Hello John, John Carmichael wrote:
> Hello All: > > Last night the four hour long TV miniseries, "Longitude" was shown on A&E > channel (in US). > > This fabulous program detailing the struggles of Harrison to perfect a > mariners clock and win the prise for determining the longitude at sea should > not be missed by anyone who is interested in the history of time, clocks, or > engineering. I have a couple of questions about its contents. > > 1. In the scenes showing the meetings of the Longitude Board with Harrison, > on the wall is a large round map of northern Europe with London at the > center. Around the perimeter of the map are the cardinal points of a > compass. A single moving metal hand is attached at the center. I'm > wondering is this might be a mechanical device attached to a weathervane > used to indicate the direction of the wind. Or it might be some sort of > indoor sundial which shows the sun's hour angle. Does any body have a clue > what this instrument does? > > 2. To determine longitude at sea, Harrison's clock was carried on board a > ship traveling west. The clock was set before the journey to show apparent > solar time in Greenwich. The difference between local solar time on the > ship and the time on the clock was used to determine longitude. > > To determine local solar time on the ship, Harrison used a sextant to look > at the sun. How can a sextant, by itself, indicate the time? To determine > high noon, wouldn't he only need a compass to know when the sun "souths" at > high noon? > > 3. Where are the four Harrison clocks today? Are they still functional? > > Thanks > > John Carmichael > Tucson Arizona 1. I didn't notice the device you mention. Cannot comment. A weathervane linked to a pointer sounds plausible. 2. You are right that a single sextant sighting cannot give longitude directly. What it can do is establish that you are somewhere on a particular circle on the globe. In practice, you know roughly where you are from some previous known position and your speed(s) and heading(s) since then. This is called dead reckoning. With luck, the circle established by the sextant will pass close by your assumed position from dead reckoning. What you do is some trig calculations to calculate how far the nearest point of the circle is from the dead reckoning, and in what direction. You then plot on your chart the fragment of this circle that is closest to your dead reckoning position. The circle is so large that you actually plot the fragment as a straight line. If you can see two or more heavenly bodies, you can get two or more sightings. Each gives a different circle. By plotting them on your chart in the same way you should find they intersect. Where they do so is, of course, your estimated position. You can also get two sightings from, say, the sun by waiting a few hours. Same principle applies, but you must move the first line to allow for the distance you've sailed in the meantime. 3. All four Harrison marine chronometers are at Greenwich, in south London. Numbers one and two are going, so you can see the wonderful grasshoppers in action. From memory, number three, and certainly number four, are not, as they are regarded as too precious, and wear would, over centuries, destroy their value to future students. To come upon them at the end of a visit to the Royal Greenwich Observatory is magical. Worth a pilgrimage. Chris Lusby Taylor 51.3N, 1.4W
