Now, for a bit of astronomical trivia. I am giving a final at the moment, and am taking the time to blog a bit before going back to grading a mountain of papers.
I saw an interesting book at a used book store a few months back. It was entitled In Search of Planet Vulcan. Now, this is not the Star Trek Vulcan. Rather, it is a hypothetical planet that was postulated to lay between Mercury and the Sun. I had heard bits and pieces of the story over the years, but this book collects the odds and ends nicely.
The story is set in the Nineteenth Century. In the early part of the century, astronomers noticed that there was a discrepancy in the orbit of Uranus. Uranus had been discovered less than a century earlier, but had already made it around the Sun once. However, in one part of the planet’s orbit, there was a slight distortion of the orbit. Mathematical astronomers Adams and LeVerrier computed that this distortion must be due to another planet farther from the Sun than Uranus. A search for the new planet turned up Neptune right where it was predicted to be. Now, this discovery is a very interesting tale of hard work, politics, heartache, and hurt feelings that makes up another piece of astronomical history. This may be another entry in the future.
Well, LeVerrier had previously noted another discrepancy in Mercury’s orbit. As it turns out, Mercury has a very elliptical orbit. But, as the planet goes along its orbit, it does not follow precisely the same path around the Sun. Rather the orbit precesses. This means that if you were to look down on the Solar System and saw Mercury’s elliptical orbit, then the orientation of the ellipse would gradually change. At first, it would be pointed to your left, then up, then to the right, etc. If you looked very carefully at Mercury’s orbit, the path taken by the planet would look like a giant Spirograph pattern. Newtonian physics actually predicts a slight precession, but the precession was too big to be accounted for using plain orbital dynamics. So, LeVerrier proposed that there must be another planet closer to the Sun than Mercury to account for this effect.
Throughout most of the rest of the Nineteenth Century astronomers searched for LeVerrier’s missing planet. They even named it before they found it! The chosen name was Vulcan, after the Roman fire god. There were several mistaken reports of a discovery. Usually these turned out to be sunspots. Unfortunately there is no planet there. It took a long time for many astronomers of the Nineteenth Century to accept that, given the problem with Mercury’s orbit. However, not finding the planet did not deter many from thinking that it was still there, but simply hard to see. It is actually very difficult to even see Mercury, since it is so close to the Sun. Seeing anything even closer is even tougher. We know that some comets and asteroids pass very close to the Sun in great elliptical orbits. There is speculation that there may be some asteroids in more circular orbits in there somewhere, but the proximity to the Sun makes them hard to find. Any time that there is a solar eclipse, astronomers hunt for vulcanoid asteroids.
So, if there is no Vulcan, how do we explain the excess precession in Mercury’s orbit? Well, it turns out that we had to wait for Albert Einstein to come up with General Relativity to explain it. You see, mass distorts space and time. More mass distorts space and time more. The Sun has lots of mass, so it distorts space and time quite a bit — enough, in fact, that the orbit of Mercury is skewed a bit with each passage close to the Sun (Remember that Mercury has a very elliptical orbit. At its farthest distance from the Sun, it is nearly 50% farther than at its nearest distance to the Sun.) Including the effects of general relativity, we can now calculate the precession of Mercury’s orbit. The calculated value and the observed value agree to extremely high precision.
So, if there is a planet Vulcan, then it is not orbiting our Sun!