Depending on which calculation system one uses, the planet Neptune reached an important milestone somewhere between July 9 and 12 of this year. It has finally completed a single orbit of the Sun since its discovery on September 23, 1846.
For Neptune, one orbit around the Sun takes nearly 165 Earth years!
For most of the 165 years since its discovery, we have known precious little about this distant planet. In 1989, Voyager II changed that by sending back glorious detailed up- close photos of the blue world, which has the fiercest winds in the entire solar system. We saw the Great Dark Spot, a storm akin to Jupiter’s Great Red Spot, found thin rings circling the planet, and discovered six new small moons. Neptune was long thought to have only two moons, the large Triton and much smaller Nereid.
The story of Neptune’s discovery is inextricably linked with that of Pluto’s discovery. In the early 19th century, astronomers studying Uranus, which was discovered in 1781, found that its actual orbit did not match the orbit they predicted for it. Uranus’ orbit was “perturbed,” suggesting the planet was experiencing gravitational influence from yet another large object even more distant.
A lot of the same issues we see today in the field of astronomy loomed large in the mid-19th century regarding Neptune. Independently of one another, a young British astronomer named John Couch Adams and a young French astronomer named Urbain Leverrier mathematically calculated the position of the supposed planet affecting Uranus. Both faced ambivalence from the established community of astronomers. Adams tried three times to get England’s Astronomer Royal to look at his calculations and each time was unsuccessful. Leverrier finally turned to the Berlin Observatory with very specific coordinates for where the planet should be found, a feat accomplished within an hour by observer Johann Gottfried Galle.
Later, some astronomers came to believe that Neptune, too, was experiencing perturbations in its orbit, and this notion led directly to the search for yet another planet even further out. It turns out there were no perturbations, just human error in calculating Neptune’s orbit. This was not known until the 1989 Voyager II flyby. Yet the erroneous notion of perturbations directly set in motion the sequence of events that led to the discovery of Pluto, prototype of a third class of solar system planets.
Galle turns out to not be the first person to have observed Neptune. The planet happened to be near Jupiter when Galileo turned his telescope on the giant planet. Neptune actually was recorded by Galileo as a star, probably because it moved so slowly against the other background stars. There is some question as to whether Galileo recognized Neptune as something other than one of the “fixed” stars.
Triton, Neptune’s largest moon, is compositionally similar to Pluto. It orbits Neptune in the direction opposite Neptune’s orbit around the Sun, suggesting it was once a planet in its own right orbiting the Sun directly that was somehow captured by Neptune. Triton is believed to have originated in the Kuiper Belt, driving home the notion that Pluto is not a loner, that there have always been Kuiper Belt planets.
The sequence of events leading to Neptune’s discovery in 1846 also illustrate that the rivalries and personality conflicts so prominent in the Pluto debate are hardly new. The players are different now, but the behaviors are very much the same—professional rivalries between individual scientists, astronomy being drawn into political conflicts between nations (England, France, and Germany in the case of Neptune), and an elitist attitude by “established” scientists when faced with challenging ideas by newcomers viewed as “upstarts.”
While many people know me online as the “Pluto lady” or “Plutogirl,” my first planetary fascination was actually Neptune, that hypnotically beautiful aqua-blue world, when the Voyager II pictures were initially released. I was captivated by the strange, faraway world and still am. I kept every Voyager II photo of Neptune from the headlines and even started painting the planet with watercolor. To this day, images of Neptune, including a painting I copied from the front cover of Newsweek magazine, still adorn the walls of my bedroom.
It was Neptune that first brought me to Amateur Astronomers, Inc., the club I would join many years later. Back in 1989, I did not own a car and had to ask a friend to drive me to the club’s observatory on one of its open public nights. As the volunteers led everyone to the club’s two telescopes for observations, I turned to one of them and specifically asked that he show me Neptune. The stunned volunteer acted as if I had asked to see a planet orbiting another star. In all his years of volunteering, no one had come in asking to view Neptune, he said. My request was not possible.
In retrospect, it is unlikely that no one ever asked to view Neptune. Experienced observers in the club likely had seen it many times while newcomers are usually shown the most common and frequently visible objects, which for planets means Mars, Jupiter, and Saturn. More likely, Neptune was not visible from Earth at that particular time.
Twenty years later, as a club member, I finally fulfilled my wish. Through the eyepiece of a member’s telescope, on a clear night, I took my first direct look at the tiny blue dot that was a world that had captured my heart.
Happy Anniversary of Discovery, Neptune!
