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- 1 Some questions
- 2 Possible error
- 3 What happened...?
- 4 Polynesian celestial navigation
- 5 "Sights on the moon, planets and stars allow navigation to occur at night or when clouds obscure other objects"
- 6 "Celestial navigation...was the first system devised to help sailors cross the featureless oceans without having to rely on random chance to enable them to strike land."
- 7 Merge of Celestial Navigation Trainer into Celestial Navigation
- 8 are these 57 stars visible to the naked eye?
- 9 Text Removed
- 10 "Beyond the scope of this article"
- 11 Common practise today
- 12 Setting chronometers?
- 13 The Link Celestial Navigation Trainer
- 14 Night and day navigation
- 15 Spaceflight
- 16 Aviation
Can you not use a theodlite to make the measurements? Can we please have something on this.
Would you not measure 3 stars/ objects as opposed to two and then use triangulation?
Isn't spherical trigonometry involved? in which case I would not call it "simple".
- A sextant has some special adaptations to shipboard use. First, it minimizes cosine error, the error caused by the length of the measurement device. It does this by measuring differentially from the celestial object to the horizon. That is, the cosine error is limited by the distance from the user to the horizon. The other important difference is that a theodolite's angles would move with the ship's motion. A sextant's two-mirror system, and use of the horizon prevents this problem as well. Ray Van De Walker 23:48, 18 July 2012 (UTC)
- 3 stars vs. 2?; Well, one, actually. The main tool of a practical navigator is a dead-reckoning track, periodically corrected by GPS or celestial navigation. The theory is that "shooting" a celestial object places you on a great circle centered on the instantaneous ground position of the celestial object. Given a reliable DR plot using a distance measuring device (a "taffrail log") and a magnetic compass, the single great circle is often enough. A second sight on another object places you on one of two intersections, one near the DR track, and the other usually thousands of miles away. A third sight creates a triangle of error.(i.e. it exposes the errors) Ray Van De Walker 00:05, 19 July 2012 (UTC)
- Spherical trig? Actually, no. Common sight reductions are done with ordinary trig, because the angles are in the sky, or fixed between the horizon and the sky. They don't usually lie in the surface of a sphere. The sight reduction tables are logs of trig constants, so that the angles and distances can be calculated by adding. Ray Van De Walker 00:05, 19 July 2012 (UTC)
I don't think this is quite right:
- "If you measure the angle to Polaris and find that it is 60 degrees away from directly overhead, then you are on a circle 60 degrees away from the north pole. This circle coincides with a circle of 60 degrees of geographic latitude."
If you are on a circle 60 degrees away from the north pole, then that would correspond to 30 degrees of geographic latitude, since latitude is measured from the equator.
for every degree moved away from the equator (in this case north), the star in question (Polaris) will "move" one degree towards the zenith. => if standing at the equator, Polaris will be located on the horizon, as you move towards the pole, Polaris will slowly seem to move at the same rate.
This article is totally empty! There's nothing after the title and the words "From Wikipedia, the free encyclopedia."
Erik 22:21, 6 October 2006 (UTC)
Should this topic include a paragraph on Polynesian navigation? Using 'star paths' of stars on the horizon, rather than stars near the zenith?
This statement, and the omission of the actual process of taking sights may be misleading to the reader. Sun sights, and if it is visible, sights of the moon can be taken during daylight. However, since the measurement of the angle between the celestial body and the horizon requires that both be visible at the same time, the impression given by the quoted sentence that observations can be taken at night, is not correct. Since stars are generally not visible during daylight (not completely true, but a fair generalization) and the horizon can't be observed in darkness, star sights are taken only at twilight a period which is several minutes long (longer at temperate latitudes and less in the tropics). The routine one would follow, if relying on celestial navigation and wanting to do the full process is to prepare in advance for the dawn twilight by figuring out in advance the approximate azimuth and elevation of several stars that produce good non acute angles in the lines of position they will produce. By having them figured out in advance, the sextant can be preset, and, together with a compass, the sought star can be picked out easily, several sights can be taken, and then you can move on and shoot the other selected stars, the more the better (two gets you a fix of limited value, 3 that coincide nicely tell you it is likely a good fix, 4 will give you an idea which line is to be ignored if one is out, etc), in the few minutes that twilight allows. You do the same process at the evening twilight. You maintain a dead reckoning position during the day and take sun sights, say every hour, advancing the lines of position along your course by the distance covered to produce a series of running fixes through the day. A noon shot, gets you your latitude. If you happen to have a moon during the day, you can get actual two body fixes. In cloud or darkness though, you depend on dead reckoning or any other method you have available.
This is a good article, but I wonder if something along these lines would improve it? At a minimum, I think the referenced sentence should be deleted. As a latecomer to this fine article though, I thought it might be better to post this first.
Actually, no. A sailor can drop a line to the bottom to find its depth and character (muddy, sandy...). He can look for clouds (which form differently over land than over water) or land birds. And he can use dead reckoning. I think the article should rather emphasize that celestial navigation was the first accurate method that can be used anywhere in the world. Jrvz 11:04, 8 August 2006 (UTC)
I'm qualified in Celestial Navigation aboard both aircraft and watercraft, for both magnetic and grid navigation. I concur. Mugaliens 20:00, 28 September 2006 (UTC)
are these 57 stars visible to the naked eye?
