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Exploring the Solar System for Kids
A History with 22 Activities
By Mary Kay Carson
Chicago Review Press Incorporated Copyright © 2008 Mary Kay Carson
All rights reserved.
Prehistory-1900: Spying on the Heavens
The next time you're outside on a clear night, look up. You won't be the first person to marvel at the Moon and stars. Studying the lights in the night sky is something that humans have always done. People have used recognizable star patterns, called constellations, to mark the passing of time for thousands of years. Ancient peoples used star calendars to help time crop plantings and to move to new hunting grounds as the seasons changed.
The night sky's pattern of stars, or starscape, is like a background of lights out in space. Our view of the starscape shifts during the year as the Earth travels around the Sun. The Big Dipper, for example, appears handle up in the sky during the summer and handle down during the winter. But the Big Dipper always keeps its ladle shape because it's not the stars, but Earth, that is moving. This changing view allows us to use the constellations as a kind of calendar.
The bright evening star near the Moon isn't a star at all. It's the planet Venus.
If you counted all the stars you could see while looking up at the night sky, you'd get to about 3,000 before running out of bright dots. But you would have miscounted by a few. That's because some of the very brightest dots aren't actually stars.
The ones that shine without twinkling are really planets. Depending on when you look and how much city light is around, you can see the planets Mercury, Venus, Mars, Jupiter, and Saturn with just your eyes.
WONDERING ABOUT WANDERERS
In ancient times the Chinese, Babylonians, Greeks, and Egyptians recorded their observations of stars. They noticed that five "stars" were different from the thousands of others — they didn't twinkle. They also noted that these brightly shining "stars" seemed to move differently, too. On most nights, these five "wandering stars" travel from east to west. But they show up in different places on the starscape from one night to the next. And their speed and direction change, too. Sometimes they move quickly, but other times slowly — or even stop, then go backward! The odd movements of the "wandering stars" seemed purposeful, or intelligent, to some ancient cultures. Many believed that the wanderers were gods moving back and forth as they went about their heavenly business.
The five "wandering stars" are, of course, not stars at all. They're the planets Mercury, Venus, Mars, Jupiter, and Saturn. They seem to "wander" across the night sky because, unlike stars, planets really do move. Planets don't twinkle like stars because planets are so much closer to us. The strong, steady light of the nearby Sun reflects off a planet's entire lit side, causing it to shine a beam of light toward Earth. By comparison, from Earth faraway stars look like single points of light. Those tiny points of weak starlight get bounced and blurred coming through Earth's atmosphere. That's what causes stars to twinkle.
The nearest planet, Venus, is 67 million miles (108 million km) from Earth. That seems far, but not compared to the nearest star, Alpha Centauri. It's 25 trillion miles (40 trillion km) away! That's the difference between walking a single step and hiking across the state of Indiana! These five planets are not a part of the unchanging starscape background. They're part of our solar system.
Everything in the solar system — planets and their moons, dwarf planets, asteroids, and comets — travels around the Sun. But each planet revolves, or orbits, around the Sun at its own uneven pace — all while the Earth is doing the same. Looking at moving planets from a world that is also on the go makes for some odd tricks of perspective. It's like watching a truck as you're passing it on the highway. The truck can look like it's standing still or even slipping backward, but it isn't really. Your car is just moving faster and passing it by (see "Why They Wander," page 2).
Sometime around the sixth century B.C., ancient Greek scholars decided that the five "wandering stars" were not really gods who were out for heavenly strolls. The scholars began to carefully chart the paths of the planets, create tables of measurements, and work on ideas that would explain the planets' movements. They were some of the world's first astronomers.
FINDING THE COSMOS'S CENTER
By the second century, ancient scholars had come up with an explanation of how the planets moved that didn't involve gods. It was hammered out by a Greek astronomer, mathematician, and geographer working in the great Egyptian city of Alexandria. His name was Ptolemy (TALL-uh-me).
Spy the Evening Star
There are five planets visible to the naked eye. But Venus is by far the easiest to see. Often called the "Evening Star," Venus is the third-brightest object in Earth's sky, after the Sun and the Moon. Look for Venus around sunrise or sunset, not in the middle of the night. It will appear close to the horizon near the Sun. (Remember, never look directly at the Sun!) When and where Venus appears in the sky depends on where it is in its orbit around the Sun. Check a night-sky calendar in a magazine about astronomy or telescopes, in the weather section of many newspapers, or on a sky calendar Web site (see page 165).
If you have a pair of seven-power (7×) or stronger binoculars you can see Venus change shape over time. You can even track the shapes Venus goes through (called phases) and prove that Venus orbits the Sun — just like Galileo did. Just sketch Venus's shape night after night and see how it changes phases. Hope for clear weather!
