An Earthling's Guide to Outer Space

by Bob McDonald

  You might be wondering why the rings of Saturn circle the middle of the planet. You can find out why that is by using a tennis ball—any soft, unbreakable object, actually—and a long string.

  Tie the ball to the end of the string and hold it straight out in front of you so the object is dangling just above the ground. The ball represents a ring particle, and the string is the planet’s gravity. You’re the planet! Think of your head as the North Pole and your feet as the South Pole.

  All planets spin, but Saturn rotates very fast, twice as fast as Earth does, to be exact. So spin your body around on one spot while holding the end of the string out in front of you. Watch how the ball at the end of the string swings around with you. It’s trying to fly away, but the string pulls it back, the same way that ring particles are trying to get away from a planet but gravity pulls them back. The faster you go, the higher the ball rises, but it won’t go any higher than your arm holding the string, because that is where they can get the farthest away from the center of your body while spinning around. That’s why the middle of a planet—its equator—is where rings are always found.

  If you want to find out why rings don’t rotate around the North or South Poles of a planet, hold the ball straight over your head. Now spin your body around the same way you did before. Maybe don’t let the ball go, though—you’ll probably get hit in the head!

  When Saturn was becoming a planet, it formed from a huge cloud of gas and dust that was rotating like a flying disc. The stuff that was above and below the equator fell inward and became part of the planet itself. The stuff around the middle circled as fast as it could, but it wasn’t drawn all the way into the planet. Instead, it remains circling around, caught in this gravitational tug-of-war.

  The Earth doesn’t have a ring today, but it did when it was younger. Billions of years ago, long before there was any life on our planet, scientists believe that a very large object, as big as the planet Mars, struck the Earth with a glancing blow.

  The force of the impact almost ripped the Earth apart. A huge piece of our planet was sheared off and thrown into space, along with what was left of the object that hit it. For millions of years after this collision, an enormous ring of dirt and debris surrounded the Earth. Eventually, these ring particles clumped together into a ball.

  That ball grew into what we now see as the moon. Hard to believe that the peaceful object that hangs so serenely in our night sky had such a violent beginning. Because our moon is far enough away from us, it isn’t torn apart by Earth’s gravity. But had the moon spiraled toward the Earth for some reason or formed much closer to the Earth than it is now, it would have been torn apart by our planet’s tides, and we would have had a lovely ring system. In fact, it might even have been more spectacular than Saturn’s—the moon is so large, it would supply a lot of material to spread out in a ring.

  Amazingly, the rings that surround the four largest planets in our solar system are all different. Saturn’s are made of ice and are very bright and wide. The rings around Uranus and Neptune are thin and dark, like pieces of charcoal, and they’re arranged in long, thin lines with little moons running between them. The one around Jupiter is orange but barely visible. That ring is thin and made of dust that continually blows out of volcanoes on one of Jupiter’s moons, forming a kind of smoke ring around the planet.

  Still, the award for the weirdest rings in the solar system goes to Uranus. It has North and South Poles like the other planets, and it turns on its axis like the rest, but for some unknown reason, the planet is lying on its side. It’s as though the planet fell over and is rolling around the sun like a bowling ball. Uranus is the only planet that does this, and, oddly enough, its moons and rings go around the same way, circling the planet’s equator.

  So, the Earth once had a ring, which became our moon, and the rings around the other planets are all different from one another. That suggests rings form for different reasons. You can get a ring if a big object hits a planet, like the one that struck the Earth long ago. Rings might be leftover material from when the planet was forming, stuff that did not become a moon, or they can form when moons collide with one another. We know for sure that big planets have lots of moons. Jupiter has seventy-nine moons going around it, and Saturn has more than sixty. That creates a lot of opportunities for some of those moons to run into one another and get blown to bits, or for one of those moons to wander in too close to a planet, where the gravity of the planet will tear it apart and turn those pieces into a ring.

  No one knows how long rings last. Perhaps they gather together into moons like the Earth’s ring did, or maybe they eventually spiral into the planet and disappear. Recent data from a spacecraft called Cassini, which flew right between Saturn and its rings, found a “rain” of ring particles constantly falling into the planet. Scientists calculated that at that rate, the rings of Saturn will eventually disappear… in about 100 million years. That might mean that planets without rings are too small and don’t have enough gravity to hold on to them, such as Mercury or even Venus, which has neither a ring nor a moon and has never been hit by something large. Or their rings might have disappeared over time.

  Whatever the reasons, rings are among the most beautiful and mysterious objects in our solar system, so we’re lucky we’ve got some in our backyard.

  YOU TRY IT! Put a Ring on It

  You can build a scale model of Saturn and its rings.

  WHAT YOU NEED

  A large, clear glass bowl

  Water

  A spoon

  A small sponge ball

  Pepper

  WHAT TO DO

  Fill the bowl halfway with water.

  Using the spoon, swirl the water around really fast until it’s spinning in the bowl.

  Drop the ball into the center of the spinning water.

