“Either you repeat the same conventional doctrines everybody is saying, or else you say something true, and it will sound like it’s from Neptune.” –Noam Chomsky
Except back before it was discovered, the very idea of Neptune… well… sounded like it was from Neptune!
The planets out to Saturn are visible with the naked eye, and have been known since at least the time of the ancient Greeks. While the telescope was first invented in the early 1600s, it wasn’t until 1781, nearly 200 years later, that the next planet — Uranus (above) — was discovered.
But unlike the other known planets, Uranus was doing something very weird. Specifically, it wasn’t obeying Kepler’s Laws of planetary motion!
More specifically, it wasn’t obeying the second law: the one that states planets sweep out equal areas in equal times as they move in ellipses around the Sun.
Sure, the other planets all do, but Uranus? For the first two decades, it moved too swiftly, later it slowed to move at the predicted speed, and finally, in the 1830s and 1840s, it started moving even more slowly than Kepler’s Laws predicted.
Well, some people believed that Newton’s Law of universal gravitation — the law from which all of Kepler’s Laws can be derived — might be wrong for Uranus. After all, it had never been tested out at distances past Saturn; perhaps things were just different over there.
Perhaps there was another planet, out beyond Uranus, that was causing these changes in speed. Perhaps the gravitational force of this additional, as-of-yet undiscovered planet was pulling on Uranus: pulling it faster when it was ahead, pulling it slower when Uranus passed it in its orbit.
When Urbain Le Verrier published his predictions of where such a planet would need to be on August 31, 1846, the world took note. On September 23, his letter arrived at Berlin Observatory, and that very night, Johann Galle and his assistant, d’Arrest, pointed their telescope towards the exact location Le Verrier predicted.
(A rich account of the story can be found here.)
And — less than 1 degree off from his prediction — the new planet was discovered. With an orbital period of nearly 165 years, Neptune takes twice as long to orbit the Sun as the next longest planet, Uranus. Think about that. Nearly 165 years for one revolution, and it was discovered on September 23rd, 1846.
Do the math?
Last night, at 22:47 Universal Time, Neptune completed its first complete orbit around the Sun since humanity first discovered and identified the eighth and final planet in our Solar System.
Its 18 billion mile journey — sweeping out an area 900 times greater than Earth’s orbit — was completed, as far as humanity is concerned, yesterday.
Neptune, despite being a “small” gas giant, holds one distinguished record for our Solar System that is unlikely to be broken: wind speed.
With a raging storm on its surface — the size of the entire planet Earth — known as the “Great Dark Spot” (as compared with Jupiter’s Great Red Spot), Neptune’s atmosphere sees wind speeds up to 2,400 km/hr (1,500 mph).
The spot seems to have no problem migrating, as it disappeared in 1994, only to re-emerge in the Northern Hemisphere under a new name: the Northern Great Dark Spot. We think this spot is generated from completely internal processes on Neptune; Neptune’s surface receives 160% more energy from its own core than it does from the Sun!
So here’s Neptune’s birthday pictures — released yesterday — courtesy of (who else?) the Hubble Space Telescope. After 18 billion miles (or 28 billion km), this marks the last planetary “first birthday” we’ll (likely) ever have in our Solar System, and for those of you Pluto-philes who cannot wait for Pluto to complete its first orbit, keep waiting. You’ve got until 2178 until that happens.