tion, ocean depth, and atmosphere begin) weighs more than two pounds. When a bal- loon is filled with a gas like helium, which weighs less than air, it floats.
Could a balloon float into outer space?
The air that makes up Earth’s atmosphere becomes thinner and lighter the higher up you go, so a helium-filled balloon would stop rising once the air surrounding it weighed the same as the helium gas inside it. Scientists think that 20 miles (32 kilo- meters) above Earth is about as far as any balloon could travel.
What is outer space?
Outer space refers to the area that exists beyond Earth’s atmosphere. Our atmosphere is divided into several layers based on the temperatures found in each of those layers.
The troposphere is the layer closest to Earth; it extends about 5 to 10 miles (8 to 16 kilometers) above the planet’s surface. Most of our weather—rain, snow, sleet—comes from the troposphere. Temperatures in the troposphere can fall as low as -112 degrees Fahrenheit (-80 degrees Celsius). The next layer, called the stratosphere, stretches from 11 to 30 miles (17 to 48 kilometers) above Earth’s surface. The stratosphere con- tains the ozone layer, which protects all life on Earth from the Sun’s harmful ultravio- let rays. Temperatures gradually rise in the stratosphere, reaching a high of around 28 degrees Fahrenheit (-2 degrees Celsius).
The stratosphere is followed by the mesosphere, which goes to about 50 miles (80 kilometers) above Earth. Temperatures drop well down into the negatives in the mesosphere, but in the next layer, the thermosphere, the Sun’s radiation heats the air to around 1,100 degrees Fahrenheit (600 degrees Celsius). The thermosphere ends at about 250 to 300 miles (400 to 480 kilometers) above Earth. The final layer is called the exosphere, and at that level, the atmosphere is so thin as to be virtually nonexis- tent. There is no line drawn in space marking the end of Earth’s atmosphere and the beginning of outer space, but many scientists agree that outer space begins some- where around 600 miles (960 kilometers) above Earth.
How old is Earth?
Scientists estimate that Earth is between 4.5 and 5 billion years old. They have reached this conclusion by studying Moon rocks and meteorites (rocks that have fall- en from space to Earth) that they believe were formed at the same time as our planet.
How big is Earth?
Earth, which is almost round in shape, measures 24,901 miles (39,842 kilometers) around at its widest part, the equator. (The equator is the imaginary line that crosses 2
the planet midway between the North and South Poles.) A measurement through Earth at the equator—in other words, the planet’s diameter—reveals that it is 7,926 miles (about 12,700 kilometers) across. Earth’s weight, or mass (the amount of matter that makes it), is around 6 sextillion tons. That is 6 with 21 zeros after it! Because Earth cannot be put on an enormous scale to find its weight, scientists use the laws of gravity and mathematical equations to figure this out.
Is it possible to dig deep enough into Earth’s surface to come out on the other side?
It’s a journey that only a superhero could make. Earth is made up of different layers of rock. The outer layer, or crust, is solid layered rock that is about 20 to 30 miles (32 to 48 kilometers) thick under the continents and about 3.5 to 5 miles (5.6 to 8 kilome- ters) thick beneath the oceans. Earth’s mantle, which is made up of a different kind of layered rock, extends for another 1,800 miles (2,880 kilometers) below that. Although scientists cannot penetrate this deep into the planet, they know that the mantle’s composition is different from the crust because shock waves from earthquakes travel very differently through it. At the center of Earth is its core, which is more than 2,000 miles (3,200 kilometers) deep. The core consists mostly of melted iron and nickel, with a solid metal center. Rock melts near the center of Earth because the great pres- sure of so much weight above raises temperatures there to between 5,000 and 7,000
degrees Fahrenheit (2,760 to 3,871 degrees Celsius). The very center of Earth may 3
EARTH,SKY,AND BEYOND
Outer space begins about 600 miles (960 kilometers) above Earth.
reach temperatures as high as 13,000 degrees Fahrenheit (7,000 degrees Celsius). Scien- tists believe that eruptions of volcanoes, with their hot gases and lava, or melted rock, relieve pressure from Earth’s interior; that pressure escapes through the shifting plates of rock that make up Earth’s crust.
