Why use Schwarzschild metric for light deflection and planetary precession

[davidge]

Why one uses Schwarzschild metric instead of FLRW metric when deriving things such

- deflection of light by the sun
- precession of perihelia of planets

Also, as our solar system is not isotropic nor static, it seems that by using the Schwarzschild metric we would get only an approximation on the results. So why we still get accurate result for the precession of perihelia of Mercury?

 

[Ibix]

Why one uses Schwarzschild metric instead of FLRW metric when deriving things such

- deflection of light by the sun
- precession of perihelia of planets

Because the FLRW metric assumes that spacetime is filled with uniformly distributed matter. A bad approximation on the scale of the solar system.

Also, as our solar system is not isotropic nor static, it seems that by using Schwarzschild metric we get only a approximation on the results. So why we still get accurate result for precession of perihelia of Mercury?

Because it's a very good approximation because the mass of the Sun is such a huge percentage of the total mass of the solar system.

 

[davidge]

Ah, ok. Thanks.

A bit off topic... but, the name precession of perihelia is due to the fact the we use the nearest distance $r$ of the planet from the sun when computing the angle? Is there a special motivation for non using the aphelion distance instead?

 

[vanhees71]

No, it's just common use in astronomy.

 

[davidge]

No, it's just common use in astronomy.

Thanks.