  Sprinkle the pepper around the ball until it covers the surface of the water.

  You have just made a working model of Saturn’s rings.

  Can you see how all the particles swirl around the ball in the middle of the bowl? If you look closely, you can see that they move at different speeds: the ones close to the ball are moving faster than the ones farther away. That’s exactly what happens around a planet. The ring looks like it’s one solid piece, but all the little particles are moving on independent orbits.

  Look through the side of the bowl. Notice how thin the rings are when you see them from the edge. They almost disappear when you look at them from the side. That’s what fooled Galileo.

  PART 2 Home, Sweet Home

  Our Planet Earth

  9 What Is Our Cosmic Address?

  We live on a planet…

  That is one of seven other planets…

  That circle our sun…

  Which is only one star among billions of others…

  That form our galaxy, called the Milky Way…

  Which is only one galaxy among billions of others…

  That are scattered across the expanding universe.…

  Another way of putting it: we live on a ball.

  It took a lot of science to figure out that the Earth is round, because to all of our senses, it looks flat. The ground is always beneath our feet, and the sky is always above our heads. Continents are basically just big islands, so if you walk in any direction and keep going for long enough, you’ll come to an ocean. And when you look out across the ocean, the horizon where the water meets the sky appears to be very flat. In fact, it looks like you could fall off the edge if you went too far.

  Thousands of years ago, ancient sky watchers believed that the Earth was an island in a huge ocean, and that the sky was an enormous dome that rotated over our heads every day. Half of the dome was blue, with the sun stuck on it; the other half was black with the moon and stars on it.

  This model of the Earth makes sense only if you think about how we see the world with our five senses. But we are limited as human beings. We can’t see far ahead of us, and we’re very small compared to the Eart
h itself. And if you are just a small being on a very large ball, the surface of the ball will look flat.

  Thankfully, we learned to sail. As a result, we realized we could travel all the way around the world in one direction and still return back to the point we started at, proving the Earth is round. Later, we flew around it in airplanes, and eventually, we left Earth’s atmosphere altogether and saw our beautiful little planet from space. And yes, it really is a ball!

  Even though we have all of this scientific evidence to show that the Earth is a ball, there are still some people who believe the Earth is flat. Members of the Flat Earth Society believe that our planet is shaped like a pancake, with the oceans running around the outside. It’s similar to the ancient view of the Earth, which is based only on what we can see with our own two eyes. But if that’s all you use to see the world, you are missing a lot. Science is another set of eyes that lets us see beyond the flat horizon, out into space, and back at our beautiful blue ball floating in the blackness of the universe.

  To get a sense of our place in the galaxy, it helps to imagine a scale model. In fact, if you wanted to build the model yourself, just about everything you need can be found in your kitchen.

  Find a grapefruit, some poppy seeds, some sprinkles used to decorate cakes, and some round candies, such as Smarties or M&M’s. Once you have everything, head to a park with a baseball diamond, along with some of your friends.

  At home plate, compare the size of the grapefruit to one of the candy sprinkles. That’s about the size difference between the sun and the Earth.

  Place the grapefruit on home plate to represent the sun at the center of the solar system. Walk toward the pitcher’s mound and stop halfway. Place one poppy seed on the ground there. That’s the planet Mercury. You might want to place it on a piece of paper so you can see it.

  Continue walking and place one candy sprinkle on the pitcher’s mound. That’s Venus. Drop another one at second base to be the Earth, and another about twenty paces out to be Mars. Now walk all the way out to the fence at the edge of the outfield and place one of the round candies there to represent Jupiter.

  Look back at how far away the grapefruit is and how tiny all the planets are. And you are still not out of the solar system!

  Saturn will be another round candy about a block away from the park; Uranus and Neptune will each be another block after that. You should now be about a kilometer away from the grapefruit sun. Congratulations—you’ve created the ballpark of the solar system!

  The next-closest star to our sun is Alpha Centauri. If Alpha Centauri were a grapefruit, it would be four thousand kilometers away from home base in our model—that’s the distance between Toronto and Vancouver!

  Of course, we’re not sitting still in space. The Earth is always moving. In fact, the Earth moves in seven different ways:

  The Earth spins 360 degrees once every day (twenty-four hours). That means the Earth is carrying you around like you’re riding a carousel. You’re traveling very fast around the center of the Earth, no matter where you live. Most cities in Canada are moving at about eight hundred kilometers per hour—the speed of a jet airliner. A person living near the equator is moving 1,600 kilometers per hour—faster than a supersonic jet fighter!

  The Earth wobbles like a top, with the North Pole tracing a circle in the sky once every twenty-six thousand years.

  Our planet nods up and down like a big head slowly saying yes. It does this every month because the moon goes around the Earth at an angle and the gravity of the moon pulls the Earth up and down.

  The Earth circles around the sun every year, moving at a speed of one hundred thousand kilometers per hour. That’s thirty kilometers every second, which is faster than most rockets.

