FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Neptune and Newton's Laws

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.

Now You See 'Em, Soon You Won't

Seeing the stars at night, year after year, generation after generation, it seems that all is quiet up there. The stars appear fixed on that black canopy above. Nothing ever changes.

But stars are born, they live, they die. And it is because stars have a life cycle that we can make these bold statements: The skies we see tonight are our skies alone. Earlier in our history they were different. In the future there will be a whole new menage of constellations.


Most of the stars we see in the sky have short lifetimes. Regular readers here will recall that most of the naked eye stars are intrinsically very bright, thus very energetic. This means they burn through their hydrogen fuel in short order, many in just tens of millions of years.

Now the Earth is over four-and-a-half billion years old. It follows then that most of our present-day stars have been up there only recently in our history.

Another factor in our changing sky is that stars are formed in nurseries. Here they interact gravitationally with each other. In doing so, some get tugged and flung out of the nursery, jettisoned into lonely space.

All this movement means that even if stars could live longer, they would not stay in the same place in the skies. They continue to move through it. Some are moving very fast, tens or hundreds of kilometers per second, others not so.

They don't appear to be moving because they are so very, very far away. But they do move. And this is another reason the skies are changing, however slowly.

Not only do stars appear in our skies over time, not only do they move through them, but they also vanish.

The biggest stars exit with the proverbial bang. They exhaust their nuclear fuel and collapse, then explode, into a supernova, vanishing from the sky altogether. Medium-sized stars fade away with a whimper, eventually shedding their outer layers and leaving nothing behind but a miniscule, hot corpse of a core.

On top of all this appearing and disappearing, in the future, because hydrogen gas in our universe is being depleted, the birthrate of stars will continue to decrease. There will be fewer stars to take the place of the dead and dying ones.

Stars are here for just one scene of one act of the Great Play, in really nothing more than a mere cameo appearance. So enjoy Orion and Sagittarius and the rest of the troupe while you can. In a couple millions of years from now, our heavens above will not be recognizable.
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me