RE: "Longitude" miniseries
On 10 July 2000 15:15 John Carmichael wrote: > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > To: sundial@rrz.uni-koeln.de > Subject: "Longitude" miniseries > 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. This is indeed an internal weathervane driven by a shaft and gears from the actual vane on the roof. Several British buildings have or had them, including the East India Company offices and even some private houses (very large ones!) > 2. 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? By determining times of equal altitude either side of noon and indeed the time when the sun reaches its highest altitude, noon is obtained (as well as the latitude, knowing the date). A compass would a) need to be accurate and corrected for magnetic deviation and b) would only be useful as described if the sky were clear at noon, whereas observations of altitude a short time either side of noon can be reduced to find the actual time of noon even if no sight is possible at that moment. For taking sights either side of noon of course a watch (a deck watch) is necessary but it is only its reasonable accuracy in the short term that matters, not its timekeeping over many days. Such a watch was used anyway to carry the time information to and from the chronometer(s) which were never moved from their resting place below deck. > 3. Where are the four Harrison clocks today? Are they still functional? The four timekeepers H1 to H4 are all on show at the Old Royal Observatory, Greenwich, London, where they are indeed functional. H1 to H3 are kept running. However H4 requires lubrication - unlike H1 to H3 - and this means dismantling, cleaning, lubrication and reassembly every few years. The risk of damage inherent in this process, even though - in the hands of the most skilled and careful watchmakers to whom alone it would be entrusted - it may be slight, and the slight wear which running might cause, are nowadays felt to be good reasons why H4 - arguably the most important watch in the world - is no longer kept running. It was running until a few years ago. There are videos and simulation of its fascinating action. Several copies have been made of H1 over the last 50 years, and Malcolm Leach has almost finished a copy of H2, while Don Unwin has very recently made a copy of H3 which I saw running a week ago along with the H2 copy at Upton Hall, the British Horological Institute headquarters. The copy of H4 which Harrison made, H5, almost identical except with much less decoration, is in the Collection of the Worshipful Company of Clockmakers at Guildhall, London. It is also in working order though I am not sure whether it is now kept going - I rather think not. Harrison's late regulator clock is also at Greenwich in the same room as H1 to H4. One of his early wooden clocks was in the Time Museum and is now in the Chicago Museum. Another of his clocks is in Guildhall together with another clock movement. Apologies that all this is a bit off sundials but the Longitude film and story IS very interesting! Andrew James
RE: "Longitude" miniseries
3. the Harrison clocks are at the National Maritime Museum in Greenwich. They are functional, but are kept unwound in order to avoid wear-and-tear. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Monday, July 10, 2000 10:15 AM To: sundial@rrz.uni-koeln.de Subject: "Longitude" miniseries 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
Re: "Longitude" miniseries
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
