# Why is it difficult to measure the perihelion advance of the planet Venus? Help!

1. Mar 28, 2005

### kiwikiwi79

2. Mar 29, 2005

### Andrew Mason

Venus' orbit is practically circular so the perihelion location is less distinct. Venus' orbital plane in inclined more than 3 degress to the plane of the earth's orbit. I don't know if that makes it more difficult to measure. The main reason is probably that the rate of advance is so small. Mercury's perihelion advance is about 10 minutes/century. Venus' is about 8 seconds of arc per century.

AM

3. Mar 30, 2005

### pinkie

The curvature of space time closer to the sun is also a factor, right?

4. Mar 31, 2005

### SpaceTiger

Staff Emeritus
If you're talking only about GR corrections, then yes, but the dominant source of perihelion advance in planetary orbits is perturbations from other planets.

5. Mar 31, 2005

### Astronuc

Staff Emeritus
So, it would appear to be a matter of precision of either measurement or calculation, and a matter of the geometry of the orbit (almost circular). I imagine the perturbations in the orbit due to other planets are on the order of a few seconds of arc.

6. Apr 5, 2005

### Ian

KiwiKiwi,
The best way to measure the perhelion advance of any of the solar system planets is to first eliminate the peturbations of the other planets, then calculate the length of advance as opposed to the angular advance.
For Mercury the angular advance is about 43 seconds of arc, less for Venus and even more so for the earth; if we work with the length of advance instead (the length of the arc subtended by the angle of advance) it is the same for all planets and is approximately 27833.859 metres.
This length is determined only by the mass of the sun, G, and the velocity of light. Since this length is the same for all the planets in their orbits after the corrections for orbital ellipticity are removed we really ought to say that the perhelion advance is the same for all planets regardless of their distance from the sun.

7. Apr 8, 2005

### Mariko

i dont know weather this helps but maybe it could have some thing to do with the fact Venus is in retrograde?

8. Apr 8, 2005

### Mariko

i dont know weather this helps but maybe it could have some thing to do with the fact Venus is retrograde?