Earth, planets and orbits

1859

Mercury misbehaves

Urbain Le Verrier

What do you do when the best theory in history is off by a hair?

Fresh from Neptune, Le Verrier found that Mercury’s orbit slowly turns a little faster than the other planets’ pulls can explain: about 43 arcseconds per century today. He predicted another hidden planet, Vulcan. People searched for decades. It does not exist.

First, make a guess

Adding Einstein’s extra pull near the Sun makes the orbit…

How they figured it out

Fresh from his Neptune triumph, Le Verrier studied Mercury and found by 1859 that the point of its closest approach to the Sun crept round about 38 arcseconds per century faster than all the planets’ pulls could explain (the modern figure is 43). It worked last time, so he proposed another unseen planet, Vulcan, inside Mercury’s orbit.

Months later an amateur astronomer, Edmond Lescarbault, claimed to have seen a dot cross the Sun. Le Verrier believed him. Astronomers hunted for Vulcan at eclipses for decades. It was never there. The fault was in Newton’s law itself.

The equation

Δϖextra≈43′′ per century\Delta\varpi_{\text{extra}} \approx 43'' \text{ per century}

The orbit’s closest point creeps around the Sun slightly faster than Newton predicts.

Everyday echoSometimes the data is right and the theory needs to change.

See it in real life

A leak, or a wrong formula?

If your car uses a little more fuel than the manual says, it might be a hidden leak (another planet) or the manual’s formula might be incomplete (new physics). With Neptune it was the leak; with Mercury it was the formula.

Where engineers use it

  • Precision testing

    Tiny leftover differences between model and measurement are how engineers and scientists find missing physics.

Where it came from

  • A planet found on paper 1846

    Led to

    After Neptune, Le Verrier used the same reasoning on Mercury and predicted a planet, Vulcan, that does not exist.

What it led to