As the students in my current astronomy class know, this fall is a particularly tough time for those interested in observing the planets of our solar system. Saturn and Mercury lie close to the Sun in the late afternoon sky, all setting less than an hour after sunset. Mars is low in the western sky at sunset, difficult to see between the trees, and blurred by the thicker layer of atmosphere in the direction of the horizon. Jupiter rises at 9:30, but isn't high enough in the sky for observing until nearly midnight. Venus rises at the inconvenient hour of 3am, and isn't easily observed until about 5.
But there are two planets that are high in the evening sky, and can be observed in small telescopes, or even binoculars. Uranus and Neptune are admittedly not much to look at from your backyard, but perhaps after knowing the stories of their discovery, you may be tempted to go on a personal quest to see the most distant planets of the Sun with your own eyes. This week I relate the first of these stories.
The pre-scientific civilizations knew only of 5 "wandering stars". Mercury, Venus, Mars, Jupiter and Saturn were known to be points of light which, unlike the rest of the stars, changed their position in the sky over periods ranging from days to months. Since the time of ancient Greece, for over 2000 years, it was generally thought that the planets, along with the Sun and Moon and stars, orbited a motionless Earth once each day. The drift of the planets among the stars was explained by an increasingly complicated set of decentered circular orbits, describing motions quite like the Spirograph toys which you may recall from childhood.
It was only in the 1500's that planetary astronomy resumed a scientific advancement. In the early part of that century, Nicholas Copernicus formulated a simpler description for the motion of the planets by assuming that they and Earth orbited the Sun in circular orbits. His theory was not published until his death, mostly for concern over religious objections. It was not until the first years of the next century that Johannes Kepler determined a more complete description of planetary motion, but this is a story to be told some other time...
The nature of the wandering stars was not revealed until the invention of the telescope in the early 1600's, and the first use of a telescope to observe the planets Jupiter and Saturn by Galileo in 1610. Galileo was the first to see the planets as spheres very unlike stars, which under the highest levels of magnification always appear as points of light (due to their immense distance from Earth).
The advancement of astronomy after Galileo was continual, with increasingly detailed catalogs of stars and other celestial objects being created throughout the later 1600's through the 1800's. The telescope was dramatically improved after Newton invented an all-mirror telescope in 1668, and both the number of telescopes and their size and quality advanced rapidly in the 1700's.
Sir William Herschel was one of the early great telescopic observers, and manufactured over 400 telescopes of various sizes in his lifetime. His largest was a 49" diameter Newtonian reflector, which rapidly proved impractical to use because of its stupendous size (over 40 feet in length). Herschel, assisted by his sister Caroline, performed a grand survey of the heavens starting in the 1770's, building catalogs of double stars and nebulae that are still consulted today by amateurs and professional astronomers alike.
It was during his survey of double stars that on the night of March 13, 1781 that Herschel observed a pale white disk at a position not matching any known fixed star in the constellation Taurus. During this period in the history of astronomy, with the steady improvement in telescopes, the discovery of comets months before they became bright objects in the night sky was becoming an exciting possibility. Herschel was convinced that he had discovered such an object after returning the next night to the same area of the sky and confirming that the object he observed the previous night had moved slightly relative to the surrounding stars.
After making several more careful observations of the pale disk, recording the exact motion of this body through the background of stars, it became possible to roughly estimate the orbit of the object and begin to predict its future motion. Herschel worked out the preliminary orbit estimate, finding that newly discovered object was incredibly over 1.5 billion miles from the Sun - nearly twice the distance of Saturn, the farthest known planet in the Solar System. To be at this immense distance, and yet easily observed by Herschel through even small telescopes, it was quickly understood that this object was far larger than any comet, and indeed much larger than the Earth.
Herschel named the object Georgium Sidus (George's Star) after the reigning king of England, George the Third (the same king of early American history infamy). Although this royal flattery did help Herschel become appointed the "King's Astronomer", the name was not well received outside England, where it was widely referred to as Herschel's Star.
However, it was not yet proven that Herschel's Star was a planet. At its huge distance from Earth, the new object changed position in the sky so gradually that an exact orbit was very difficult to calculate, and the possibility of error was large. The Russian mathematician Anders Lexell, a master of the field of mathematics now known as celestial mechanics, became interested in this problem, and worked for several months using the accumulation of observations being made throughout Europe. Despite his efforts, he was unable to determine if the object was orbiting in a closed elliptical path, or in an open parabolic orbit typical of comets.
Finally, Lexell referred back to previous sightings of an object of similar brightness made in back in 1759, and also not matching any known star position, by the astronomer Christian Mayer. Conjecturing that these observations, made in the nearby constellation Pisces, were of the same object seen by Herschel in the constellation Taurus some 22 years later, Lexell recomputed the orbit, finding it to be a nearly circular ellipse, and confirming the object as a very massive planet.
Herschel went on to discover two large moons orbiting the new planet. European astronomers debated the proper name for the new planet, suggesting both Neptune, to commemorate Britain's recent naval victories, and Uranus, the father of Saturn. The element uranium, discovered in 1789, was named in support of this choice. Uranus became the accepted name for Herschel's planet, though it was only in 1850 that the Royal Society of London officially abandoned the name Georgium Sidus.