Are you tired? Maybe you should be. After all, you traveled almost 1.6 million miles yesterday. In fact, even as you read this, you're moving at 67,000 miles per hour. That's almost 19 miles every second! And it doesn't stop there. You're moving that fast every moment of your life.
Where are you going in such a hurry? Around the sun. We live on a huge, round rock that spins and flies through space - a planet we call Earth. And as the people who live on Earth, we go everywhere it goes, moving with it as it zips around the sun. It's a journey the Earth has made for billions of years, and, with a little luck, will continue to make for billions more.
Why do we travel so fast and far each day? Earth, like all objects, moves in space, and, like other planets, has two special movements: revolution
. Revolution is the movement around the sun. Rotation is the spinning of the Earth, which creates day and night.
Each revolution of the Earth around the sun takes one year. Humans have divided the year into 12 months
. Originally there were 10 months, but then July and August were added by the Romans.
There are 365 days in a year, right? Not exactly. It actually takes the Earth 3651⁄4 days to get around the sun. We make up the difference every four years by adding one day - February 29. These special years are called leap years
As the Earth flies around the sun, it also spins (rotates) on its axis, a movement that causes day and night. As the Earth spins, half of it always faces the sun - the half that's experiencing daylight. The other half, meanwhile, faces away, toward space. Guess what's happening there? A lot of sleeping, because it's night.
The law of inertia
was discovered by Isaac Newton. He realized that a body (which is a scientific word for "thing") in motion will stay in motion unless it's acted upon by another force. So a baseball that you threw would travel forever if gravity didn't pull it down and the air didn't push against it.
The Earth is likewise acted upon by the force of gravity
. The sun's gravity pulls on the Earth, bending its path into a circle. That circle is the Earth's orbit
Another scientist, Johannes Kepler
, discovered the three Laws of Motion
, which further explain our revolution around the sun. Kepler's Third Law of Motion, for example, predicts how long a planet's revolution takes, noting that planets farther from the sun take longer than planets close to it. Mercury, the closest planet to the sun, takes only 88 Earth days (compared with the Earth's 3651⁄4 days) for one revolution, while Neptune, the most distant planet, takes 165 Earth years
to do the same.
Kepler also noticed that most planets do not have completely round orbits, but travel in more oval-shaped paths around the sun. One end of the oval, like the top of an egg, brings the planet closer to the sun, while the other end puts it much farther away. This oval path is called an elliptical orbit
, or an ellipse
. Kepler also observed that when a planet is closer to the sun, it speeds up; when it's farther away, it slows down. That means that we sometimes travel faster or slower than 19 miles per second around the sun. Can you feel the difference?
The Earth spins or rotates on its axis, an imaginary line through its center. We call the top the North Pole and the bottom the South Pole. A day is the length of time needed for the Earth to rotate one full time on its axis.
Because the Earth is tilted on its axis, either the top or bottom is always leaning closer toward the sun. This is what causes seasons
: During our summer
, daylight lasts longer and we are warmer, while in winter
, we have less daylight, so we're colder.
Midway between the two poles is another imaginary line that circles the very middle of the Earth - the equator
. South of the equator, seasons are reversed, so that in South America and Australia, there's snow in July and beach weather in December.
In the morning, when the sun rises, it isn't really rising - the Earth is just rotating. Again, the Earth's rotation on its axis is what causes day and night.
Imagine you're an explorer. You're sailing on your ship in the open sea, looking for gold. Soon you find an island and, to your surprise, it's full of gold, more than you can fit on your ship. You take what you can, but naturally, you want to go back and get the rest of it. You sail back, but you can't remember where the island was. You sail around looking for it for weeks, then for years. Finally, a storm at sea sinks your boat and kills you. Could this unfortunate ending have been avoided?
But of course! To do so, early navigators invented a system of coordinates, imaginary horizontal and vertical lines, that covered the whole globe. With these, you could use the stars or a map to figure out where you were. The horizontal lines give your latitude
, or your distance from the equator. The vertical lines provide your longitude
We tend to think of north as the top and south as the bottom, but in space there really isn't any up or down, top or bottom. You could print a map of the world "upside down" and it would still be perfectly accurate. In fact, you can buy maps in Australia that feature Australia at the top and the U.S. close to the bottom.
The sun is very large compared with the other objects in our solar system. It is 13,500,000 kilometers in diameter - that's more than 100 times the diameter of the Earth — and has more than 300,000 times the mass of our planet.
There are billions and billions of other suns in space, but we don't call them "suns." We call them stars. Yes, the sun is a star, the closest one to the Earth. And other than its location, it's really not all that special. It's important to us, though, because all the objects nearby, in what's called our solar system, revolve around it. The sun is the center of our solar system.
