Talk:Longitude

From Wikipedia, the free encyclopedia

	This article is part of WikiProject Maritime Trades, an attempt to build a comprehensive and detailed guide to Merchant Shipping and closely related topics on Wikipedia. If you would like to participate, you can edit the article attached to this page, or visit the project page, where you can join the project and see a list of open tasks.
Start	This article has been rated as Start-Class on the assessment scale.
Mid	This article has been rated as mid-importance on the assessment scale.

Are Dava Sobels' books "Longitude" (which I thought was history wrapped in a historical novel) and "A True Story...." the same book? DJ Clayworth 15:07, 20 Oct 2003 (UTC)

I have Longitude, and I can assure you it is purely non-fiction. You may be thinking about the television programme, which was a dramatized and fictionalized (several characters were merged into one etc. similar liberties taken) version of the events covered in the book. -- Cimon Avaro on a pogostick 15:21, Oct 20, 2003 (UTC)

According to Dana Sobel, John Harrison didn't actually win the Longituded prize. The rules were changed under him so that he no longer qualified for it. Instead, parliament granted him a sum of money in lieu of the prize. --Ezra Wax 16:53, 25 Jul 2004 (UTC)

I thought her first name was Dava, not Dana. Was I mistaken? Michael Hardy 02:35, 3 Jan 2005 (UTC)

Contents 1 Positive and negative longitudes? 2 History 3 Discrepancy between this and Latitude 4 Coordinate system 5 Mile conversion

[edit] Positive and negative longitudes?

This article says longitude runs between +180° and −180°, but it does not say whether east or west is positive. I had always thought that it was standard that west is positive, never suspecting until a couple of hours ago that anyone used the other convention. Then I saw Wikipedia's list of earthquakes, which does use the opposite convention. As I stated on that article's discussion page, I once heard the head of the math department at MIT (David Vogan, who has since been succeeded as department head), speaking before about 150 undergraduates, state that it's appropriate that longitudes in Europe are negative since Europe is a cultural cesspool. No one responded that east longitudes are positive and west negative. (Nor did anyone complain about that characterization of Europe.) So I'm not the only one to think that's standard. Michael Hardy 02:35, 3 Jan 2005 (UTC).

According to the IAU/IAG Working Group On Cartographic Coordinates and Rotational Elements of the Planets and Satellites:

The planetographic longitude of the central meridian, as observed from a direction fixed with respect to an inertial system, will increase with time. The range of longitudes shall extend from 0° to 360°.

Thus, west longitudes (i.e., longitudes measured positively to the west) will be used when the rotation is prograde and east longitudes (i.e., longitudes measured positively to the east) when the rotation is retrograde. The origin is the centre of mass. Also because of tradition, the Earth, Sun, and Moon do not conform with this definition. Their rotations are prograde and longitudes run both east and west 180° instead of the usual 360°.

Thus, Earth longitude should really run westward from 0° to 360°, but because of tradition it runs westward from 0° to +180° and eastward from 0° to -180° (which makes no difference from a geometric point of view).

Urhixidur 04:21, 2005 Jan 3 (UTC)

So whoever wrote list of earthquakes had it wrong, then. Michael Hardy 03:21, 4 Jan 2005 (UTC)

Wait --- the web page you're citing does say longitude is to be measured in an easterly direction. Michael Hardy 03:55, 4 Jan 2005 (UTC)

It is standard for west longitude to be negative;

Standard in what communities? Among professionals in certain fields? Which ones? And do the standards differ from one discipline to another? Michael Hardy 04:05, 4 Jan 2005 (UTC)

I'm yet to see a single computer application that defaults to west positive. It's possible that west positive is standard among some scientific disciplines, but I've never seen it, and I work with a lot of them.

