by Bob McDonald
Right before noon, he put the gnomon on the ground and waited for the sun to pass overhead. If the shadow of the stick disappeared entirely at noon, it meant that the Earth was flat—end of experiment. But of course, the shadow didn’t disappear. As the sun passed overhead, the shadow shrank to a little sliver, but it never completely went away. When Eratosthenes measured the angle of the shadow, it was seven degrees. It wasn’t much, but it proved that the Earth was curved.
What Eratosthenes had measured was a thin slice of a circle, like a piece of pie. Seven degrees is about one-fiftieth of a circle (a circle has 360 degrees). That meant that the distance between the towns of Syene and Alexandria was about one-fiftieth of the distance around the Earth. He knew how far apart the two towns were, so he just multiplied that by fifty.
In ancient Greece, length was measured in units called stadia—as long as a stadium, or about two hundred meters. (The stadium was where people gathered to watch races, just as we still do today.) Eratosthenes figured that the Earth had a circumference of two hundred fifty thousand stadia, which translates to roughly fifty thousand kilometers. The actual distance around the Earth is forty thousand kilometers, so he was pretty close!
Eratosthenes was the first person we know of to get a sense of how large our planet really is. Imagine what went through his mind more than two thousand years ago, realizing that the world was round and that it was far bigger than what anyone knew at the time. The ancient Greeks didn’t know that North America, South America, Australia, and Antarctica existed, because ships at that time could not cross the oceans.
Later, other civilizations built upon the discoveries of the Greeks. In 1000 AD, a Muslim mathematician named Al-Biruni found a new way to measure the curvature of the Earth by sighting along the ground to the top of a mountain. Using trigonometry, the branch of mathematics involving triangles, Al-Biruni was able to calculate the distance to the center of the Earth and, from that, figure out the planet’s size.
Despite how far we’ve come since Eratosthenes, we need to keep things in perspective. Although the Earth seems big to us, it is actually small as planets go. Just look at the Earth compared to Jupiter, the largest planet in our solar system. A thousand Earths would fit inside Jupiter. And if we could place the Earth at one side of Saturn’s rings, the moon would be on the other side.
Most planets that have been found orbiting other stars in the universe are big, like Jupiter and Saturn. Our planet may be small, but it is the only one we have!
YOU TRY IT! Eratosthenes’s Experiment
WHAT YOU NEED
Half of an 8x10 picture frame that forms an L shape
A friend in another location with the same thing, same size
Measuring tape or a ruler
Phone
WHAT TO DO
On a day that is sunny at both your location and your friend’s, call your friend.
Both of you should go outside and place the picture frame on something flat, such as a windowsill or a patio. It is very important that the surface is level so one arm of the frame points straight up.
Turn the bottom part of the L until the shadow of the upright piece falls along it.
At the same moment, both of you measure the length of the shadow using the ruler.
Compare your measurements.
There will be a difference in the length of the two shadows because of the curve of the Earth. The farther away you are from each other, the better.
11 Why Are Planets Round?
Look at anything in space and what do you see? All the planets are shaped like balls. So are the sun and all the other stars in the universe. Planets circle the sun in round orbits, and the sun follows a circular path around the Milky Way Galaxy. Everything seems to be round. And there’s one force out there making it work that way: gravity.
Everyone on Earth feels the pull of gravity. It doesn’t matter where you are—you feel a pull toward the ground, and you think you’re at the “top” of the planet. In fact, there is no single up or down; each one of us has our own personal sense of where down is, and all of us are correct! Down is simply toward the center of the ball.
Look at a globe of the Earth and find your home country—say, Canada. If you turn the Earth over, you’ll find Australia on the other side. That means at this very moment there are people under your feet, on the other side of the world, who are calling “down” what you call “up.”
Everything on the planet is pulled toward the middle of the Earth. The only shape that allows every part to be as close to the center as possible is a ball. Take a piece of Silly Putty, or if it is winter, make a snowball, and try to make it as small and compact as possible. Squeeze everything toward the center as best you can. What do you end up with? A ball.
Gravity works the same way, always pulling toward the center of an object or a group of objects, gathering everything into the smallest possible shape. That’s why large objects in space, no matter what they’re made of, are round.
The bigger an object is, the more gravity it possesses. By the way, we don’t know where gravity comes from. We just know that the more mass you have, the more gravity you get. As gravity continues to pull objects in toward the center, we get a larger and larger object… and the larger it grows, the more gravity it has, which pulls even more stuff to it.
Our whole solar system works this way—small objects gravitationally circling big ones. The sun is the biggest of all, thousands of times bigger than all the planets put together, so it has the most gravity. All the planets—which are little by comparison—are caught in the endless circle of the sun’s gravity. At the same time, moons are smaller than planets, so they loop around the planets as the planets circle around the sun. Everything is swinging around something in a giant circular swing dance in space.
All of those forces have an effect on the way the stars and planets move around the galaxy, which is also spinning around its center. But, although all planets are shaped like balls, they’re not always perfectly round. Sometimes they can have a little bulge in the middle.