Our solar system has many different kinds of objects in it, and all of them revolve around the sun. The largest of these space objects are called planets. Our solar system has eight of them. Planets must meet four requirements to be called planets: 1. They orbit the sun. 2. They are round or almost round. 3. They have cleared the area of space nearby of other objects. 4. They are not satellites or moons of another planet.
The first four planets share a lot of characteristics: They are modestly sized (not huge); made mostly of rock (not gas); and have either two, one, or zero moons. These four planets are known as the inner planets. Between Mars and Jupiter, we find the asteroid belt
, where various smaller rocks orbit the sun.
Things get a lot bigger past the asteroid belt. The next four planets, the "gas giants," together make up 99% of the mass that orbits the sun. These giant, heavy, gaseous planets all have rings (although Saturn's is the most visible) and many moons. Jupiter alone has 63 moons.
A dwarf planet
is a large object that does not meet the four requirements to be a planet, but is still part of our solar system. Eris
are the most distant dwarf planets that we know of, though there may be many more in an area called the Kuiper Belt
, out past Neptune.
A satellite is an object that orbits a planet. The moon is the Earth's satellite. It is also called Luna, so moon events are called lunar events. Mars has two small satellites or moons, named Phobos and Deimos. Jupiter has more than 60, the largest being Io, Europa, and Ganymede. Io is the most volcanically active body in the solar system; Europa is believed to be covered by an icy ocean; and Ganymede is the largest satellite in the solar system, larger even than the planet Mercury.
Since our moon orbits around the Earth, it also receives light from the sun, which is why it looks lit up to us. But the moon rotates much slower than the Earth does - just one rotation per revolution around the Earth. Therefore, the same side of the moon faces the Earth all the time; we only see one half of it at any given moment.
The moon goes through several phases each month. When we see the entire illuminated side of the moon, it's called a full moon
. When we can't see that side at all, it's a new moon
. Each night after a new moon, as more of the lit portion comes into view, we have a waxing moon
. Each night after the full moon, as we see less and less of what's lit, it's called a waning moon
A lunar eclipse
occurs when the Earth's shadow blocks the sunlight reaching the moon, thus darkening it. A solar eclipse
is the reverse — when the moon's shadow darkens the Earth.
Because of the laws of gravity, the moon tugs on the Earth. Its gravity is not enough to affect the land or the people, but it does cause the oceans to rise and fall a few feet twice each day. We call this rise and fall high
and low tide
An asteroid is another kind of rocky object, or small solar system body. Most asteroids orbit the sun between Mars and Jupiter; the largest, named Ceres, is also considered a dwarf planet. Comets are objects made of rock, ice, and frozen gases from the far edge of the solar system, with very elliptical orbits. We can see them from Earth when they are close to the sun. Some have long tails of dust that resemble feathers in the night sky, and others have such large elliptical orbits that they come near us only once in a thousand years.
Meteors are smaller rocks and pebbles that sometimes hit the Earth's atmosphere, though most burn up from friction before they can reach us. We call their streaks of light "shooting stars," or meteorites. Sometimes, about once every hundred thousand years, a large meteor hits the Earth; the impact can blow open a huge crater on the Earth's surface. Such huge meteor strikes can change the atmosphere of the planet. A meteor crash millions of years ago may have made the dinosaurs extinct.
Distances between the sun and the planets are very, very large. The Earth is 93 million miles from the sun; the planet Neptune is 2,794 million miles from it. To simplify the numbers we use different ways to measure distance in space. One astronomical unit, or AU, is the average distance from the Earth to the sun: about 93 million miles. Earth, therefore, is one AU from the sun, and Neptune is more than 30 AU from the sun. Now isn't that easier?
But even AUs are too small to measure the distance between stars, so we use the speed of light for that. Light travels at approximately 186,000 miles per second; in a year, a beam of light will travel 63 astronomical units. That means a light year, one of the largest units we have for measuring distance, equals 63 AUs.
By the standards of deep space, the distances between Earth and the other planets are small. The nearest star to our own sun, called Alpha Centuri, is more than four light years away. Our solar system is only one of millions in a galaxy called the Milky Way. Galaxies extend for millions of light years.
Our Milky Way galaxy was named by ancient people, who saw a milky trail of stars in the dark night sky. Some stars in the Milky Way are millions of light years from Earth, which means that when you stare into the night sky, you are looking into the past: Some of the starlight you see at night has been traveling to you since the birth of the galaxy.
The Earth's orbit around the sun is a very big circle - 600 million miles each year. How far have you traveled with the Earth in your life? Multiply your age by 600 million to find out how many miles you've journeyed so far.
"We are just an advanced breed of monkeys on a minor planet of a very average star. But we can understand the Universe. That makes us something very special."
— Stephen Hawking, Physicist
"Somewhere, something incredible is waiting to be known."
— Carl Sagan, Astronomer