When giving coordinates, it is normal to describe the coordinate system precisely--e.g. LL84 (lat-long World Geodetic System 1984) or LL84-WP (lat-long World Geodetic System 1984 west positive) -- as well as whether the coordinates are in decimal degrees, degrees minutes seconds, or something else. Antandrus 04:16, 4 Jan 2005 (UTC)

however a lot of applications and hardware (such as GPS units) allow LL-WP (west positive) as a preference. Think of a mapping exercise with the x axis going E-W and the y axis going N-S: with west longitude negative, numbers get larger as you go east and larger as you go north, which conforms to Cartesian geometry (with north at the top). Speaking as a GIS professional we never use west positive. HTH! Antandrus 03:37, 4 Jan 2005 (UTC)

What in the world is "HTH"? Michael Hardy 04:05, 4 Jan 2005 (UTC)

Hope that helps. Antandrus 04:16, 4 Jan 2005 (UTC)

Going back to the IAU/IAG paper (a PDF version is found here), now I'm confused. If « the planetographic longitude of the central meridian, as observed from a direction fixed with respect to an inertial system, will increase with time », then this means the longitude is positive eastward, because that's the direction in which the meridian will be seen to be moving (for a prograde planet). So why then say, in the next paragraph, that « west longitudes (i.e., longitudes measured positively to the west) will be used when the rotation is prograde and east longitudes (i.e., longitudes measured positively to the east) when the rotation is retrograde »? It seems a contradiction.

If the aim is to convert the planetographic coords into celestial coords, by using the equation for W (the celestial longitude of the prime meridian, more or less) for the specified time, and then adding the planetographic longitude, then the latter ought to run eastward in all cases (prograde and retrograde). So I guess that's not the aim, then. It also seems to me that the planetographic longitude of the prime meridian is, by definition, always zero --it is only its celestial longitude that spins around. I'll have to sleep on this.

Urhixidur 04:33, 2005 Jan 4 (UTC)

Ah, now I understand. I was confusing prime meridian with central meridian. The central meridian is the meridian above which the observer is hovering --this changes with time as the object rotates underneath the observer, obviously, and that is what is meant in the first quote. Prograde objects have their longitudes labeled from east to west. Obvious, now.

Urhixidur 14:04, 2005 Jan 4 (UTC)

WGS84 has longitude positive eastward, negative westward. Urhixidur 14:11, 2005 Jan 4 (UTC)

From a strictly mathematical point of view, our usual standard is to use a right-hand coordinate system, in which angles are measured in a counterclockwise fashion. --Jacobolus 00:10, 15 Feb 2005 (UTC)

And counterclockwise viewed from above the north pole equal to clockwise viewed from above the south pole, so that doesn't help unless you say which pole is preferred. Michael Hardy 01:26, 15 Feb 2005 (UTC)

I added an explanation of the east-is-positive convention. Also note that the apparent annual motion of the Sun is eastward along the ecliptic, and many of the planets (Earth, Mars, Jupiter, Saturn) rotate west to east. I think maybe Neptune, too. It might be worth adding a remark about the Sun's motion along the ecliptic.

I am curious, however about all the fuss with clocks. Yes, clocks are great and solve the longitude problem. I have taught that. But the motion of the Moon in relation to the Sun and to the stars can be used to tell time, too. Just occurred to me that though it's a bit complicated and not quite so accurate, it might work to low accuracy, but good enough for simple navigation, say within a few degrees longitude.

Pdn 05:19, 25 Feb 2005 (UTC)

[edit] History

I have removed the reference to shipwrecks in Western Australia, since only 5 are known of in early times: English East India Company's, Trial (1622), the Dutch East India Company's Batavia (ship) (1629), Vergulde Draeck (1656), Zuytdorp (1712) and Zeewijk (1727). For these early wrecks I dont know to determine the role of longitude navigation errors as opposed to chart errors (uncharted reefs) and other causes. Anyway ship wrecks were occuring in many parts of the world, motivating the search for a solution. - Op. Deo 20:18, 23 August 2005 (UTC)

The clocks were not so good, but could not the ancient mariners have measured time by the motion of the moon, at least when it's visible at the same time as the sun, so you can get the angle between the two? Pdn

