EARTH Earth is one the nine Planets in Milky way Galaxy. HOW DOES EARTH MOVE IN SPACE? Earth spins like a top on its axis. Earth’s axis is an imaginary line that goes through Earth from the North Pole to the South Pole. Earth’s axis is slightly tipped, like a spinning top leaning to one side. Earth travels around the Sun at about 67,000 miles per hour (about 107,000 kilometers per hour). One year is one trip around the Sun. Earth’s path around the Sun is slightly oval-shaped. This oval shape causes Earth’s distance from the Sun to change during the year. WHAT MAKES DAY AND NIGHT? The Sun seems to rise in the morning, cross the sky during the day, and set at night. However, the Sun does not actually move around Earth. Earth’s turning on its axis makes it look as if the Sun is moving. Earth makes a complete turn on its axis every 24 hours. As Earth turns, half of the planet faces the Sun, and the other half faces away. It is daytime on the half facing the Sun. It is night on the half facing away from the Sun. WHY ARE THERE SEASONS? Earth has seasons because of the tilt of its axis. For part of the year, the top half of Earth is tipped toward the Sun. The top half of Earth is called the Northern Hemisphere. During another part of the year, the bottom half of Earth is tipped toward the Sun. The bottom half is called the Southern Hemisphere. It is summer in the half that is tipped toward the Sun. It is winter in the half tipped away. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. During spring and fall the hemispheres are tipped neither toward nor away from the Sun. The equator is an imaginary line around Earth’s middle. The farther you are from the equator, the greater the difference in temperature between seasons. The equator never tips far from the Sun. Near the equator it is warm enough to go swimming all year long. The average temperature barely changes from month to month. In Alaska, far from the equator, the average temperature in January can be more than 60 degrees colder than it is in July. WHAT IS THE INSIDE OF EARTH LIKE? Earth is made of layers. The top layer is called the crust. It is made of hard rock and soil. More than 70 percent of Earth’s crust is covered with water. Most of the water is salt water in the ocean. Pieces of dry land called continents rise above the ocean. The part of Earth’s crust under the ocean is called the seafloor. Under the crust is a layer of partly melted rock called the mantle. Under the mantle is Earth’s core. The core is mostly iron. The outer part of the core is liquid metal. The inside of the core is solid metal. Scientists believe that the liquid metal makes Earth a giant magnet and creates Earth’s magnetic field. Earth’s crust is made of gigantic slabs of rock called plates that move over the mantle. Plates crash together to make mountains. They pull apart and let red-hot rock ooze up from inside Earth to make new crust. HOW DID EARTH FORM? Scientists think that Earth and the rest of the solar system formed from a spinning cloud of gas and dust. Gravity pulled most of the gas and dust together to form the Sun. Some leftover gas and dust formed Earth and the other planets. Scientists think that Earth and the Moon formed about 4.6 billion years ago. SPACE: The Milky Way Galaxy belongs to a cluster of about 30 galaxies called the Local Group. Within the Local Group, the Andromeda Galaxy is the galaxy closest to the Milky Way. The Andromeda Galaxy is so far from Earth that it takes about two million years for its light to reach us from there. The Sun joins about 400 billion other stars to form a galaxy called the Milky Way. The Milky Way Galaxy is shaped like a pinwheel, with curved arms that circle around the center. The Sun and solar system are located in one of the pinwheel’s arms out toward the edge of the galaxy. After the Sun, the closest star to Earth is Proxima Centauri, more than 20 trillion miles (30 trillion kilometers) away. While light from the Sun takes only about eight minutes to reach Earth, other stars are so distant that their light takes billions of years to reach Earth. Earth is one of nine planets that circle around the Sun to form a solar system. The Sun’s gravity is so powerful that it can hold objects in orbit that are billions of miles away.