According to his theory of the universe, Earth is a sphere that never spins or moves. Instead, it is fixed in the center of the cosmos, and all the other planets and the Sun orbit around it. Ptolemy explained the wandering paths of the planets by claiming that these planets moved around in their own mini-orbits within different layers of celestial stuff. Ptolemy's theory may not sound that convincing today, but it was then. If you accept the Ptolemaic system of circles and spheres as true, the system can be used to predict the paths of the planets across the night sky pretty well. Maybe this explains why the Ptolemaic system was widely accepted in both Europe and the Middle East for more than a thousand years!
It took a Polish clergyman to finally change people's ideas about the center of the cosmos. Nicolaus Copernicus (Coh-PER-nih-cus) was born Mikolaj Kopernik in 1473. After studying law and medicine in Italy, Copernicus took up math and astronomy. Then he moved back to Poland, became a church official, and started studying the night sky. Most astronomers during the 1500s worked on fine-tuning the Ptolemaic system.
But Copernicus decided that Ptolemy's system was too ridiculously complicated to be true. He decided that the simplest way to explain how the cosmos moved was to put the Sun in the center, with all of the planets, including Earth, revolving around it. He thought the Earth must spin itself around once every day. Copernicus wrote up his ideas in a book called On the Revolutions of the Heavenly Spheres.
It's unlikely that Copernicus knew that his ideas would soon start the age of modern astronomy. But he did know that saying the Sun was the center of the cosmos could get him into trouble. Copernicus was an official of the church, after all. And the church stated that the Earth was the most important thing in the cosmos — that it was unlike any other planet and that it rightfully belonged in the center of the universe. That's why Copernicus put off publishing his book until he was dying. He died in bed after seeing the first copy of it on May 24, 1543.
Copernicus's Sun-centered, or heliocentric, view of the cosmos helped bring about the scientific Renaissance. By 1600 most astronomers accepted that the Sun was the center of the cosmos, that all the planets circled around it, and that the Earth spun around, creating day and night. But Copernicus's theory had a big problem. It didn't actually predict the path of the planets very well. Why didn't Copernicus's cosmic model match what astronomers were seeing in the night sky? It was a question that pestered Johannes Kepler for many years. Kepler was the German-born assistant of Tycho Brahe (BRA-hey), the greatest observer of the planets at the time (this was before Galileo and the invention of the telescope). For many years Brahe made detailed records of where each planet was in its night-by-night path through the dark sky. After Brahe died, Kepler replaced him as the astronomer at an observatory in Prague.
Kepler knew firsthand that Brahe's observations were absolutely accurate. So why didn't they match Copernicus's theory of how the planets should move across the sky? Kepler decided to study the problem by concentrating on the movement of just one planet — Mars. Kepler had Brahe's detailed records of Mars's movements — and he knew they were right. For six years, with failing eyesight, Kepler tried combinations of circular orbits that would put Mars in the positions that Brahe had observed. Finally, in 1609, Kepler figured out that there was no magic combination of circular orbits. Mars's orbit was not circular. It was oval shaped, or elliptical.
Copernicus's theory had the planets orbiting the Sun in simple circles. But Kepler discovered that all the planets have elliptical orbits. Once he made this breakthrough, Kepler solved other mysteries about how and why the planets move as they do. In an elliptical orbit, a planet is sometimes nearer to the Sun than it is at other times. Kepler discovered that a planet's movement speeds up when it's closer to the Sun. He also discovered that the longer it took a planet to orbit the Sun, the farther away it was from the Sun. These ideas about how planets move became known as Kepler's laws (see the Kepler biography on page 5). Kepler's laws backed up Copernicus's theory of a Sun-centered cosmos. But it would take a colleague of Kepler's to actually prove it to the world.
Johannes Kepler's discovery that the planets move around the Sun in elliptical, not circular, orbits led the way to his laws of planetary motion. Create and compare a circular orbit and an elliptical orbit in this activity.
8½" × 11" (22-cm × 28-cm) sheet of white paper
Pencil or pen
Piece of cardboard (or an old magazine) that is at least 8½" × 11" (22 cm × 28 cm)
5" (13-cm) length of string tied into a loop
Colored pencil or pen
1. Fold paper in half, then fold that half again. Open the paper up and use a pencil or pen to draw a line in the longest horizontal crease.
2. The spot where the unlined crease intersects with the line you drew is the midpoint. Label the midpoint "Sun." Put the paper on the cardboard or old magazine and tape down the corners so it doesn't slide around.
