(3) Tristan Gooley, Time, Sun and Longitude. The Natural Navigator, July 12, 2013
https://www.naturalnavigator.com ... -sun-and-longitude/
Quote:
(i) "The sun rises in the morning and sets in the afternoon. When it is rising, the sun is east of you. When it is setting, the sun is west of you. When it is neither rising, nor setting it is neither east nor west of you, it has the same longitude as you do. This means that at this exact moment that the sun is highest in the sky [relative to you] * * *
(ii) "Let's say a friend told you that they were going on a touring holiday of the world, but you then said, 'Don't tell me exactly where yet. What I’d like you to do is this...
"Put a stick in the ground and mark the end of the shadows.
"When the shadow reaches its shortest length each day, this means the sun has reached its highest point and I'd like you to give me a telephone call.
"Now on the first day they kindly give up their holiday plans to mark shadows and your telephone goes at 11.00am GMT.
"You quickly do the mental arithmetic. Where your friend is the sun reached its highest point one hour before it did in Greenwich. One hour is 1/24th of a day, therefore they must be 1/24th of the way round the world from you, in the direction of the sun rising. 1/24th of 360 degrees is 15.
" 'You're 15 degrees east of Greenwich.' You reply.
(iii) "The sextant is just a fancy shadow stick in reverse. It may be quite a bit more accurate, but it is not much more complex in principle. All a sextant does is measure angles. Once the angle between your horizon and the sun stops growing [ie, atop you at 90 degrees], you know the sun is highest where you are, therefore it is your local midday.
"A chronometer keeps track of time at your home, or traditionally Greenwich for most navigators. By looking at the time on the chronometer when the sun is highest in the sky, there is no need to ‘phone a friend’. You can therefore work out your longitude on a ship at sea.
"The picture at the top shows the visible time signal that ships moored near Edinburgh used to set their chronometers. The [wooden, per en.wikipedia.org] ball on the Nelson Monument is raised halfway at 12.55, to give a warning to the ships’ navigators to get ready. It is raised all the way at 12.58 and then drops at 13.00 precisely. When it was foggy, the ships would listen to the gun fired at the Castle each day. Sound is not a bad second option, but it travels so much more slowly than light that it is prone to greater inaccuracies. (The gun has traditionally been fired at the Castle, not to keep tourists happy, but because the vibrations would harm the instruments at the Observatory.)
"Tourists take the time to come to Edinburgh, they take the time to listen out for the Castle gun and some even watch the ball drop from the Nelson Monument. But very few take the time to understand the relationship between the sun, time and the place they are.
Note:
(a) City Observatory
https://en.wikipedia.org/wiki/City_Observatory
(1776-2009; on Calton Hill in Edinburgh, Scotland)
(b) Edinburgh Castle
https://en.wikipedia.org/wiki/Edinburgh_Castle
(since on Caste Rock (elevation 140m); table: Built 11th century through to the 21st century)
(c) City Observatory is separate from Edinburgh Castle by two miles in air distance.
So you need the precise time, say GMT, as a reference time. The local time at high noon is always 12 noon; however with GMT, you can not know the longitude of where you are.
(4) The above sounds good, but in reality not precise.
(a) longitude by chronometer
https://en.wikipedia.org/wiki/Longitude_by_chronometer (section 1 Noon sight for Longitude: "It is impossible to determine longitude with an accuracy better than 10nmi by means of a noon sight. * * * At noon the sun's change of altitude is very slow, so determining the exact time that the sun is at its highest by direct observation is impossible * * * By taking a sextant reading within 15 to 30 minutes prior to local noon (culmination) and noting the time, then leaving the sextant set to the same angle and subsequently observing the moment in time at which the sun passes through the sight tube on its descent from its highest altitude between a half-hour and hour later, the two times can be averaged to obtain a longitude sufficiently accurate for navigation (within 2nmi)" ) (citation omitted).