What is gravity?
Gravity, or gravitation, is the force of attraction that exists between any two particles of matter (or any two objects). It is the force that holds planets in their orbits around the Sun, or the Moon in its orbit around Earth. (As the dis- tance between two objects increases, their gravitational attraction decreases.) Gravity is also the force that holds any object to Earth—or to any other heavenly body—
instead of allowing it to fly into space. The larger an object, the greater its gravita- tional pull. That explains why the American astronauts that landed on the Moon could leap about with little effort.
With the Moon much smaller than Earth, its gravitational pull is one-sixth as strong as that of our planet.
Gravity also explains why Earth—and other planets and heavenly bodies—are fair- ly round in shape. When our solar system was formed, gravity drew the dust and gases hurtling through space into lumps. When a great amount of matter is pulled together at one time, it crowds together into the shape of a ball because gravity pulls every- thing toward a center point. Still, Earth is not perfectly round. As it rotates on its axis, the spinning causes an additional force to pull against gravity, making Earth bulge out a little around its middle.
4
The needle of a compass is actually a magnet, and it will always point toward Earth’s magnetic North Pole.
Why does a compass needle always point north?
A magnet—made of iron or other special metals that are electrically charged—has two poles, or ends, where its magnetic strength is greatest. Each end has an opposite electrical charge. When two magnets are held near each other, the poles that have the same charges repel each other, while the ends with opposite charges attract. The nee- dle of a compass is a magnet and, believe it or not, so is Earth! Earth’s greatest mag- netic strength is concentrated at the magnetic North and South Poles (which are dif- ferent from the geographical North and South Poles). So a compass needle is attracted to the opposite electrical charges of Earth’s poles, with the tip of its needle always pointing north and the bottom of its needle always pointing south.
Where is Earth located in the universe?
This is our cosmic address: Earth is the third planet from the Sun in what we call our solar system. The center of the system is our Sun, which is orbited by nine planets, several dozen natural satellites like our Moon, and other heavenly bodies like asteroids, meteoroids, and comets. Our solar system is part of the Milky Way galaxy, a group of some hundred billion stars that are arranged in a vast disk-like shape held together by gravitational forces. Our solar system is located about halfway between one edge of the Milky Way and its center, so all the stars that we can see from Earth belong to our galaxy. But with giant tele- scopes, scientists have been able to observe many other galaxies in our universe;
some scientists believe there are 100 billion other galaxies.
What is the order of the planets in our solar system?
Starting with that closest to the Sun, the order of the planets is: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. If you have trouble keeping track of this order, remember this sentence (in which the first letter of each word is also the first letter of a planet): My Very Educated Mother Just Served Us Nine Pizzas.
How did the planets get their names?
All the planets in our solar system, with the exception of Earth, are named after ancient Greek or Roman gods and goddesses. Gigantic Jupiter, for instance, is named—fittingly—after the Roman king of the gods. The ancient Greeks and Romans believed that their gods and goddesses lived in the heavens. Astronomers of long ago—who thought that Earth was the center of the universe and that the planets and Sun orbited around it—decided to use the names of these mythical heavenly dwellers when they labeled new planets. Because our planet was not considered a part of the
heavens it was called Earth, which means “of the ground.” 5
EARTH,SKY,AND BEYOND
Are any other planets in our solar system like Earth?
Although scientists believe that all the planets in our solar system were formed at the same time, from the same giant cloud of gas and dust, each is very different from the others. The four planets nearest the Sun—Mercury, Venus, Earth, and Mars—are known as terrestrial planets, because they are made of rock and metal. The next four planets—Jupiter, Saturn, Uranus, and Neptune—are called the gaseous planets, as they are made mostly of such gases as helium and hydrogen; their inner layers are liq- uids, and they may have rocky cores. Not much is known about Pluto, the farthest planet from the Sun (about 3.66 billion miles away). A small, solid planet, Pluto is thought by some to be made mainly of ice. Some scientists have suggested that Pluto may not be a planet at all; it may simply be a large piece in a band of rocky objects, called the Kuiper belt, that orbits the Sun.
