“It isn’t what we don’t know that gives us trouble. It’s what we know that ain’t so.”—Will Rogers

I’m an astronomer at the Sun Valley, Idaho meeting of the High Energy Astrophysics Division of the American Astronomical Society. It’s a good meeting, but I’m really here for the total eclipse of the sun.

Astronomers know the motions of the Earth and moon and accurately predicted this eclipse decades ago. Our professional society shrewdly made reservations 5 years ago at a hotel in Jackson Hole, Wyoming. Modern commerce trumped astronomical prediction, however, when some greedy person bought that hotel and purged the unprofitable reservations. The astronomers were on the hit list! Urgent call for Plan B!

Plan B: The scholars of exploding stars, neutron stars, black holes, and clusters of galaxies chose Sun Valley, Idaho for their 4-day meeting. But the schedule for Monday morning was left open for something even more important than “Hyperluminous Wandering Massive Black Holes Discovered in the XMM-Newton Catalog” scheduled for later in the day. A total eclipse of the sun.

Despite deep knowledge of astronomy, or perhaps because of it, our motives mirror everybody’s in the crowd gathered on the lawn of the resort. We’re here for physical sensations: feeling the chill in the air as the moon covers the sun, seeing the weird not-quite daylight as totality approaches, and experiencing the disorienting seconds when the sun goes black, with the pearly corona revealed. Intellectually, we’re pretty familiar with the descending nodes of the moon’s orbit, but we’re here for the thrill.

Just as predicted, the air cooled, while light from the last kiss of the moon’s disk made crescent shadows under the aspens and illuminated the crowd with an eerie light. The ducks in the resort’s ornamental pond waddled ashore and twisted their beaks into their down. Dutifully keeping my eclipse glasses in place (one of a million pairs sponsored by the Gordon and Betty Moore Foundation that were distributed through public libraries), I waited, waited, waited. Then, just as the disk of moon covered the last bit of the Sun, I whipped them off, the Sun went black and this month’s wispy Y-shaped opalescent corona emerged from the dark sky. This spectacle bypasses your cortex and goes straight down to the part of the brain where fear emerges. A black sun? Not normal!

After a few seconds, I mastered myself, picked out Venus, searched for Mercury, and then much too soon, there was a bright flash at the edge of the black disk and the eclipse was over. This brief and disorienting experience made me think of the search for the planet Vulcan, strangely woven into the story of eclipses and their role in helping understand gravity.

Profound aspects of the Universe have been revealed by eclipses. It is a well-known story that the 1919 eclipse was used to detect the deflection of light from stars by the gravitation of the Sun. This is told as a triumph of Einstein’s gravity. First he computed the change in direction of the long axis of Mercury’s slightly elliptical orbit: the precession of Mercury, which had vexed astronomers for decades. He found it very exciting when he got exactly the right answer and reported palpitations of the heart. It’s good when you account for something known. Then he predicted the deflection of star light grazing past the limb of the Sun. It’s even better when you predict something that hasn’t been measured and it came out right, too.

But there was another possibility to account for Mercury’s orbit that astronomers considered for decades in the 1800s. Newton’s theory of gravitation was a huge success in accounting for what was known in Newton’s time and (even better) allowing predictions of further phenomena. One of the most spectacular successes was the 1846 discovery of the planet Neptune, whose existence was inferred from deviations from the expected orbit of its solar system neighbor, Uranus. A mathematical analysis, using Newton’s theory, led to a prediction so accurate that the new planet was discovered on the first night of searching!

The idea that an as-yet undiscovered mass might affect the orbit of something you can see was a spectacular demonstration of the power of Newton’s gravity. As described in Thomas Levenson’s wonderful book, The Hunt for Vulcan, there was another motion in the solar system that was bothering astronomers like a piece of corn between their teeth. Mercury’s orbit doesn’t quite close. It forms a kind of spirograph that slowly changes its direction in space.