3. Push a pushpin into the Sun midpoint. Place the string loop around the pushpin. Hold the pencil upright inside the loop of string until it's taut. Move the pencil around inside the string loop to make a circle, as shown below. This creates the path of a circular orbit, which no planet has!
4. Now push the other pushpin somewhere on the horizontal line you drew. It can be either to the left or the right of the Sun; it doesn't matter. Place the string loop around both pushpins. Use the colored pencil or pen to draw an oval inside the string loop, as shown. This path shows an elliptical orbit, which every planet has!
5. Take the pushpins out, remove the string, and compare the two orbits. Notice how a planet traveling on this elliptical path wouldn't always be the same distance from the Sun, like a planet traveling on a circular path would.
SEEING NEW WORLDS IN A NEW WAY
In the spring of 1609, an Italian scientist heard about a new instrument that showed faraway things as though they were nearby. Remarkable! At 45 years of age, Galileo Galilei set out to build such an instrument himself. Within months Galileo had built a device that magnified objects to twenty times their size. By the fall of 1609 Galileo was doing what no one had ever done before. He was observing the heavens with a telescope.
What Galileo saw through his telescope proved that Copernicus and Kepler were right. When Galileo observed Venus through a telescope, he saw that it went through phases — just like the Moon does. This proved that Venus orbits the Sun, just like the Moon orbits the Earth. Galileo also discovered that the Moon wasn't smooth, like everyone thought. He could see craters and mountains on the Moon with his telescope. Galileo also saw four never-before seen moons circling Jupiter. And he spotted something odd near the edges of Saturn. (It would later turn out to be the planet's rings.)
Galileo's discoveries changed everything. They not only provided the proof needed to forever push Earth out of the center of the cosmos, but the discoveries also showed that the Moon and the planets weren't godlike points of light, made of celestial material and beyond the understanding of humble humans. These were real places–actual worlds with rocks, mountains, and moons of their own. Earth wasn't the unique center of the cosmos. It was simply one of many worlds that orbited the Sun. Earth was in the Sun's realm. Our world belonged to a solar system.
REASON BEHIND MOTION
When the plague hit Cambridge, England, in 1665, Isaac Newton decided to leave town. While waiting for the outbreak to pass at his family's country home, an apple caught Newton's eye. He watched as the fruit fell from its tree to the earth below. It got him thinking. Could the force that pulled the apple to the ground be the same force that makes the Earth orbit around the Sun? It is the same force. Newton had discovered gravity.
Gravity is the force of attraction among all matter. How the gravitational attraction of one thing affects another depends on mass and distance. Objects that are far apart have less gravitational attraction to each other than objects that are close together. And more massive objects create a greater gravitational force of attraction than smaller ones do.
Newton published these ideas in his 1687 book Principia Mathematica. In it both Kepler's laws of planetary motion and Galileo's observations are explained by one simple law of universal gravitation. The puzzle of why and how the planets moved was now solved. Astronomers left the mystery of planetary movements behind. It was time to begin exploring the nature of the planets themselves — up close.
ZOOMING IN ON THE HEAVENS
Discovering how gravity holds the universe together wasn't Isaac Newton's only contribution to astronomy. He also created a better telescope. Galileo's biggest telescope was a metal tube less than two inches (5 cm) wide and about three feet (1 m) long. Inside the tube were two lenses, one at each end. The lens that Galileo looked through is called the eyepiece lens. It was concave, or curved inward like a bowl. The lens at the far end is called the objective lens. It was convex, or curved outward. This combination of lenses zoomed in so well that Galileo could only look at a fourth of the Moon at a time! The telescope had what's called a very small field of view. Johannes Kepler improved on Galileo's telescope design by using a concave lens for both the objective and eyepiece lenses. This produced an upside-down image, but the field of view was larger. Kepler could see the whole Moon at once with his telescope.
Another problem of early telescopes like Kepler's and Galileo's was that the edges of the crude lenses acted like prisms. This caused a rainbow halo to appear around the image. During the 1660s Isaac Newton discovered that sunlight is actually made up of many colors of light. While studying light, Newton figured out that if he replaced his telescope's lenses with curved mirrors, the rainbow halo vanished. Newton had built the first reflecting telescope. Newton's first reflecting telescope was only six inches (15 cm) long, and its primary mirror was just an inch (2.5 cm) wide. But the telescope was so powerful that he could see Jupiter's moons with it!
Excerpted from Exploring the Solar System for Kids by Mary Kay Carson. Copyright © 2008 Mary Kay Carson. Excerpted by permission of Chicago Review Press Incorporated.
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