive. They were expected (at this time) to cost around 500 pounds each. Chronometer readings are not commonly mentioned in British log books until the 1780's, when the prices had finally dropped to a meer 45 to 65 pounds. Considering that the captain's had to purchase these chronometers out of their own pocket, their usefulness and popularity with the practical seaman is evidenced in the regularity with which the readings appear. |
| Want to try your hand at it then? NOVA (PBS) has provided more background, and a way to try the method out. With Shockwave or Without Shockwave |
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| photo copyright R. Gay. All rights reserved |
| photo copyright R. Gay. All rights reserved |
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