Yes this is the method of Lunar Distances which was the practical competitor to Harrison's chronometer in C18. It was at first limited because the orbit of the moon was not known by ship's officers. But eventually suitable tables were produced which made it easy to measure the moons position relative to certain stars on the ecliptic and determine local time, and therefore longitude. It was then cheaper to provide ships with a set of tables and a sextant, rather than a very expensive chronometer modelled on Harrison's design, So in some ways the impact of Harrison's successful toil was limited. - Op. Deo 22:29, 23 August 2005 (UTC) Thanks. I just saw that Galileo devised a method of using the Moons of Jupiter. see [1] which says: "For Galileo, his admonition marked the beginning of a period of silence. He busied himself with such tasks as using tables of the moons of Jupiter to develop a chronometer for measuring longitude at sea. He endured his rheumatism, ..." Pdn 04:40, 24 August 2005 (UTC)

[edit] Discrepancy between this and Latitude

Latitude says that they are not "exactly" equal to 1 nautical mile whereas this article says that Latitude is "exactly" equal to 1 nortical mile! This seems to be to be something that needs clearing up. —The preceding unsigned comment was added by 58.105.29.26 (talk • contribs).

The latitude statement that one minute of latitude is very close to one nautical mile is correct for the reasons it states. Nevertheless, a minute of latitude was indeed originally exactly one nautical mile at the time it was defined, when it was thought that the Earth was a perfect sphere (rather than an oblate spheroid with an undulating geoid). — Joe Kress 01:17, 16 June 2006 (UTC)

The 4th paragraph should be changed, both because it is factually wrong (twice) and because it contradicts the entry under "Latitude." This creates confusion and introduces uncertainty as to which is correct. The differences may seem small but I was led to this entry while trying to resolve an apparent error in the ground track of a satellite orbit simulator. The error was resolved by using the correct definition of latitude as shown by the Wikipedia entry for that parameter (as opposed to the one implied here.) Mennochio 23:38, 30 July 2006 (UTC)

I agree. I also suspect it is worth pointing to nautical miles, statute miles, and the Great Circle Distance Formula in the same discussion. As is, the formula for equivalency in one long. deg. is misleading (i.e., are degrees in radians? do I get km, really?) 71.236.220.71 16:31, 25 October 2006 (UTC)

[edit] Coordinate system

I think the article should emphasize the primary purpose of longitude: that together with the latitude it uniquely identifies any point on the surface of Earth (or other celestial body). This is mentioned only in the third paragraph, saying

A specific longitude may then be combined with a specific latitude to give a precise position on the Earth's surface.

and in the link to Geographic coordinate system in the See also section. I am not sure yet how to change the article to emphasize this more, so I did not edit it. – b_jonas 21:08, 19 June 2006 (UTC)

[edit] Mile conversion

How many miles is 15 degrees (1 timezone) of longitude? --Homfrog^{Tell me a story!} 00:41, 30 December 2006 (UTC)

A time zone is similar the section of an orange—it is fat at the equator and slims down to a point at either pole, so its width varies from 1670 km (1037 statue miles) at the equator to 0 km at the poles. Mathematically, 15° of longitude = (1669.800 + 5.595sin²φ)cosφ km, where φ is the latitude, varying from 0° to 90°. Multiply by 0.621 for statute (land) miles, and by 0.540 for nautical miles. Technically, each time zone is a gore, 24 of which can be combined to form a flat world map which can be cut so that it can be glued onto a sphere to create a globe, covering the entire sphere without any gaps or overlaps. The fattest part of a gore touches both neighbors at the equator even when lying flat, but their points all touch at the poles when glued onto a sphere. An example of such a world map/globe with only twelve gores (rather than 24) is here. — Joe Kress 21:19, 31 December 2006 (UTC)

You might want to work that explanation into globe, and link to it from gore. Andy Mabbett 00:45, 17 March 2007 (UTC)