This week every year we celebrate the first day of fall, the autumnal equinox. We are headed for winter, summer is behind us (a hard sell here in Southern California, to be sure). But there is one historical event that annually gets overshadowed by this equinox business - the unique discovery of Neptune. Let's go there today.

For thousands of years, there were the Big Five planets above us: Mercury, Venus, Mars, Jupiter, and Saturn. Then, in 1781, Sir William Hershel discovered the next planet out, Uranus, while merely gazing through his telescope. Lucky guy.

But it wasn't until decades later, in 1846, that the next planet out was officially discovered. Why then, and how?

Planets way the heck out there are in no hurry to go around the sun. They do not race around in a fit like Mercury and Venus. And the farther out they are the slower they go. So, an untrained eye with no star charts with which to make comparisons will see those faint dots as nothing more than stars, not knowing they are indeed planets.

Once star charts and telescopes and mathematics became more refined in the 1800's, the odds of finding other, more distant planets shot up. And this is what happened with our remote neighbor, Neptune.

In the 1840's two men, a British astronomer named John Couch Adams, and a French one, Urbain Le Verrier, were independently perplexed by the odd movement of Uranus. Instead of moving slowly and steadily through the sky, records showed the planet speeding up for years, then slowing down.

Both men were convinced that this movement was no mistake in the calculations, that there had to be a distant planet, yet unseen, that was pulling and pushing on Uranus as it went by. The race was on to find it - but not by looking up, rather by doing the math. Using Newton's Laws of Motion on Uranus' strange movements one could narrow down where the culprit might be.

To make a really long - and fascinating - history lesson short, Le Verrier was the first of the two to not only publicly announce his mathematical findings, but also first to get someone with a telescope, namely Johann Galle at the Berlin Observatory, to go look for it. Galle received Le Verrier's correspondence with the coordinates, and on that very night, 23 Septemeber 1846, he and his assistant looked up and found Neptune, within one degree of where Le Verrier had predicted it to be.

It was a great triumph, not only in the finding of a planet, but of Newton's laws which were used to find it. Astronomy had taken a major step forward.