The planets of our solar system vary greatly in size, rotation, and other characteris- tics. The smallest planet, Pluto, has a diameter (the measurement through its widest part) of 1,430 miles (2,288 kilometers), which makes it smaller than our own Moon.
Jupiter, on the other hand, has a diameter of 88,700 miles (141,920 kilometers), which is about 11 times the size of Earth’s diameter. If Jupiter were a hollow ball, it would take more than 1,000 Earths to fill it. Jupiter also has the fastest rotation, or spin, on its axis, completing a day and night in just 9 hours, 55 minutes. It takes Jupiter a much longer time than Earth to rotate around the Sun, however: a Jupiter “year” is 4,333 days. Mercury has one of the slowest axis rotations: it takes 59 Earth days for Mercury 6
Saturn, one of the four gaseous planets, has bright rings made mostly of ice.
to make a complete turn on its axis. But Mercury rotates around the Sun much faster than Earth does: it takes only 88 days—a very short year compared to Earth’s 365 days.
Because Mercury rotates slowly on its own axis and quickly around the Sun, its days—
defined as the time from one sunrise to the next—are as long as 176 Earth days.
Saturn is perhaps the most interesting-looking planet, surrounded by seven wide, bright rings made mostly of ice. (Jupiter and Uranus also have rings, though they are harder to see than Saturn’s.) Although not closest to the Sun, Venus is the hottest planet because its thick atmosphere of mostly carbon dioxide traps in the heat of the sunlight that reaches the surface of the planet. The average temperature on the sur- face of Venus is 900 degrees Fahrenheit (481 degrees Celsius). Mars is the planet most like Earth, with a thin atmosphere and a surface that has mountains and valleys, extinct volcanoes, and what looks like dried-up riverbeds. Scientists wonder if some form of life once existed there; they have been trying for years to figure out a way for humans to explore that planet.
Only Earth has liquid water on its surface, which is necessary for life as we know it. Without water, plants could not grow, and without plants, oxygen—the gas essen- tial for animal life—would not be released into the atmosphere.
How was the universe created?
There are many ideas about how the universe was formed. The most popular one is called the “big bang” theory. It is thought that at the beginning of time, all the matter and energy in the universe was concentrated in a very small space or volume that exploded. Eventually, the matter left over from the explosion formed galaxies. Within those galaxies came to be stars, one of which is our Sun, and planets, including those in our solar system. The universe as we know it continues to expand, moving away from the center of the big bang.
How old is the universe?
No one knows for sure. For a long time, scientists believed that the big bang that formed the universe occurred 15 to 20 billion years ago. This estimate was based on mathematical calculations using the current rate of expansion of the universe. But recent information collected by the Hubble Space Telescope indicates that the uni- verse may be newer—only 8 billion years old.
How big is the universe?
Scientists have demonstrated that the universe is expanding in size, with galaxies moving farther from one another (objects within a galaxy, like the planets in our solar
system, don’t move away from each other, however, because they are held together by 7
EARTH,SKY,AND BEYOND
gravity). Because distances in space are so huge, scientists often use the measurement of light-years instead of miles to describe them. A light-year is the distance that light can travel through space in one year, which is 5.88 trillion miles (9.46 trillion kilome- ters). The farthest galaxies that can be seen from Earth are thought to be 12 billion to 14 billion light-years away. That means that the observable universe has a diameter of up to 28 billion light-years. And that’s just the galaxies we can see—imagine if we could stand at the edge of one of the farthest galaxies, look through a telescope, and see galaxies extending 14 billion light-years from there. The potential size of the uni- verse is mind-boggling. It is nearly impossible to imagine the distance of one light- year, let alone 14 billion of them!
