After Giovanni Cassini first espied Tethys and Dione in 1684, satellite discoveries dried up for more than a century. Astronomers didn’t think to look for new planets either. Lack of imagination? Perhaps so, because many famous astronomers viewed them. They just didn’t know what they were seeing.
The great Galileo had a close encounter with Neptune. In 1612, two days after Christmas, he actually noted Neptune as he sketched the moons of Jupiter. Just the man’s luck that Neptune had, that very day, begun its retrograde motion and was stationary in the sky. Galileo did note Neptune’s movement in some drawings a little more than a year later. But the inspiration didn’t dawn that he was seeing a new planet. The moons of Jupiter got him into enough trouble, I guess.
We can forgive Galileo and Flamsteed for missing 30,000-mile wide planets. There’s a lot of space, even along the ecliptic to hunt for things. Once you get the planet, sleuthing the moons is one step easier. Though the moons are always much smaller and dimmer than the primary, the search is greatly narrowed.
At any rate, it took a church musician to further draw back the curtain of the unknown from the outer solar system. William Herschel (right) was an immigrant from Hanover, Germany and had a comfortable life in Bath, England. He wrote music, directed bands and orchestras, and gave concerts with his brothers and sister. In mid-life he adopted astronomy as a hobby and began building his own telescopes.
And in March 1781, he began tracking a green orb (below) moving slowly against background stars in Taurus. After consulting with professional colleagues, it very soon became clear that Herschel had found a new planet. It changed his life.
King George III gave him an annual salary, with the understanding Herschel would make himself available for sky parties at Windsor Castle. (I guess they needed a distraction from those upstart American revolutionaries.) The new “King’s Astronomer” suggested the name “Georgium Sidus” for the little green orb.
The French didn’t approve. They called the new planet “Herschel,” and were unaware one of their own made no less than a dozen sightings of this planet in the 1750’s and 60’s.
German astronomer Johann Bode suggested Uranus as a name. Two things seemed to work for its acceptance. That a new planet was named for a Roman god kept a consistency with Saturn, Jupiter, and the others. Uranus was also god of the sky, and at twice the distance of Saturn from the sun the little green body was pretty “deep” in the sky.
Herschel continued to refine his telescope-building under royal patronage. He didn’t neglect his green orb discovery, and six years after it, identified two moons in orbit. Funny thing about those moons: Herschel’s telescopes were, for half a century, the only instruments in the world powerful enough to view them. The amateur had advanced to the head of world class.
Saturn’s edge-on rings in 1789 gave Herschel a chance to test the limits of his monster 49-inch reflector. First night of viewing: Enceladus. Then a month later: Mimas. Within a decade, Herschel had tied the records of Galileo and Cassini, throwing a planet into the mix if you want to consider a tiebreaker.
Here are Herschel’s 18th century moons: Titania (magnitude 14.0) and Oberon (14.2) of Uranus, both discovered in 1787, Enceladus (11.7) and Mimas (12.9) of Saturn, viewed in 1789. You’ll notice Saturn’s moons are brighter than Titania and Oberon by one and two magnitude factors. They are as hard to spot because of the nearby glare of the planet Saturn in an Earthbound telescope. Myself, I have yet to see them.
Enceladus, of geyser fame:
And Mimas, with a giant crater named … Herschel:
It’s worth noting that the difference in magnitude between Jupiter’s bright Ganymede (4.6) and these 18th century moons is on par with the difference between the full moon and Venus.
My imagination snags on my commonality with Herschel. Hey! I’m a church musician who loves astronomy, too. Unlike Herschel, there’s no way I’d convince my sister to patiently take notes while I observed the heavens. Also unlikely I’ll be building telescopes with the technology of the mid- to late-21st century–fifty years ahead of my time as Herschel was.
Still, following in the footsteps of a great observer like William Herschel is exciting. No other branch of science is as receptive of the contributions of amateurs as astronomy is. It should give us all a sense of pride in our accomplishment to be standing with astronomers of centuries past, probing the unknowns of space, possibly with our own hand-made instruments and hand-drawn charts at our side.
And besides, if there are no planets or moons in the solar system left for me to discover, I guess I could write an oboe concerto.