PDA

View Full Version : Extraterrestrial Longitude



Simmo
2003-Jul-03, 12:29 PM
Could someone who knows more about such things explain how longitude is assigned for worlds other than the Earth ? Is there a technical procedure for calculating it - or is it just an arbitrary assignment, made when the first detailed maps of a world, moon, rock, etc, are prepared ?

I'm just intrigued as the coordinates for the landing sites of various missions to Mars have been published and while latitude is no challenge to understand and equators, tropics and polar circles can be measured, I just wondered how the zero longitude position was decided.

kucharek
2003-Jul-03, 12:38 PM
AFAIK, for Mercury, they picked a very small crater, said this crater is at 20 deg longitude and named the crater "Hun Kal", which is the Mayan? word for twenty.
I don't know how it was fixed for Moon, Mars etc.
On Earth, it was a long, political struggle. In old times, nearly every - especially seafairing - nation defined 0 longitude somewhat through their capitals.

Harald

Added:
On Mars, it's a small crater named Airy-O. Read more at
http://spaceflightnow.com/news/n0102/17mgsairy0/

Simmo
2003-Jul-03, 01:31 PM
Thanks for that - it sounds more or less what I expected.

The Greenwich observatory in London, now an interesting museum, has lots of info about how the prime meridian on Earth was decided - plus several iron bars in the ground indicating various meridia (?) used before the current one was decided. They also used to shine a laser through the night sky along the meridian to the north, but I don't think they do any more.

Argos
2003-Jul-03, 02:06 PM
How the 0 degree meridian is defined on other planets. (http://itss.raytheon.com/cafe/qadir/q500.html)

Glom
2003-Jul-03, 04:14 PM
I'd imagine that for Luna, the Zero Meridian was defined as being the meridian pointing directly at us, since Luna is tidally locked.

kilopi
2003-Jul-03, 04:28 PM
I'd imagine that for Luna, the Zero Meridian was defined as being the meridian pointing directly at us, since Luna is tidally locked.
Yes, and the same for the Sun, right? Since it doesn't have any fixed features...

Argus's link (from Dr. Sten Odenwald) says that two systems have been developed for Jupiter, and none for the other gas giants.

Donnie B.
2003-Jul-04, 12:06 AM
I'd imagine that for Luna, the Zero Meridian was defined as being the meridian pointing directly at us, since Luna is tidally locked.
Well, maybe in that general area... but libation means there's no such exact point.

tracer
2003-Jul-04, 12:52 AM
I say they ought to define 0 degrees longitude on the moon by putting a line through the center of Tycho crater. That's where the giant black alien monolith was discovered in 2001, after all.