Mercury A to Z: Vulcan

Hello, friends!  Welcome to another posting of the A to Z Challenge.  For this year’s challenge, my theme is the planet Mercury, and in today’s post, logic dictates that V is for:


As you know, Mercury is the planet closest to the Sun, but at one time astronomers had reason to believe that there was another planet even closer to the Sun than Mercury.  This hypothetical planet was named Vulcan, after the ancient Roman god of fire—a highly logical choice.

Our story begins with Isaac Newton and his law of universal gravitation.  Thanks to Newton, it became possible to predict the motions of the planets with extraordinary precision; however, in the centuries following Newton’s death, astronomers started having trouble using the logic of Newton’s law to predict when transits of Mercury would occur.

A transit of Mercury is when Mercury passes directly in front of the Sun, as observed from Earth.  This is one of the most exciting ways to see Mercury, provided you take the necessary precautions to protect your eyesight.  But in the 18th and 19th Centuries, Mercury started transiting the Sun at seemingly illogical times.  Mathematical predictions of Mercury transits were off by minutes, hours, or even by as much as a full day!

So French astronomer and mathematician Urbian Le Verrier hypothesized that another planet (named Vulcan) might exist, orbiting the Sun within the orbital path of Mercury.  Vulcan’s gravity might perturb the orbit of Mercury enough to explain why Mercury never seemed to transit the Sun on schedule.  Le Verrier had made a similar hypothesis, based on perturbations of the orbit of Uranus, which led to the discovery of the planet Neptune.  Thus, it seemed only logical to take Le Verrier’s Vulcan hypothesis seriously.

In the following years, a few astronomers claimed to have found Vulcan, proving Le Verrier’s hypothesis, but follow up observations could never confirm these discoveries.  Most sightings of Vulcan were probably just stars that happened to be near the Sun.  Most transits of Vulcan were probably just sunspots.  Perhaps, instead of a single planet, Vulcan might be a swarm of asteroids: the vulcanoid asteroids.  But it would require an absurd number of asteroids to account for the observed perturbations of Mercury’s orbit.  Logically speaking, an asteroid swarm that large would have already been noticed.

So Mercury kept transiting the Sun at the wrong times, according to Newton’s laws, and no one could explain why.  Not until 1915, with the publication of the theory of general relativity.  Thanks to the logic of German theoretical physicist Albert Einstein, we now know that the mass of the Sun curves the fabric of space-time.  This curvature affects the orbits of all the planets, but most especially the orbit of Mercury, because Mercury is so very close to the Sun.


Today I want to recommend this video from Astrum, one of my favorite YouTube channels.  If you love space as much as I do, it would be only logical to check out what Astrum has to offer.

10 thoughts on “Mercury A to Z: Vulcan

  1. Vulcan has always seemed to me like a good example of how difficult it can be to know if a scientific theory has been falsified, particularly when it’s one that’s otherwise been reliable up to that point. Is it unseen factors? Or have we reached the limit of the theory’s domain of applicability?

    Liked by 1 person

    1. I have a similar thought whenever I read about either dark matter or dark energy. Maybe we’ve reached the limits of what current physics can explain, or maybe there’s an explanation within current physics after all. I try to keep my mind open to both possibilities.

      Liked by 2 people

      1. Yeah. My assumption is that dark matter is a particle of some kind. That could be just one more thing to throw into the particle zoo, or it could mean rethinking the whole standard model of particle physics.

        Liked by 1 person

    1. It’s a very slight effect. In Newton’s lifetime, it wasn’t noticeable, but in the centuries that followed, the very small discrepancies started to add up to some pretty large errors.


  2. Thumbs up for Einstein! It’s amazing that the fabric of time and space can be bent and demonstrated so close to home…
    Saw this picture of a comet like tail emanating from Mercury today

    Liked by 1 person

      1. I clicked through and saw it. That’s really awesome! I’d read about Mercury having a comet-like tail, but I always assumed it was too faint to see it from Earth.


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