|Planetary Prime Meridians
||[Jan. 3rd, 2007|07:35 pm]
this article (and it referencing Titan's 35ºW) made me think about how prime meridians for planetary bodies are chosen. Wikipedia doesn't have much, nor on the Titan article.Reading |
Anybody know who picks them and/or how?
I would guess that a group of people get together and decide stuff like that.
They could have done a little better in some places, I think.
Judging by how Mars and Moon prime meridians were defined I'd say that decisions were quite arbitrary (same goes for good ol' Greenwich btw).
2007-01-04 10:06 am (UTC)
Actually the choice of Greenwich was not arbitrary: by the time of the meridian conference in Washington DC in 1884, about 70% of the world's shipping used charts based on the Greenwich meridian.
Agree, maybe not arbitrary but customary. Still, this choice had no real scientific meaning (compare with equator which could only be defined as it is due to obvious reasons).
2007-01-04 06:15 pm (UTC)
The scientific requirement of the prime meridian (in 1884) was that it had a good observatory with a transit instrument and time-keeping equipment, or that it was some fixed offset from such an observatory. It was also necessary for it to have a simple relationship - 0 or 180 degrees - with what is now the International Date Line. By the 1880s sailors had already established an informal date line through the Pacific, and it is positioned there by the historical accident that the Americas were colonised across the Atlantic.
The need for a prime observatory became less of a concern after time transfer mechanisms were developed after WWI, which allowed Universal Time (and therefore longitude) to be determined by averaging times from multiple observatories. (The French set up and managed this process, which let them take control of longitude despite having lost the argument in 1884.) Various transitions in timekeeping (including the move of the British Royal Observatory to Herstmonceaux, and fixes to geodetic models to make them geocentric) introduced errors into the position of the prime meridian. Its current realizations (e.g. WGS84, as used by GPS) are now about 100m away from the line on the ground at Greenwich.
So yes, it's nothing more than vaguely preserved custom now, but there were several reasons for the choice at the time it was made.
It seems to me that they are chosen based upon being readily identifiable. Since the surface of Titan hasn't been throughly surveyed, they have to pick a line that they'll remember.
The Bruce Crater wikipedia article says "there is a band of darker material cross the mid-point of the crater from west to east." Since they can easily spot that band, and they have to arbitrarily chose a longitude line anyway, it's the obvious choice because they can easily find it again.
2007-01-04 05:25 am (UTC)
that is maps of all satellites (theirs names are given into Russian) with the literature list added.
i think they built the cylindric (or somewhat else) projection of the Titan's surface and covered it by coordinate lanes, having used somewhat geological features as an origin.