The Earth
Earth is the third planet from the Sun and is 93 million miles away. Earth is the only known planet that has water and can support life. A long time ago, people believed that Earth stood still while the Sun traveled around the Earth each day. They thought the "rising" and "setting" of the Sun caused day and night. Today we know that the Sun does not move around the Earth but the Earth moves around the Sun. The Earth rotates, or turns, on an axis, which is an imaginary line that runs through the center of the planet. The spin axis is not perpendicular to the orbital plane but it is slightly tilted. The North Pole is the north end of the axis and the South Pole is the south end of the axis. It takes approximately 24 hours for Earth to make one complete rotation; one rotation = 1 day. It is daylight where Earth faces the Sun and darkness where Earth is turned away from the Sun. When it is daytime on one side of the world, on the other side is nighttime. The rotation of the Earth produces a regular succession of night and day as it comes to face the Sun and then moves on into the region that is in darkness. As one side moves away from the Sun, the Sun seems to "sink" below the horizon; as the other side of the Earth moves toward the Sun, it seems to "rise" above the horizon. Viewing above the Northern Hemisphere, the Earth always turns towards the East (counterclockwise); therefore, the Sun appears to "rise" from the East and "set" in the West. As the Earth rotates on its tilted axis, it also revolves, or moves in a circular path, around the Sun. This path around the Sun is called an orbit. It takes 365 1/4 days for the Earth to complete one revolution around the Sun; one revolution = 1 year.
Earth has an atmosphere surrounding the planet. This atmosphere is made up of gasses that are held in place by the gravity of the Earth. The atmosphere protects life on Earth by absorbing most ultraviolet solar radiation as well as reducing the differences in temperature between day and night. The higher you go in the atmosphere, the thinner the layers become until eventually the layers dissipate and you are in the realms of space. There are six named layers of the atmosphere; troposphere, stratosphere, mesosphere, thermosphere, ionosphere, and exosphere. Each layer has its own unique properties.
Since Earth has an atmosphere, when we look outwards from the surface, we see the sky, which is a combination of molecules that scatter light. The sky has a blue tint to it because the molecules scatters more blue light that red light, therefore giving a bluish tint to the sky.
The atmosphere that surrounds the planet helps to protect life from the Sun and Space. "The atmosphere absorbs the energy from the Sun, recycles water and other chemicals, and works with the electrical and magnetic forces to provide a moderate climate. The atmosphere also protects us from high-energy radiation and the frigid vacuum of space." (http://liftoff.msfc.nasa.gov/academy/space/atmosphere.html) Without this great protection, life on planet Earth would not be able to exist as we know it today.
Although the students at this level are not introduced to the problems that we are having with Global Warming and the Greenhouse effect, it is important for them to understand that we need to preserve the existing atmosphere the best way we can for the future. One way that the students understand this concept is the ideas of recycling and preventing harmful chemicals or gasses from entering the atmosphere.
Have you ever wondered how astronauts in space need to be tethered to the Space Shuttle so they do not float away, but here on Earth's surface we do not need to do that? The force that keeps us planted firmly on the ground is called gravity. Gravity is a force of attraction. We can find it in all pieces of matter in the universe. There are many things that produce gravity, including galaxies, the Sun and the planets. Gravity is called a universal force due to the fact that it affects all matter. Try throwing something up in the air as high as you can, watching it come down would be gravity in full effect!
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