Look up at the sky on a clear, starry night. What you see is part of the universe. You could never see all of the universe. No one even knows whether the universe has an edge or if it goes on forever. The universe contains all matter and energy. The universe holds all space and time. You are a part of the universe. The universe includes everything there is.
WHAT MAKES UP THE UNIVERSE?
Earth and all the planets in our solar system make up just a tiny part of the universe. Billions of other stars like our Sun all form a group called the Milky Way Galaxy. With telescopes we can see billions of other galaxies.
There are no cars on the planet Uranus. Yet Uranus is covered with a kind of smog. Smog on Earth is pollution that comes mostly from cars that burn gasoline. Sunlight changes fumes from the cars to smog.
Smog on Uranus comes from gases called ethane and methane. Small amounts of these gases are part of the atmosphere of Uranus. An atmosphere is the layer of gases around a planet, like air on Earth. Sunlight shining on methane and ethane makes smog. The haze of smog around Uranus makes the planet look like a smooth, bluish-green beach ball.
Take a really deep breath. Feel how your chest gets bigger and bigger. Your chest gets bigger because your lungs are filling up with air. You cannot see air, but air is all around you. You can feel it when the wind blows.
Earth’s atmosphere is made of air. An atmosphere is made up of the gases that surround a planet.
WHAT IS AIR?
Air is a mixture of several different gases. The main gases in air are nitrogen, oxygen, and argon.
Days on Venus seem endless. Venus is the second planet from the Sun. One day on Venus is as long as 243 days on Earth. A year on Venus is only 225 Earth days long. So a day on Venus is longer than a year!
WHY DOES VENUS HAVE LONG DAYS?
Venus orbits, or goes around, the Sun just as all the planets do. It takes Venus 225 days to go around the Sun. Earth goes around the Sun in 365 days. We call the time it takes Earth to go around the Sun a year.
Thousands of years ago, human hunters had a problem. They couldn’t get close enough to an animal to use spears or knives to bring down their prey. Finally, they found a clever solution: the bow and arrow.
No one knows just how or when the bow and arrow was invented. Some experts say it was developed more than 25,000 years ago. No matter when it appeared, it changed hunting and warfare for good.
Using a bow and arrow, humans could strike at animals and enemies from a long distance away and with more accuracy. People didn’t have to get so close that they might be killed first.