(b) nautical mile
https://en.wikipedia.org/wiki/Nautical_mile
(5)
(a) sextant
https://en.wikipedia.org/wiki/Sextant
(used in "celestial navigation. * * * The principle of the instrument was first implemented around 1731 by John Hadley (1682–1744) and Thomas Godfrey (1704–1749), but it was also found later in the unpublished writings of Isaac Newton (1643–1727). In 1922, it was modified for aeronautical navigation by Portuguese navigator and naval officer Gago Coutinho. * * * The frame of a sextant is in the shape of a sector which is approximately 1⁄6 of a circle (60°), hence its name (sextāns, sextantis is the Latin word for 'one sixth'")
Quote:
"Most sextants also have filters for use when viewing the sun
"Star and planet sights are normally taken during nautical twilight at dawn or dusk, while both the heavenly bodies and the sea horizon are visible.
• There is no need to read the rest of text. Do view the animation whose caption reads: "Using the sextant to measure the altitude of the Sun above the horizon[.]" Please do not read the rest of the text, which I could not understand.
(b) Latin-English dictionary:
* sextāns (noun masculine; from numeral sex six): "a sixth"
https://en.wiktionary.org/wiki/sextans
* sexus (noun masculine): "sex, gender" (Ultimately from this Latin word comes the English noun sex.)
https://en.wiktionary.org/wiki/sexus
(c) How a Sextant Works. PBS, undated.
https://www.pbs.org/wgbh/nova/sh ... te/escapeworks.html
(d) Navigation by Sextant. PBS, undated
https://www.pbs.org/wgbh/nova/shackleton/navigate/escapenav.html
two consecutive paragraphs:
"Finding latitude is easy enough. The first thing you need to do is measure the angle [with a sextant] between the horizon and the sun when the sun is at its highest point, which is right around noontime on your watch [but an ordinary mechanical watch is not reliable at sea; a quartz watch is (reliable)]. A quick look at your trusty tables tells you which line of latitude the sun should be above on that particular day. For example, let's say it's noon on December 21 [winter solstice], and the sun is directly overhead. Well, on that day the sun is above the Tropic of Capricorn, so your latitude would have to be 23.5 degrees S.
" * * * Now if you have a chronometer (this is just a fancy name meaning 'extremely accurate clock'), you can find your longitude. Let's say that the sun is directly overhead and your chronometer, which was set to noon when you were at 0 degrees, says it's 3 o'clock. This means that three hours ago the sun was overhead at this latitude at 0 degrees longitude. In those three hours, the sun moved 15 degrees 3 times, or 45 degrees. So you're at 45 degrees West. Of course, the fact that the sun was directly overhead (which very rarely happens) made it especially convenient for finding your longitude, but you could have found your longitude anyway, with the help of your tables.
• trusty (adj): "TRUSTWORTHY, DEPENDABLE"
https://www.merriam-webster.com/dictionary/trusty
• "the fact that the sun was directly overhead (which very rarely happens)"
The sun over one's head happens if and only of he is located between Tropic of Cancer 北回归线 and Tropic of Capricorn 南回归线, AND in season. Outside this zone, the sun willneer appear over the head. I have observed for decades that sun in Massachusetts never does even on summer solstice.
(c)
(i) Chronometer. Encyclopaedia Britannica, undated
https://www.britannica.com/technology/chronometer
("the word [chronometer] was originally employed in 1779 by the English clock maker John Arnold to describe his sensationally accurate pocket chronometer “'no. 1/36.' * * * [Englishman] constructed four marine timekeepers, the fourth of which effectively won him the reward of £20,000 offered in 1714 by the British government for any means of determining a ship's longitude within 30 geographical miles (about 34.6 miles, or 55.7 km) at the end of a six weeks’ voyage. * * * The modern chronometer is, broadly speaking, a large, well-made watch but with a detached chronometer escapement, suspended in gimbals (a set of rings connected by bearings) poised so as to remain horizontal whatever the inclination of the ship. The modern chronometer is, broadly speaking, a large, well-made watch but with a detached chronometer escapement, suspended in gimbals (a set of rings connected by bearings) poised so as to remain horizontal whatever the inclination of the ship")
(ii) How Does a Marine Chronometer Work. Sep 8, 2017
https://sailorinsight.com/how-does-a-marine-chronometer-work/
("With the advent of satellite navigation, GP and integrated navigation systems very accurate GMT can be obtained from any of thm and hence ib nidern ships the chronomeeter tends to take a backseat. However, at least one chronometer is carried on board ships and they are required to be maintained")
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