In 1859, Le Verrier, in Paris, who had successfully predicted the presence of Neptune from unaccounted-for motions of Uranus, did a similar analysis for Mercury. He found that the precession of Mercury’s orbit was almost, but not quite accounted for by the gravitational tug of the known planets, mostly Venus. But there was extra precession left over. What could account for this? Why not use the same idea that had been so successful in 1846? Posit a mass inside the orbit of Mercury to provide that tug! Invent the planet Vulcan3-D 

3-D map of the distribution of dark matter in a volume of the cosmos—presuming it exists. Credit: NASA, ESA, Richard Massey California Institute of Technology

What we have forgotten in the modern era, dazzled by Einstein’s success, is that this seemed like a reasonable explanation and it made a prediction: astronomers should see that new planet. Several observers reported seeing Vulcan’s shadow crossing the Sun’s disk in the middle of the 1800s. By September, 1876, the New York Times concluded, “Vulcan exists, and its existence can no longer be denied or ignored.” One further test would be to see the new planet shining during the brief dark moment of an eclipse. After all, Mercury peeps out of the daylight and into the darkness. Why not Vulcan?

Fully qualified astronomers reported seeing it! At the 1878 eclipse in Rawlings, Wyoming, James Craig Watson, Director of the University of Michigan Observatory and eventual member of the National Academy of Sciences said he saw something new through his telescope, something not on his chart of known stars, close to the Sun during the moments of totality. He thought he had seen Vulcan and he never gave up that idea. He moved to the University of Wisconsin and built a special subterranean telescope to spot Vulcan in the daytime. He never saw it again, and neither did anyone else.

The subsequent unraveling of this “discovery” was long, painful, and slow. The short version is that others searched at subsequent opportunities and did not see Vulcan. Over time, impartial astronomical photographs that could be examined at leisure replaced the eyewitness testimony of an excited person at the eyepiece of a telescope in the brief and disorienting moments of totality. None of those plates showed Vulcan. Vulcan was something Watson knew, but it wasn’t so.

A non-discovery often takes more time to sink in. All my pals who had perfect weather for this eclipse posted their joy on Facebook, but the people who are clouded out were not so eager. James Craig Watson never gave up the conviction that he had made a profound discovery. A prodigious calculator of orbits, using Newton’s gravity, he also used numerical skills to help set up the first insurance company in Michigan. He left a substantial bequest to the National Academy of Sciences for a medal recognizing contributions to astronomy. Maybe the guy who bought the hotel in Jackson Hole will do something similar. I doubt it.

Watson’s personal tragedy makes a poignant version of our notion of how science works. First there was a prediction by Le Verrier, based on Newton’s theory of gravity, and then an observational confirmation of that prediction with the reports of others seeing the planet Vulcan transit the sun’s disk. The next step was to see it reflecting sunlight during an eclipse. Watson confirmed that prediction. That’s how science is supposed to work!

But a good rule is that real results can be confirmed. Some people say absence of evidence is not evidence of absence. It's a good chiasmus, but after a while, careful scrutiny without a result begins to carry some weight. It’s not as exciting as claiming a discovery, but the evidence for real effects gets stronger as the data piles up. Not so for spurious results--and Vulcan’s vogue as the explanation for Mercury’s precession was already fading into obscurity when the patent clerk in Berne began his creating his new vision of gravitation. By the time of the 1919 eclipse, the theoretical explanation to be tested in an eclipse was a new one, worked out by Albert Einstein based on a different theory of gravity: general relativity.

Sometimes the correct explanation for something that doesn’t seem quite right about cosmic motions is an unseen massive object. That was true for Neptune. Sometimes the correct explanation is a new theory. That was true for Mercury.

As the light in Sun Valley comes back to normal and the band on the lawn plays “Here Comes the Sun” and “Doctor My Eyes”, a cosmologist like myself intruding on the high energy meeting might begin to wonder: Is there something similar going on today?

We see the motions of stars in the outer parts of galaxies. The speeds are much too high to be accounted for by the mass of the stars that make a galaxy glow. As in the 1870’s, we infer the presence of unseen mass to produce this effect: the dark matter. Scientists are searching for the dark matter, not in eclipses, but deep in mines where subatomic dark matter can trigger subtle detectors without interference from high energy particles emitted by the Sun or the more exotic sources studied by high energy astronomers. There have even been a few hotly disputed reports of detecting the effects of dark matter. Everybody expects the next measurement, better than the last, will finally solve the mystery.

We’ve been expecting the detection of dark matter any day now for 20 years, just as James Craig Watson expected to see Vulcan. But what if we don’t detect dark matter directly? Most likely, we’re looking in the wrong place or in the wrong way. After all, Einstein’s beautiful theory has had 100 years of success, explaining phenomena he did not think of when constructing it. And yet, perpetually elusive dark matter might eventually be a reason for to think the thoughts that a small minority of theorists think today—that we don’t need hidden mass, we need a new theory of gravity. Is the dark matter today’s Vulcan?