What causes day and night?
Besides orbiting around the Sun, Earth turns in a circle, or rotates, from west to east on its own axis—an imaginary line running through the center of the planet—spin- ning like a top. A complete rotation takes about 24 hours, or one day. When the part of Earth on which we live is turned away from the light of the Sun, we have nighttime.
At the same time, people on the other side of the world have daytime. As Earth contin- ues its movement, we move toward the Sun’s light, and day comes.
If Earth is moving all the time, why don’t we feel it?
Even though Earth rotates at a startling speed (1,036 miles [1,668 kilometers] per hour at the equator) and orbits the Sun even faster (at 67,000 miles [107,000 kilometers] per hour), we don’t feel it because the rate of motion is a constant one, never slowing down or speeding up. We can only really feel motion when the speed changes. If you were in a moving car and couldn’t see the scenery passing by, couldn’t hear the wind blowing, and couldn’t feel the car vibrating, you wouldn’t be able to tell how fast you were going, or even if you were moving at all. Objects don’t fly all over the place when Earth moves because gravity keeps everything firmly in place as the planet orbits and turns.
What causes the seasons?
Earth’s complete orbit around the Sun takes about 365 days, or one year. During the course of the orbit, Earth tilts on its axis. When the northern end of Earth tilts toward the Sun, it receives more of the Sun’s direct rays. These rays warm the Northern Hemisphere and its summer season arrives. At the same time, winter comes to the Southern Hemisphere, which then receives less-direct solar rays. As Earth continues its orbit through the year, it tilts on its axis in the other direction. Then the winter season comes to the Northern Hemisphere, and the Southern Hemisphere enjoys summer. This tilt of Earth also explains why the length of days varies throughout the year. In the summer, more direct solar rays give us longer days.
8
What is the Sun?
The Sun is a star, just like the twinkling heavenly bodies we see at night, far off in the dark sky. But the Sun looks different to us because our planet is quite close to it (just 93 million miles [150 million kilometers] away!). The Sun, a sphere or ball made of several layers of gases, gives off tremendous heat and light. It takes about eight min- utes for the Sun’s rays to cross space and reach Earth. It is believed that the Sun’s vast and continuous production of energy is due to nuclear reactions among the atomic particles of hydrogen, one of its gases. The Sun is the center of our solar system, and its gravitational attraction keeps Earth and its neighboring planets in their orbits. The Sun’s energy is responsible for Earth’s weather—solar heating causes air to move (wind) and also causes rain by contributing to evaporation, a key step in our planet’s water cycle. Without solar heat and light, plants could not grow and animal life could not survive on Earth.
How big is the Sun?
The Sun’s diameter—its widest measurement across—is estimated at 865,400 miles (1,393,294 kilometers). Its mass is 332,000 times that of Earth. Compared with the billions of stars in the universe, our Sun is considered average in size, with half of the other stars bigger and half smaller.
How hot is the Sun?
The temperature of the Sun’s surface is thought to be about 10,000 degrees Fahrenheit (5,500 degrees Celsius). That’s more than 50 times the tempera- ture required to boil water. The center of the Sun is much hotter. Scientists have estimated that the center of the Sun is 27 million degrees Fahrenheit (15 million degrees Celsius).
Where does the Sun go at night?
The Sun doesn’t go anywhere at night; it is Earth that moves. Earth makes a complete rotation on its axis once every 24 hours. So half of our constantly shifting planet is always turned facing the Sun, experiencing day, while the other half is turned away from the Sun, experiencing night.
Will the Sun ever burn out?
It is believed that the Sun—like all stars—will burn out eventually. As a star uses up the hydrogen that fuels the nuclear reactions that power it, it is thought to die, col- lapsing in upon itself. But it’s unlikely this will happen in the near future: our Sun is
expected to shine for at least another five billion years. 9
EARTH,SKY,AND BEYOND