  Our path around the sun is not a smooth circle because the moon pulls us from side to side as it circles around us. Imagine swinging a heavy weight on a string around your head while walking down the street. The weight will make you stagger from side to side. The Earth follows a wavy path around the sun, with twelve waves in it, representing the twelve months of the year.

  The same way that the Earth revolves around the sun, the sun itself is traveling around the center of the Milky Way Galaxy. The sun’s speed is just over a million kilometers per hour, yet it still takes 250 million years for it to circle the galaxy once, carrying us and the seven other planets in our system with it. The last time we were on this side of the galaxy, dinosaurs were just emerging.

  Our Milky Way Galaxy is one of a small group of about fifty other galaxies that all move around each other, and our local group is one of billions of other such galactic groups scattered across the vastness of the universe, which is itself expanding, getting bigger and bigger every moment. It’s enough to make you dizzy!

  It has taken thousands of years of sky watching, telescopes, astronomy, and spaceflight to figure out our cosmic address. But knowing our place in the universe will help us as we explore the cosmos. Just don’t expect the mail from our neighbors to arrive any time soon.

  YOU TRY IT! A Guide to Earth’s Orbit, for Aliens (and Earthlings)

  There are two times in a day when you can watch the Earth turn: at sunrise and sunset. While it appears that the sun is sinking into the horizon at sunset and rising from it at dawn, the fact is that the Earth is turning, not the sun, and it’s carrying you with it.

  WHAT YOU NEED

  A lamp (a desk lamp, or one with a shade that shines light in one direction, is best)

  A swivel chair, or a stool

  A dark room

  WHAT TO DO

  Place the lamp in the middle of the room with the chair a short distance from it. Aim the lamp at the chair. Then sit in the chair and face the light.

  Think of your head as the Earth and the lamp as the sun. When you face the lamp, it’s daytime.

  3.Slowly turn your body and your head in a counterclockwise direction and notice how the “sun” seems to be moving to the right. This is the direction the sun moves across the sky during the day when you’re facing south. It rises in the east and moves to the right across the sky until it sets in the west.

  Keep turning around until the sun is on the edge of your vision on your right. That is sunset.

  Turn more and you’ll not see the sun at all. Now you are in night, or on the dark side of the Earth. Continue all the way around and the sun will reappear on your left side, which is sunrise. Welcome to a new day.

  If you want to give the experiment a twist, try repeating it from the opposite side of the lamp. You should be looking back at where you were sitting before. Notice how now, when it’s “night” and your back is toward the light, you’re facing a different direction than you were before? That is why the summer constellations are different from those in winter. Stars completely surround the Earth, just as the walls of your room are all around the lamp. We face opposite directions in the sky (or different “walls”) in summer compared to winter, which is why we see different stars in each season.

  10 How Big Is the Earth?

  Compared to a human being, the Earth is enormous. If twenty million people (almost half the population of Canada) held hands with their arms stretched out, they would reach around it once. If you wanted to walk all the way around the planet, you would have to cover 40,074 kilometers. It would take two years of nonstop walking, twenty-four hours a day, to make the trip. That’s quite the hike!

  The Earth looks flat to us because we’re so small and stuck on its surface. If you hold a big ball, like a basketball or volleyball, right up to your eye and look at the edge, the ball doesn’t look as round. The only people who get to fully appreciate the roundness of the Earth in person are astronauts who fly high above it in space.

  Today, we take seeing the Earth from space for granted. But it wasn’t always this way. It took many scientific minds and centuries of technological innovation to get us to where we are. One of those early science pioneers was a man named Eratosthenes, who worked in the great library of Alexan
dria in Egypt more than two thousand years ago. He made the first accurate measurement of the Earth, not only proving that the Earth was round but also showing how big it was. The most amazing part? He did it all with just a stick.

  One day in June, Eratosthenes was in his hometown, Syene, along the Nile River in Egypt. As he passed by a water well, he did what we all do when we pass a well: he looked down to see the water at the bottom. Not only did he see the water, he saw a reflection of the sun, which meant that at that moment, the sun was straight overhead.

  Now, I’m not sure that walking by that well was an accident, because he did this at exactly noon on June 21, the summer solstice, the one day of the year when the sun is at its highest point in the sky. If the Earth were flat, then at noon on June 21, Eratosthenes would have been able to see the sun at the bottom of every water well throughout the world, because sunlight falls straight down from the sky. But if the Earth were round, he’d only see the sun in one well at a time and only when the sun was directly above each one.

  So Eratosthenes reasoned that if he could measure the angle of the sun at another well at the same time on the same date, he could tell how far around the curve of the planet he was… and, by a simple calculation, figure out the size of the Earth.

  Eratosthenes waited a year to do the measurement. Just before noon on June 21 the following year, he stepped outside the library in Alexandria, several hundred kilometers north of his hometown. There was no water well to see the sun in, so to find out whether the sun was straight overhead, he used a stick. Not just any stick, but a very special one called a gnomon—it’s like a sundial that measures the angle of the sun in the sky.