Venus Perihelion

  1. Why is it so hard to measure the perihelion advance of Venus?
  2. jcsd
  3. tony873004

    tony873004 1,555
    Science Advisor
    Gold Member

    Maybe because Venus' orbit is very round. It's almost a perfect circle, and a perfect circle would have no perihelion (or would it be correct to say that the entire orbit would be perihelion and apihelion at the same time?)

    I'm just guessing here, but as Venus gets tugged around by other planets, its perihelion might almost instantly jump from one part of its orbit to another without actually advancing smoothly to that point. This being because in an almost perfect circle, a point even 180 degrees from perihelion just barely misses being the perihelion itself.

    So it's kinda like the calculus problem, what is the slope at a cusp? Undefined, it can point in many different directions. For Venus, with its almost round orbit, which point is the Perihelion, and would the smallest little change affect which point would be considered Perihelion?

    Something with a higher eccentricity such as Mars would never have this "perihelion identity crisis", and its perihelion advances in a periodic fashion.

    Just my guess since I've never heard that it was hard to measure. :smile:
  4. Well to be honest, I never heard that it was hard to measure either.
    The prof was teaching us about Mercury's perihelion and how it baffled scientists for so long. Then Einstein's GR came along and solved the mystery. However, I never really understood the problem with Mercury's perihelion, so maybe if someone could clear that up that would be nice...

    But so anyway, he then asked us for an assignment question, "Why is Venus's perihelion difficult to measure?"

    The more I searched on the net, the more I found information regarding Mercury. The closest information I got towards Venus was that, "Mercury, Venus and Earth have a perihelion advance".

    I'm actually quite lost with what they mean by perihelion advance too. Man I sound stupid today.

    Well thanks for any input.
  5. tony873004

    tony873004 1,555
    Science Advisor
    Gold Member

    Mercury drifted a small amount from where it was predicted to be. Early theories suggested that a planet interior to Mercury was causing this. But Einstein's theory says that time runs at a different rate in the presence of a strong gravitational field. Mercury is close to the Sun, and Einstein's theory was all that was needed to clean up the slight discrepancy.

    As far as advance of Perihelion:
    Draw a long ellipse on a piece of paper. Make it point up and down. Then draw another one, same size and shape, but make it point sideways. Now picture all the positions inbetween. For the orientation of the orbit to get from the vertical drawing to the horizontal one, it precesses or advances. Not just the perihelion, but the whole thing. Perihelion is just a convinent point in the orbit to use as a reference. So in your vertical ellipse, if you place the Sun in the bottom focus, your perihelion is at the 6 o'clock position, and in your horizontal ellipse, your perihelion is at the 3 o'clock or 9 o'clock position depending on which way it advanced.

    With Venus, the orbit is round. Every point on the orbit is almost perihelion. One point will be the closest to the Sun, but it won't be much closer than the furthest point. The smallest little change can cause this point to drift chaotically and quickly to another part of the orbit.
  6. Thanks a lot tony!
  7. Smokie,
    As a matter of interest, try calculating the 'length of advance' of perhelion of all the planets in the solar system, as opposed to the 'angular advance' Einstein used. Try and determine the length of the arc subtended by the angle of advance and you might realise something very interesting.
    You can do this by either removing the correction term for the ellipticity of orbit from Einstein's equation ( r(1-e^2) ), or by using 6PiGM/c^2. Either way will give you dimensions of length, but the length will be the same for all planets regardless of distance from the sun.

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