Talk:Longitude

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)

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 (http://www.hnsky.org/iau-iag.htm):

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 (http://ipsapp008.lwwonline.com/content/search/4579/51/2/fulltext.pdf)), 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)