This is an intriguing question, I have read many different opinions but nothing to clearly substantiate their claims. Others in the Scientific arena when pressed are afraid to give an answer. Checking the JPL data which is an ephemeris produced by the Jet Propulsion Laboratory shows that Jupiter certainly doesn’t orbit around the SSB (although at first because of an error I made, it looked as if it did) and probably orbits around the Jupiter/Sun barycenter. After checking the Jupiter/Sun distances (which I will refer to as the radius vector which is quite different from the semi-major axis) through JPL it became obvious there was a substantial variance each orbit that was measured each time in the same place. Originally I saw this as an opportunity to look for a solid link between Angular Momentum and the Solar modulation re the planets, but soon discovered Jupiter and all the other planets have a modulating Perihelion/Aphelion distance. The quest was on the find out why and involved many weeks searching for any data I could find. Others on this site got involved and I even emailed an Astronomer but to no avail, very sparse detail available. Dr Svalgaard suggested it was a result of planetary perturbations and while correct his understanding was also far from complete. Below is an account of the progress along with what I think is the complete answer to what perturbs Jupiter and how.
My research shows the Jupiter/Sun distance varies on average 300,000 km each orbit of Jupiter. Check graphic above.
The Earth is seen to orbit the Sun directly. If we use NASA’s JPL data it shows the Earth/Sun distance is varying by 0.0001AU per year (approx 15000 km). The AU measure is the average distance between the Sun and Earth. If we measure The Earth to SSB (solar system barycenter) distance it shows a much larger variance.
What I have done is measure the Jupiter/Sun distance at exactly the same point in the elliptical orbit of Jupiter each 4331.572 days, this should isolate any aphelion/perihelion changes. I also measured the Jupiter/SSB distances.
As can be seen in the first graphic there is a large variance each orbit between BOTH measurements (Sun & SSB). This makes it hard to pinpoint any point of orbit, although the variances seem smaller on average with the Jupiter/Sun data. I also compared the Heliocentric longitude (angle away from Sun) and found each Jupiter/Sun reading had an angle of 359 deg, but the solar position at perihelion does return very closely to the same position each orbit.
The above graph was made from reliable data by blogger JimP (thanks for the spreadsheet). Once blown up it shows some remarkable detail. It shows the Perihelion distance (closest point) for each Jupiter orbit over 400 years. There is a definite pattern, what causes this modulation? The movement between the closest and furthest perihelion is 1.2 million km’s , the Sun is capable of moving 1.5 million km’s from the barycenter, Neptune’s radius vector moves by 1.4 million km. Importantly the corresponding movement at the other end (aphelion) is the same value on each orbit but in the opposite direction, if one end shortens the other end lengthens.
Jupiter/Sun distances 1600-2020 with Jup/Sat conjuncture, Jup/Sat opposition and Jup/Sat quadrature (square) positions plotted for every third occurrence. Note the phase change around 1880. If the Variance in distance is caused by angular momentum J/S conjunction & J/S opposition both produce high angular momentum. Regular reader lgl and myself suspect the other Jovians need to come into the equation, but what is needed is a new method of displaying angular momentum other than Carl’s graph. A total strength with both J/S conjuncture and J/S opposition shown as high points instead of a sine wave is required, manipulation of the current JPL data via a spreadsheet can do it. Update: There is a new article covering this.
Saturn/Sun distance showing the same 60 yr pattern as Jupiter. After 1880 they are in unison with Jupiter but not before. There is a reason for these fluctuations but I have not found any literature on this phenomena.
This diagram has been adapted from “Linkages between solar activity, climate predictability and water resource development” Alexander et al. Bailey has been criticized for claiming the Sun-Earth distance varies by the Sun-SSB distance, although not correct there is a fluctuation caused in the same manner as per Jupiter but to a smaller degree….0.0001 AU.
UPDATE 27/05/09: Here is another suggestion Dr. Svalgaard has provided to explain the modulating Jupiter Perihelion.
