The planet Mercury is difficult to observe through a telescope, and even dangerous to sight. In the nineteenth century it was harder still. Observers were staring at the magnified blinding sun. They used strong filters, and great caution, but at the time they had no photography, radio instruments, or spectrographs. Lacking sophisticated instruments, some scientists used Newton’s laws of motion and gravitation to work out where telescopes should be aimed.

Mathematician Urbain Jean Joseph Le Verrier had discovered the planet Neptune this way in 1846, using Newton’s laws to plot its path. He deduced that an unknown planet must lie near the orbit of Uranus, because its orbit was perturbed by some great mass that pulled it away from where it otherwise would have been. Le Verrier’s precise calculations told him where this object must be, and when, and though he was not the first to see the actual planet, he was given credit for its discovery.

So scientists believed him when he announced that a hidden planet was perturbing the orbit of Mercury. Mercury has an odd, eccentric orbit. It is 46 million kilometers away from the sun at perihelion (its closest approach) and moving at 56 kilometers a second. At aphelion (its greatest distance from the sun) it is 70 million kilometers away, and slows to 37 kilometers a second. This orbit is also tilted about 7 degrees out of the plane Earth’s is in, which makes its movements seem even more eccentric to terrestrial observers.

Le Verrier used the best physics of his day, and his great mathematical ability, to calculate how the orbit of Mercury must run. But he found that the orbit did not match his calculations. Taking into account the pull of the sun, and of every known planet, Mercury did not reach perihelion where it should. In fact, he calculated it was off by 43 arc seconds per century. This is not a great distance. An arc second is 1/1296000th of an orbit. (Another way to look at it is as the apparent diameter of a dime seen from 2.3 miles away.) This is a tiny difference, and later calculations would shrink the difference further. Nevertheless, Le Verrier searched for an explanation of the discrepancy.

What he came up with was the planet Vulcan. He hypothesized a small planet, perhaps half the size of Earth’s moon, at about one third of Mercury’s distance to the sun. Astronomers searched, and quite a number of them found what Le Verrier had told them they would.

The two best times for observing Mercury are when it transits, passes between the sun and the Earth, and when the sun is eclipsed, when Mercury can be observed in the corona. Under both these circumstances, reputable astronomers, as well as amateurs, thought they saw Vulcan. It is no wonder that Urbain Le Verrier died believing he had discovered two planets.

However, there was considerable controversy, even in his lifetime. The orbit of Vulcan was recalculated every time there was a new sighting, because the orbit required for the newest transit never seemed to match the orbit calculated from the previous sighting. The estimated mass of Vulcan also did not seem quite right either, considering the amount of perturbation it was supposed to be causing.

Einstein finally solved the mystery. The orbit of Mercury does not precisely fit the results obtained by Newtonian physics, but it fits perfectly the results of the theory of relativity: energy equals mass times the speed of light squared. This means that the huge ball of energy that is the sun is equivalent to a much smaller amount of mass, in addition to its Newtonian mass. This extra mass, other than the mass that Le Verrier had known about and accounted for, is the source of the gravitational pull on Mercury that changes its orbit.

No one can doubt that Urbain Le Verrier was a great mathematician. He discovered a planet, after all. He was led into error by following a method which had been shown to work beautifully. Then many astronomers produced what seemed to be proof. Only the genius of Albert Einstein finally disproved the existence of Vulcan, and, actually, there are people who still believe in hidden Vulcan, the sun’s close companion.

This is the true science story that gave birth to the science fiction name of the planet Vulcan (or Vulcanis) home of Spock and the Vulcan race, in the Star Trek universe.