The biggest planets in our solar system have the biggest bulges. Jupiter, Saturn, Uranus, and Neptune are called gas giant planets because they’re giant compared to the Earth and they’re made mostly of gas and liquids.
Imagine a liquid ball floating in space. Gravity is pulling it together into a sphere, but big planets rotate very quickly, twice as fast as the Earth. And when a ball of gas or liquid spins around, the equator is moving faster than the poles, the same way that the tire of a bicycle moves faster than the hub. That causes the planet to bulge out at the center because its material is being flung outward by the spin.
The Earth also bulges at the equator—when you stand on the equator, you are forty-two kilometers farther away from the center of the Earth than you would be if you stood at the North Pole. But that’s nothing compared to Jupiter, which is made of gas. Measured across its middle, Jupiter is 143,884 kilometers. But when you measure the planet from north to south, it’s 133,709 kilometers. That means that Jupiter’s equator sticks out an extra 10,175 kilometers farther away from its center. If it didn’t spin at all, Jupiter would be perfectly round.
Really small masses—say, less than one hundred kilometers across—come in crazy shapes. Space masses that small don’t possess enough gravity to pull everything toward the center and form a ball. Phobos, a moon of Mars that is only twenty-two kilometers across, looks like a potato. And an asteroid named Eros, which is about the same size, is shaped like a peanut. There’s even a comet that looks like a dog bone! These odd-shaped little moons and asteroids could be pieces that were chipped off larger bodies, or stuff that just never formed into a big moon or planet.
Recently, a robot spacecraft called Rosetta caught up with a comet that looked like a rubber duck. Philae, a small lander that was dropped off by Rosetta, was sent to the surface of the duck comet to try to take a sample. Philae had a harpoon in its belly and claws on its feet that were supposed to dig into th
e comet so the lander would not bounce off in the low gravity.
But there was a bit of a mishap. The harpoon didn’t fire, and the claws didn’t hang on, so the lander hit the ground and bounced back up. With almost no gravity at all on the comet, Philae drifted back up a kilometer high above the comet, then slowly floated back down and ricocheted up a second time. Each bounce took about a minute to happen. Finally, Philae ended up tilted at a crazy angle against a cliff.
If astronauts ever visit a comet or asteroid, hopefully they won’t bounce like Philae, but they will be able to take giant, slow-motion leaps and fly just like superheroes. Wouldn’t that be fun?
For now, we know how gravity works—it pulls you down, tries to make you round, and keeps you stuck here on the ground. But we still don’t know what it is beyond a mysterious, invisible force. If we understood it better, maybe we’d be able to turn it off, or turn the volume up and down like on a TV. Imagine that! Maybe one of you will become the scientist that figures out gravity and makes it work by remote control!
YOU TRY IT! Garbage Gravity
You can see how gravity makes objects round by using a garbage can gravity well.
WHAT YOU NEED
A large, round garbage can
String
A thin tablecloth or sheet
A bag of marbles
WHAT TO DO
Drape the cloth over the top of the garbage can and tie it tight around the rim with a piece of string so it looks like a drum. Make sure the surface is smooth and the fabric is taut. The surface of the cloth represents space.
Roll two or three marbles across the drum and notice how they move in fairly straight lines. You will also see how they end up stopping in the center and their weight causes the fabric of the drum to bend downward, forming a little well. Albert Einstein described gravity as the curvature of space—every object causes space to bend inward, causing other objects to follow that curve until the two come together.
Repeat the above step with the rest of the marbles one at a time until they all cluster in the center.
When you have created a deep well, roll one marble around the cluster so it goes into orbit. You have just made a model of a planet going around a star!
The more marbles you add, the more the fabric bends. Look down from above. Can you see how all of the marbles gather together and make a round shape? That’s how gravity makes stars and planets round: by pulling everything as close to the center as possible.
12 How Do We Know the Earth Moves?
The ground beneath your feet doesn’t feel like it’s moving. Today, we know that the Earth is spinning like a top, whizzing around the sun, and circling the galaxy at dizzying speeds. But hundreds of years ago, people thought that the Earth was at the center of the universe because everything in space seemed to be moving except us!
Galileo was an astronomer who lived more than four hundred years ago and was the first to use a telescope to look at the moon and the planets. He was also the first to prove that the Earth really does move around the sun. His biggest clue came when he saw Jupiter through his telescope and picked out four tiny dots on either side of the planet. As he observed Jupiter night after night, the dots changed their positions. He measured their motions and calculated that the dots were moons orbiting the big planet. Those four moons are now called the Galilean satellites.
Galileo reasoned that if small moons orbit a big planet, then it makes sense that small planets would go around our big sun. He was right, but his discovery got him into trouble for the rest of his life.
Back in Galileo’s time, most astronomers—and the Pope, who was the powerful head of the church—believed that the Earth didn’t move at all. They thought that the Earth was at the center of the solar system, with the sun, moon, and all the planets going around it. It’s a nice idea, thinking we’re so important, but it’s wrong.