“The general principle is that to change the orbit, you have to apply a force along the orbit, so Saturn’s effect is largest when it is ‘to the side’ (approximately quadrature I think the astrological term is), rather than in conjunction. All of this has been understood for 250 years, and there are no other forces involved. Saturn doesn’t ‘push’ (gravity is attractive, not repulsive). And the Sun is not ‘dragging’: the force is always along the line connecting two bodies.
To recapitulate how Kepler’s second law works: To move an orbit out a bit (increase perihelion distance if you are near perihelion), you apply a force along the orbit in the same direction as the movement of the planet, e.g. by Saturn being ahead of Jupiter. That force produces a ‘delta-v’ (google it), which ‘lifts’ the orbit a bit out of the gravitational well, to another (higher) orbit, where it actually moves slower than before. Because of the periodic movements, the lifting is counteracted over time by a similar but oppositely directed movement, so that the semi-axis stays constant. This takes a couple of orbits to accumulate, so you have to consider the integrated effects over many years.”
There is substantial merit in Dr. Svalgaard’s rejigged explanation, but there still remains some pertinent questions. The explanation works well for perturbations occurring in the 1/2 to 3/4 region before Perihelion, but perhaps not so well in others. Dr. Svalgaard explains its a matter of the smaller background oscillations that make it hard to track the source of the perturbation. This is plausible but maybe a little weak and may be the reason it is near impossible to find any reliable data on this topic via the web. In particular I have questions why the “force along the orbit” fails to work when Jupiter approaches Perihelion and actually looks to have an opposite effect. Remember, when Jupiter & Saturn are in conjunction the Perihelion distance is at its shortest, but as you can see in the following diagrams the substantial pull along the orbit fails to elevate Jupiter’s orbit to a higher state.
The red dots signifies Perihelion in this 1762 example:
I wanted to see the perturbation affects plotted with successive orbits to see how the planets contribute to Jupiter’s orbit changes. Although fairly easy when you know a couple of tricks (thanks to Dr. Svalgaard) this task took me many days. I plotted the JPL data into excel and once the orbit was expanded to a size too big to display here it was easy to see each individual orbit. I was lucky with my data selection because it included a very strong perturbation of the 1987 Perihelion. The orbit has Jupiter doing a fly past Saturn, Uranus and Neptune before Perihelion which manages to take it away from its more normal path by 6 million kilometers. Once Jupiter passes all 3 planets its orbit is dramatically “braked” which brings it back into line around Perihelion, but still manages a very long perihelion distance. This is a good example of the perturbation theory in practice.
The corresponding planetary view shows the relative positions of planets which line up with the above average perturbation.
So do we have a situation where the perturbation theory works in some parts of the orbit only? Could other forces be overriding this perturbation? The orbit changing mechanics have been described as speeding up the planets velocity (I have serious reservations that this occurs) which raises it up the gravity well to a higher orbit before settling into a slower overall velocity.
UPDATE:After much research I think I finally have the cause for Jupiter’s Perihelion distance modulation….and it surprising how little is known or published in this area. The above example is not worded very well and leads you to think an acceleration lifts the orbit higher then slows the velocity (that’s at least how I read it), from what I now understand it actually happens in reverse…the acceleration moves the orbit closer (following Kelper’s 2nd Law) but then unlike a rocket using boost, Jupiter is then immediately subjected to deceleration from Saturn’s gravity which slows velocity in turn lengthening the radius vector or raising Jupiter to a higher orbit. As Jupiter moves further away from Saturn the Sun takes over and reins in its sibling and waits for the next perturbation. So now it becomes obvious, and completely explains why a Jupiter/Saturn conjunction causes a short Perihelion, Jupiter is always in acceleration mode during this lineup which causes it to move closer and the plotted orbit agrees. Also of note, Neptune and Uranus also help out and can be strong when together as we see elsewhere on this site.
The orbit spreadsheet is able here: http://users.beagle.com.au/geoffsharp/jup_orbit1940-2009.xls