Re: Does Jupiter peturb the earth's orbit when it is closest?

Yes, one can detect the effect of Jupiter's gravity on the Earth's orbit. The amount of the deviation is too small to be visible as an obvious bulge in the path of a single year's orbit. The perturbation is more easily visible by plotting the difference in the Earth's trajectory between two different years: as Jupiter moves in its orbit, its effect on the Earth's orbit will change from year to year.

I don't know the exact distance of the perturbation, but I have run a simple computer model of Earth's orbit, once with and once without the perturbing presence of Jupiter. This model shows a difference of roughly 0.001 astronomical unit (150,000 kilometers) in the shape of Earth's orbit. between the two experiments. This difference is almost impossible to see when you plot the whole orbit.

By taking careful measurements of the positions of planets in the sky and comparing them with precise mathematical models of the gravitational attraction of the various planets upon each other, scientists in the 19th and early 20th century were able to deduce the existence of Neptune and then Pluto by observing their perturbing effect on the inner planets. Observers then discovered the planets by looking in the region of the sky where the models predicted the planets could be found. Given the small mass and great distance of Neptune and Pluto, this is an extremely delicate experiment, requiring very precise observations and computation.

[Moderator's Note: Everything Jason said is true, but actually the "prediction" of Pluto's existence based on its gravitational effect turned out to be an error! An astronomer at the beginning of the 20th century examined the orbit of earth and found a discrepancy that led him to predict the existence of a planet outside of Neptune. However, his calculations had an error in them, and there really was no discrepancy. Without knowing of the error, Clyde Tombaugh searched for the planet and found Pluto, completely fortuitously!]