When William Herschel discovered that giant green world a little over 200 years ago, it created a sensation that had - and still has - no equal; for even in our modern era, when scientific revelations are commonplace and expected, we never have our very sense of reality ripped to shreds. The recent discovery that the universe is blowing itself apart at an ever-increasing pace, or that our cluster of galaxies is being yanked by a massive "great attractor" - these are fascinating revelations, but their effect is to enhance our store of knowledge rather than deeply disturb us.
But back in the mid-18th century, there had always been five planets (plus our own, which through most of history was seen not as a planet at all, but as of a separate essence). Five planets were woven into the fabric of human consciousness. It was basic reality, like birth and death and the Sun and Moon. Nobody through history in their wildest dreams imagined that there were additional unseen planets. Imagine if scientists announced that Earth is hollow and inhabited by a race of genius monkey-people. It was that kind of news: beyond belief.
Looking back, we're entitled to be a bit surprised ourselves. For a few months every year, Uranus can be glimpsed without too much difficulty with just the naked eye. Its slow-moving track across the heavens, requiring 84 years to complete one orbit, causes it to appear dimly in each zodiacal constellation in the course of a long lifetime.
Why hadn't anyone ever noted it? And why did it require more than a century-and-a-half since the invention of the telescope for someone to claim its sighting? After all, ordinary binoculars make it seem almost brilliant.
Uranus is currently experiencing one of its rare Equinoxes, when its Equator points edgewise to Earth and the Sun. Equinoxes happen twice a year here; on Uranus it's once every 42 years. At such times, its dozens of moons appear to move up and down, since its Equator now looks vertical to us, and its moons circle its Equator.
Uranus orbits the sun sideways. From the '70s until a few years ago, its vertically aligned Equator was on the leading side of that planet, while we and the Sun faced its South Pole. Its moons made concentric circles as we watched. Now we instead face its vertical Equator, and the moons rise and fall while going around it. Either way, Uranus is weirdly unlike any other world.
Uranus is the third-largest planet: a globe of mostly hydrogen and helium like the Sun, with enough methane gas to absorb the Sun's red light and reflect only its blue and green. If you know the constellations a little, this is how you find it: At 10 p.m. any night, in the southeast, under the Great Square of Pegasus, you'll see the dim circle of stars called the "Circlet of Pisces." Follow the two rightmost of these stars downward a bit more than their same distance apart. At that spot, use binoculars to find a distinctly blue-green star. That's Uranus. Then try to see it with the naked eye.
If you do, salute that day two centuries ago when human thoughts about the universe underwent an unexpected leap that to this day has seen no equal.