FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Kepler's Three Laws - Part 2

The story so far: It is the beginning of the 17th century. The idea that the entire cosmos is going around the earth is still prevalent but is showing its age. A new breed of astronomers is challenging the old ways, and with fresh vision is looking at the cosmos in entirely new ways.

One of these astronomers is Johannes Kepler who, in the early 1600's, has acquired decades of planetary data from the late astronomer, Tycho Brahe. It is armed with all these data that he will mark his place in the history of astronomy by giving us three laws of planetary motion, all of which will put some of the final nails in the coffin of the old ways. This week we focus on Kepler's laws.

First Law: The orbit of every planet is an ellipse with the sun at one of the foci.

Translation: Up until now it was believed that all planets - every heavenly body - traveled in perfect circles at constant speed. That was the unquestioned way. Using Brahe's mountain of data, Kepler showed that planets actually travel in ellipses. In layman's terms, the ellipse is like an oval shape, a slightly squished circle. And the sun is not at center, but slightly off center. This means that planets in their annual orbits would spend some of their year closer to the sun, at other times farther.

The great significance here was that the planets were now unshackled from the "circles only" orbits they had had for millennia. Moreover, the sun wasn't at dead center; it is off center in relation to the planet's orbit.

Second Law: A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

Translation: When a planet is closer to the sun in its orbit it goes faster, when farther it slows. This explained why the time from our fall equinox to spring equinox is a week shorter than from spring equinox back to fall. During winter we are closer to the sun and move through our orbit quickly. During summer we slow down a bit.

The significance here is that no longer were planets bound by a constant speed as they were under the old regime. Heavenly bodies could actually speed up and slow down!

Third Law: The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

Translation: This relatively simple but elegant equation tells us that planets near the sun whip around pretty quickly - Mercury takes just 88 days. Planets farther out take longer. Neptune, for example, takes 165 years!

These three laws were all paradigm busters and helped take western civilization into a new realm of astronomy. Galileo would soon follow with his great works, and of course Isaac Newton would round out this golden age of astronomy by showing us that most all these laws would find a common thread in the mysteries of gravity.

Four centuries later we are still indebted to Johannes Kepler's perseverance and willingness to think outside the box.

Until next time, clear skies!

What's There to be Thankful for? Look up.

It's Thanksgiving time again. And for some of us it is not as easy to come up with things to be thankful for this year. There seems to be less peace in the world, more law-breaking and disorder - things just don't seem to be getting much prettier. It doesn't help that we humans have a propensity to miss the gifts right in front of us, those we look at every day but don't really see.

Let's a take a cosmic look at - let us see - our place in the universe. Let's find things to be genuinely thankful for this Thanksgiving.

One thing we take for granted is that we have been blessed with fixed laws. Not the man-made type, but those in nature. There are laws pervading the universe which underpin our whole life. For example, the law of gravity is such that we can have a planet perfectly orbiting a perfectly sized star. Our star itself is held together by this law. Our ability to deftly move about this planet is because of this law. Our atmosphere is the perfect mix of the right gases because gravity is such that we hold onto the right-sized molecules like nitrogen and oxygen. One could write a book about the perfect fine-tuning of the law of gravity for life.

And there are many other laws tweaked so precisely that if any of them were any much different at all we wouldn't be here: electromagnetism, forces within the atom, laws of motion, etc.

And these laws are fixed; they are unchanging. Imagine if the laws of gravity, or the laws that hold the atoms and nuclei together, spontaneously varied and were not fixed. The sun might not rise tomorrow, the moon may slip away from us. We might instantly vaporize - literally. The chemistry that runs our body would fail. The atmosphere might vanish.

I am thankful for the fixed and awesome laws that run this universe.

There may be violence all around but we live in about the most peaceful place imaginable. No nearby supernovae, no destructive twin stars, no threat from a supermassive black hole, no other planets invading our orbit, no violent baby stars forming nearby. You want peace? We are immersed in it. And I am thankful for that.

Though things can get ugly in the world of men, the universe still surrounds us in absolute beauty - the full moon rising, the red sun setting, the beauty of the rainbow, the stunning starry, moonless nights. And as far as we can see into the universe with our telescopes there is indescribable grandeur in the collisions of galaxies, star nurseries and nebulae, and the death throes of stars.

Just a moment's reflection on the universe and our place in it should fill us with awe and thanksgiving. There we can find stability in its laws, peace within the violence and beauty all around. We are blessed.