We now know that the sun is at the center of our system, and we go around it. Although all of the planets in our solar system orbit the sun, those orbits aren’t completely even, and we occasionally pass the other planets, like one car overtaking another on the highway. Take Mars, for example. It’s farther away from the sun than we are, so it takes longer to complete its orbit. Every now and then, the Earth passes Mars like we are on the inside lane of a racetrack, and when that happens, the red planet can be seen closer in our night sky, and sometimes it seems to move backward as we pass it. Jupiter and Saturn do the same thing, although they’re farther away and don’t seem to back up as much.
That’s why early models that tried to duplicate the motion of the planets with the Earth at the center got so complicated. Ancient scientists were trying to represent this strange motion with a model that had the Earth standing still. Oops.
Galileo was different. He believed the Earth did move. He wasn’t the first to come up with the idea, but he was the first to prove it using a good scientific method. He knew the key to understanding how the Earth moved was gravity, but he wondered: Would gravity work the same way on a large mass like the Earth as it does on a small mass?
To test his idea, he went to the Leaning Tower of Pisa.
The story goes that Galileo threw a bunch of objects off the tower to see if they would fall at the same speed. He was trying to better understand the laws of gravity. He figured that if objects of different sizes fell at the same speed, it would mean that gravity worked the same way on all objects on Earth, no matter what size they were. And if that was the case, Galileo figured that gravity would also work the same on the planets in the solar system.
Galileo’s experiments proved his hunch. With that theory of gravity in mind, Galileo found that the motions of the planets around the sun all made sense. The gravity of the sun held on to the planets as they orbited around it. In other words, the Earth did not sit still; it moved.
Galileo wrote the results of his experiments in a book that became an instant bestseller. Scientists liked what they read. They started pointing their own telescopes into the sky and agreeing with his idea that the sun was at the center of the solar system. Galileo, in other words, started modern astronomy.
There was just one little problem with Galileo’s new idea that the Earth moved. The Pope at the time, Urban VIII, also happened to be an astronomer.…well, sort of. He believed the official church position that the Earth was unmovable, and therefore all the stars and planets had to go around us. In the Pope’s eyes, we were the center of the solar system, not the sun. And the Pope did not like being told he was wrong.
Galileo was put on trial for his discoveries. The Church threatened to throw him in jail or even torture him if he didn’t take back what he said. Fortunately, Galileo was famous enough that he escaped torture and imprisonment, because the Church and state feared the people would protest.
Galileo was forced to take back his ideas and appear contrite. Still, he never actually said he was wrong. He merely said his observations could be interpreted differently. That satisfied the Church, but deep down, Galileo knew he was right.
His punishment was to be confined to his home, a fairly nice place in the hills outside of Florence, Italy. In fact, you can still visit his house today. The only problem was that he wasn’t allowed to leave. Imagine being told you can’t leave your house for the rest of your life. It was way better than jail or torture, but it was still limiting. Beyond confinement, Galileo also wasn’t allowed to do experiments or use a telescope. Of course, that didn’t stop him from working on his ideas and quietly passing them on.
Galileo had young students visit him at his home, and he gave all of his knowledge to the newer generation, hoping that they would one day carry on his research. And that’s proof that he was truly a great scientist. He, like so many other gifted and learned men and women of science, passed his knowledge down to others.
There’s an important lesson that Galileo taught us. He believed he was right while others did not. He stood up for his beliefs and did everything he could to spread his ideas to other
s. We remember him for that.
So if you know you’re right about something, stand up for it. Don’t let people who are wrong try to change your mind. You might be criticized for your thinking, but in the end you’ll be remembered. You might even change the course of history!
That’s certainly what happened with Galileo. Those little moons that he saw with his telescope in 1610 have turned out to be four of the strangest places in space. All of them are roughly the size of our moon, but they’re very different from what we typically consider to be a moon. And one of the robot spacecrafts that went to Jupiter to study these moons was called Galileo!
First of all, the moons come in different colors. The one closest to Jupiter is orange. The next one out is white. And the other two are shades of gray.
Each moon has been given an interesting name—Io, Europa, Ganymede, and Callisto. Much better than the name we gave our moon… “the moon.”
Io is the strangest of the four moons. It’s covered in orange and yellow crystals made of sulfur that blows out of volcanoes and falls like multicolored snow on the ground. The whole moon seems to be turning itself inside out as hot material from the inside spews out of holes in its surface, forming umbrella-shaped clouds that rain particles down onto the ground.
In some places on Io, the ground is so hot, it has melted into black lakes of liquid goo with yellow islands in them. It looks like something out of science fiction, but it’s all real. It’s a colorful, violent place.
Europa is the opposite of Io: it’s a frozen world. The whole surface of this moon is one huge sheet of ice. If you could hold Europa in your hand, it would feel like a perfectly round ice ball. There are no big mountains or valleys on this moon, just cracks in the ice. Some of the pieces of ice look like they’ve moved around; some of the cracks look fresh and new.
