It would be the coolest thing to discover a planet, don’t you think? A moon wouldn’t be too bad either. For the inner eight planets of the solar system, as of the early twenty-first century, humankind has pretty much uncovered everything down to about thirty miles wide. Any subsequent real estate anybody picks out will be no larger than the size of a city. And probably a lot smaller.
The nineteenth was a heady century for solar system discoveries. It was also the last in which new satellites were found by direct observation. Astronomy was being conquered on two fronts: by the emerging art of photography and especially mathematics.
William Herschel, that one-time church musician and bandleader, built the most powerful telescope in the world. He claimed to have found other moons of Saturn in the early 1800’s, but these were never verified. Nobody had a telescope as good as his. It would be left to another generation to probe the outer reaches of the solar system. They did so by eventually matching and exceeding Herschel’s optics. They were helped by another academic discipline mathematics.
Curious things were happening in the Uranian neighborhood. The planet seemed to be a bit off from where it should be, slowing down and speeding up at odd times. Astronomers combed through early records of Uranus-sightings–the ones where people didn’t realize it was a planet. Mathematicians set to work on a notion that another large planet was out beyond Uranus. Newton’s gravity equations were put to the task. But decades passed with no sighting. Either the math was off, or the planet was super faint, or Uranus was just a strange duck orbiting the sun at 1.7 billion miles.
In these years John Herschel (left) took up his father’s mantle. John recorded what he thought was a star on July 14th in 1830. Just his luck he was unable to tie together emerging Newtonian models for Uranus’ peculiar orbit with his dad’s keen eye. It would have been a great coup for the Herschel family: father and son planet discoverers. For that summer “star” was indeed the planet later to be known as Neptune.
In the 1840’s John Couch Adams was sure another planet was out there. His numbers led him to believe. But he could not convince his arrogant countrymen at the Royal Observatory in London to take him seriously. James Challis did attempt to pin down a trans-Uranian planet in the summer of ‘46. He actually viewed Neptune twice in August, but he lacked the imagination (or perhaps the diligence) to claim a discovery for England.
It was left to a French mathematician, Urbain Le Verrier, (above, right) to tip the balance. Once Sir George Airy saw Le Verrier’s published paper on the mathematics of a possible eighth planet, he took his compatriot Adams more seriously. However Le Verrier also told astronomer Johann Galle (left) in Berlin. Galle had no problem with letting mathematical analysis guide his imagination. On his very first night of observing, only one degree (twice the span of a full moon) from Le Verrier’s predicted position, he found a small blue orb. Adams’ estimate was off by 12 degrees (a little wider than a fist at arm’s length).
Once Neptune was nailed down, other observers had no problem with some new satellites. For the British, Neptune was lost. Astronomer William Lassell (left) managed to salvage some homeland pride with his discovery of Neptune’s largest moon, Triton, seventeen days after Galle first spotted the planet. I suppose it made him the toast of Britain. Appropriate for the guy who made his fortune brewing and selling beer so he could devote his time to his real love: astronomy.
That small image at the top right of the post is an earthbound telescope view of Neptune with Triton from the Xanadu Observatory web page. It’s a little bit better than what you might see in an 1840’s telescope, but it gives a good idea.
Two years later, while the American father and son team of William Cranch Bond (left) and George Phillips Bond spotted Hyperion tumbling between Titan and Iapetus, Lassell found the same moon from his vantage point in Britain.
The brewer wasn’t finished, as he probed to the depths of fifteenth magnitude to discover two Uranian moons, tying the record of four satellite discoveries set by Galileo and repeated by Cassini and William Herschel.
Courtesy of modern space probe close-ups, here are these four satellite discoveries, none of which, by the way, I’ve ever seen through a telescope.
Triton (1846) of Neptune, magnitude 13.6:
Hyperion (1848) of Saturn, 14.2:
… and Umbriel (1851, below) both of Uranus, 14.4 and 15.3 respectively.
What about you?
These finds left astronomers with a curious 1-4-8-4-1 symmetry of known satellites among the solar system’s five largest planets. Earth: 1 satellite, Jupiter 4, Saturn 8, Uranus 4, and Neptune 1. Would the world’s astronomers summon the imagination to look further and break the pattern? You’ll have to read about it in the next installments as the Americans take up the task, looking near planetary glare to ferret out more orbiting bodies.