Have a great Thanksgiving!

Kepler's Three Laws - Part 1

We all are aware that there are different laws that run the universe. Most of us are a little fearful of those laws. It might be a carryover from our innate fear of manmade laws, like those involving speed limits or Happy Meals. Or it may be that those laws probably involve complicated equations and terrifying math functions that can be solved only by men in white lab coats and disheveled hair.

Let's look at three simple laws given us from a complicated man who died 380 years ago today. His name was Johannes Kepler and his laws of planetary motion are still taught in schools even now, four centuries after he figured them out. This week we'll set the background for the laws, next time we'll look at them more closely.

Kepler lived in Europe at a time when astronomy was breaking free from its astrological roots. This was the time of Copernicus and Galileo, a golden age of astronomy. This is when the infant science world was starting to contemplate the starry heavens as being natural, physical phenomena orchestrated by a Creator God, rather than as being merely "other worldly" or quintessential.

And there was a new perspective on the heavens, called heliocentricism - one in which our system of planets orbits our sun rather than everything up there going around us - that was about to blossom. These new views freed up Kepler to derive his three famous laws.

Kepler was not only fortunate to live in a time when the new views were gaining traction, he was also in the perfect place at the perfect time to have decades of planetary data fall right into his hands.

It turns out that Kepler fortuitously found himself working for a man called by many the most important astronomer to live before the invention of the telescope. That man was a Dane named Tycho Brahe.

Tycho was eccentric to say the least. His personal life was like a circus at times - he had elks as pets, he employed a dwarf as a jester, and he was the bearer of a false metal nose, a replacement for the real one he lost in a duel. But Tycho was dead serious about his starry observations. He had literally decades of data of planetary positions using the best sky measuring equipment of the time.

Tycho was determined to use this treasure chest of data to prove his own eponymous planetary system, the Tychonian system, in which the planets go around the sun, but the sun itself goes around the earth. But, alas, he died an untimely death and Kepler soon inherited Tycho's trove.

Kepler would use the data not to prove Tycho's view correct, but to try to prove a heliocentric universe true. And what fell out of his investigations were three laws that appear to govern orbiting bodies everywhere we look.

The specifics of those laws? We will have to tackle those next time. Until then, clear skies!

It's a Big Universe! (but not that big)

We all know that when the sun is up it lights our atmosphere to a fair-thee-well. Hence the "day." And when that same sun sets, when its blazing glory disappears over the horizon, our atmosphere goes dark and the other stars out there, those not nearly as close as our own, sparkle in the heavens.

But that it gets dark at all was a quiet mystery for astronomers for the longest time. Really. It is not a paradox easily seen at first, so I'll give you a classic analogy to shed some light on the problem, traditionally called Olbers' paradox.

Imagine you are deep in the middle of a vast, sprawling woodland. Around you, close by, are trees of course. But there are also trees a little farther away. And still more trees a little farther than that. In fact, when you look around there are trees literally in all directions. Important here is that if you look as far as you can in any given direction, there is a tree in that line of sight, somewhere, whether close or far.

Our universe is kind of like a forest of stars. And there are stars in all directions for as far as the eye can see. Or are there?

For centuries it was believed that the universe was infinitely big and infinitely old. But if that were so, then we should be able to see stars literally everywhere, and I mean everywhere. Any point on our sky should have some star somewhere along the line of sight, no matter how far. If the universe is infinitely old, then all starlight has had plenty of time to reach us. There should be light literally anywhere our eye looks.

Which means it should not get dark at night. That's assuming of course that the universe is infinitely old and big. But maybe - just maybe - it is not.

Maybe, thought some astronomers and philosophers in the 1800's - including the unexpected likes of Edgar Allen Poe - maybe the universe is not infinite after all. Maybe there was a beginning to it. That the skies get dark at night implies there is a limit to the universe. Maybe the prevailing philosophy of the last centuries was... wrong.

That was a profound paradigm shift, that the universe might be finite. The theological implications are deep, and for some disturbing. And many in science would not accept the conclusion that the universe was finite, that it had a beginning.

It would take another century - and the great contributions of men like Edwin Hubble and Albert Einstein - for scientists to finally accept that there was a beginning to it all.

Sometimes taking the time to examine and question even the most common phenomena can bring us to the most amazing conclusions.
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me