What decides longitude zero ?

[Externet]

Watching some images of Pluto and Charon (http://www.space.com/11431-photos-pluto-charon-moons-dwarf-planet.html), see that longitude lines have already been decided.

Should be recent, as close photos are.

What features are taken in account and who decides were to set longitude zero position ? I assume other planets have been already determined.

 

I wished they had decided for a different method/scale instead of the same archaic/arbitrary 360 degrees on earth...

 

[John Cuthber]

It looks like they make a pretty arbitrary choice in most cases.

https://en.wikipedia.org/wiki/Prime_meridian#Planets

It looks like Charon and Pluto are tidally locked so the "obvious" choices for their prime meridians are the middles of the sides facing each other.

Edited July 14, 2015 by John Cuthber

[imatfaal]

It looks like they make a pretty arbitrary choice in most cases.

https://en.wikipedia.org/wiki/Prime_meridian#Planets

It looks like Charon and Pluto are tidally locked so the "obvious" choices for their prime meridians are the middles of the sides facing each other.

 

My obvious choice would have been something else...but that's typical of me.

 

I assume that once a rough position is chosen then a surface marking will be decided to mark a point on the prime meridian. Even tidally locked partners will librate due to the non-circular orbit and any axial tilt - a theoretical point marking closest point between two tidally locked bodies will describe an analemma around any fixed physical point. On Pluto this is taking precision a bit too far - but there are other tidally locked bodies that are more accessible

[MonDie]

 

On Pluto this is taking precision a bit too far - but there are other tidally locked bodies that are more accessible

 

Not when you consider Pluto's so slow. Its motion is mostly attributable to its ~10 degree parallax (retro/prograde cycle).

 

Externet, if you mean celestial longitude AKA ecliptic longitude, then it's the vernal equinox, the point at which the Sun crosses the celestial equator every spring. This point does move due to the precession of the equinoxes, but I don't think by much.

Edited July 15, 2015 by MonDie

[Externet]

 

...Externet, if you mean celestial longitude AKA ecliptic longitude...

Just plain meridian zero. Seems answered as arbitrary to someone's eye or consensus.

[imatfaal]

 

Not when you consider Pluto's so slow. Its motion is mostly attributable to its ~10 degree parallax (retro/prograde cycle).

 

Externet, if you mean celestial longitude AKA ecliptic longitude, then it's the vernal equinox, the point at which the Sun crosses the celestial equator every spring. This point does move due to the precession of the equinoxes, but I don't think by much.

 

Could you explain those points? You countered my point by saying that Pluto's motion is slow - which of it's many motions?

 

Parallax? You are using that term in a way I do not understand - I think of parallax as the apparent displacement of a distance object when viewed from two different perspectives; ie a the definition parsec is the distance required to give a annual parallax angle against distant stars of one arc second. It is an observational artefact rather than a physical characteristic

 

In reply to Externet you seem to be conflating co-ordinate systems which give viewing angles from earth with the arbitrary picking of a prime meridian which John has already provided links to. So I am confused here too.

[MonDie]

 

Could you explain those points? You countered my point by saying that Pluto's motion is slow - which of it's many motions?

 

Parallax? You are using that term in a way I do not understand - I think of parallax as the apparent displacement of a distance object when viewed from two different perspectives; ie a the definition parsec is the distance required to give a annual parallax angle against distant stars of one arc second. It is an observational artefact rather than a physical characteristic

 

I was talking about motion through Earth's ecliptic plane, which necessarily involves retro/prograde cycles from the parallax shifts due to Earth orbiting the Sun. Looking at photos 31-35 in Externet's link, that's obviously not what was meant because Pluto is currently in Saggitarius around 275-295 ecliptic longitude.