Why perihelion shift and not aphelion?

[krishna mohan]

Hi..

The explanation of the perihelion shift of Mercury was one of the major victories of GR.
But since the whole orbit precesses, why do people always talk about the perihelion shift and not, say, the aphelion shift?

Is the perihelion shift easier to observe or something?

 

[bcrowell]

I think it's just arbitrary terminology. People who do celestial mechanics are in the habit of tabulating their data in such a way that the orientation of an orbit is described by its perihelion. For instance, there is an angle called the argument of perihelion: http://en.wikipedia.org/wiki/Argument_of_periapsis

However, in an extremely elliptical orbit (not a nearly circular one like Mercury's), most of the precession will occur near perihelion, where gravity is strongest and relativistic effects are strongest.

 

[mathman]

The angle of a perihelion shift is exactly equal to that of an aphelion shift. However the perihelion shift can be measured more accurately, since the distance involved is greater.

 

[bcrowell]

However the perihelion shift can be measured more accurately, since the distance involved is greater.

Huh?

The linear distance would be greater at aphelion, not perihelion. And in any case (a) astronomers measure angles on the celestial sphere, not distances, and (b) you don't determine the orbital elements of a planet from a single observation at a single point in its orbit.

 

[mathman]

Huh?

The linear distance would be greater at aphelion, not perihelion. And in any case (a) astronomers measure angles on the celestial sphere, not distances, and (b) you don't determine the orbital elements of a planet from a single observation at a single point in its orbit.

I had my definitions backwards, I presumed perihelion is furthest from the sun, so that a year to year shift would be a greater distance for the same angular shift. You are correct, the aphelion is furthest distance.