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catloaf
2010-Apr-10, 11:56 PM
What criteria was used to determine the location of 0 degrees longitude on Mars? They just did it to honor some guy at Greenwich making it fairly arbitrary or was there a method in place for the determination?

grant hutchison
2010-Apr-11, 01:52 AM
It was fairly arbitrary, but long predated the naming of the crater Airy after some guy at Greenwich.
Early maps of Mars, drawn in the 19th century, used a meridian plotted through an albedo feature called "A", which was subsequently named Sinus Meridiani. That meridian has been in use ever since, being tied to smaller features as increasing accuracy was required. The name Airy was applied to a crater on the meridian, as a nod to Airy's work on the Greenwich meridian, and then a small crater within Airy was adopted as the marker for zero longitude.

For bodies in synchronous rotation, the zero meridian is usually defined as the centre of the hemisphere that faces the primary. For other bodies, like Mars, the choice is necessarily arbitrary.

Grant Hutchison

chrlzs
2010-Apr-11, 02:43 AM
It was fairly arbitrary, but long predated the naming of the crater Airy after some guy at Greenwich.
Early maps of Mars, drawn in the 19th century, used a meridian plotted through an albedo feature called "A", which was subsequently named Sinus Meridiani. That meridian has been in use ever since, being tied to smaller features as increasing accuracy was required. The name Airy was applied to a crater on the meridian, as a nod to Airy's work on the Greenwich meridian, and then a small crater within Airy was adopted as the marker for zero longitude.

For bodies in synchronous rotation, the zero meridian is usually defined as the centre of the hemisphere that faces the primary. For other bodies, like Mars, the choice is necessarily arbitrary.

Grant Hutchison

Don't all such bodies experience libration? Surely you would still need to anchor it somehow..

EDG
2010-Apr-11, 03:18 AM
Libration is a back and forth motion of the satelilte as viewed from the primary - there's a point halfway between the extremes of the libration that can be used as a zero meridian.

grant hutchison
2010-Apr-11, 12:22 PM
Don't all such bodies experience libration? Surely you would still need to anchor it somehow..Yeah, you need to use some sort of mean centre. If you plot the position of zero latitude, zero longitude on the Moon, as viewed from the centre of the Earth, and follow it over time, you'll see it perform a series of Lissajous-type loops, with the Earth-Moon line at the centre of the loops.
What seems to happen in practice is that a mean sub-primary point is calculated (over some time period, using some model), and then the coordinates are tied to a reference feature on the surface. The reference feature doesn't need to be at zero longitude: it just needs to define one value of longitude. So, for instance, the longitude reference for Europa is the crater Cilix, at 182 degrees longitude.

Grant Hutchison

catloaf
2010-Apr-11, 11:49 PM
for instance, the longitude reference for Europa is the crater Cilix, at 182 degrees longitude.


That's interesting. I thought Europa resurfaced its surface (heh) on a regular basis, like Io, and using impacts as points of reference would be futile. I imagine future astronauts arriving on Europa, looking for Cilix and remarking "Well it was right here!"