Yes, but some are faint, and cannot be seen if the Moon is bright. Modern sextants multiply the illumination slightly through optics. - Mugs 18:41, 10 January 2007 (UTC)
I removed the following text for two reasons: "Sights on the moon, planets and stars allow navigation to occur when clouds obscure other objects." Reasons: First, if clouds are obscuring objects on the surface of the planet, they're usually obscuring celestial objects, as well! Second, celestial navigation is not normally performed in the vicinity of other landmarks due to it's lack of accuracy. Rather, those landmarks are used, instead. - Mugs 18:41, 10 January 2007 (UTC)
"Beyond the scope of this article"
Why is this statement here? "the size and location of this circular line of position can be determined using mathematical or graphical methods that are beyond the scope of this article." Why is it "beyond the scope of this article?" Wiki contains thousands of far more technical articles than celestial navigation! The methods are sufficently discussed in the remainder of the article. Text removed. - Mugs 18:44, 10 January 2007 (UTC)
Common practise today
"Modern practical navigators nearly always use celestial navigation in combination with satellite navigation to correct a dead-reckoning track, that is, a course estimated from a vessel's position, angle and speed."
My current experience at sea aboard oceangoing ships indicates this should read something like "With widespread availability of GPS, celestial navigation serves two primary function today. First is the determination of compass error in the absence of terrestial references. Secondly, celestial navigation is used as a check on the reliability of GPS positioning data." --Captclbecker 03:02, 29 January 2007 (UTC)
- Isn't GPS a kind of celestial navigation? The GPS unit detects photons (radio instead of optical) from objects at known locations in the sky (man-made objects, rather than natural) and determines its location on the earth's surface. Seems to be "celestial navigation" to me. Spebudmak 22:19, 12 April 2007 (UTC)
- I humbly suggest that the entire GPS network is based on calculations and comparisons of "when exact solar noon" would be at various waypoints in question. — Preceding unsigned comment added by 188.8.131.52 (talk) 21:06, 2 September 2011 (UTC)
- By your definition, yes. But the term "celestial navigation" is specifically reserved to refer to navigation by the Sun, Moon, Planets, and the stars via a means that provides uncorrected height of the body, and for the purpose of determining true heading.
The article says "Traditionally, a navigator set his chronometer from his sextant...". As far as I know, chronometer are never set "in the field". I assume what is meant is that the procedure described is used to determine the current time, and that is compared with what the chronometer shows, so the chronometer's error at that instant is known. Paul Koning 15:47, 27 April 2007 (UTC)
- Chronometers were reset at regular intervals, usually each week, using techniques such as lunar distance or taking observations at known points.
- I have a question. How do I begin editing the article?
- Never mind. The help pages are very good. Celestialmechanic 19:39, 3 August 2007 (UTC)
- The marine chronometer section of Navigation specifically says that chronometers were not reset in the field, and certainly that seems to be a sensible practice. For one thing, it requires taking the cover glass off the chronometer. For another, you wouldn't reset the second hand, there is no provision for that. You need to know the "rate" of the chronometer (its systematic error) in any case to correct the time shown, so there is no conceivable value in resetting a chronometer. Do you have a reference supporting "reset at regular intervals"? Paul Koning 16:35, 6 August 2007 (UTC)
- Here's a reference for keeping track of and compensating for chronometer error; it in no way recommends resetting the chronometer. http://books.google.com/books?id=W_cAAAAAYAAJ&pg=RA2-PA227&dq=marine+chronometer&output=html --SV Resolution(Talk) 14:58, 8 August 2007 (UTC)
- Actually, the article says the navigator set the ship's CLOCKS, not the chronometer(s). The naval day began at noon, which was made daily by observation of the sun at zenith. Any ship's clocks would have been set to noon daily. --SV Resolution(Talk) 16:45, 8 August 2007 (UTC)
- When I originally made my comment (back in April) the article did claim that chronometers were set. I subsequently removed that claim when I found confirmation that this isn't done, so the current text talks about checking but not about setting. Your additional references are helpful to confirm that the current text is the right one, thanks! Paul Koning 16:05, 9 August 2007 (UTC)
The Link Celestial Navigation Trainer deserves an article in its own right. Lumos3 11:47, 27 August 2007 (UTC)
There seems to be no disctinction in the article, I suggest changing the into to
During daytime; celestial navigation generally uses "sights," or angular measurements taken between a celestial body (the sun, the moon, a planet or a star) and the visible horizon. The sun is most commonly used. At night, navigators can also use the moon, a planet or one of 57 navigational stars whose coordinates are tabulated in the Nautical Almanac and Air Almanacs. At night, distances between celestial objects can be measured and calculated, or the person can simply decide to travel in a specific direction (north, south, west, east) using one of the navigational stars. — Preceding unsigned comment added by 184.108.40.206 (talk) 08:42, 19 March 2012 (UTC)