Which planet is closest to us? We are standing on it…the Earth of course! To fully understand the Earth as a planet, students need to grasp both the Earth's shape and gravity concepts.
Our planet is the largest of the terrestrial group. It is the third planet from the sun and it has a diameter of 7,926 miles (12,756 km) at the equator and orbits the Sun at 1.0 Astronomical Units, or 93 million miles (150 million km). Earth is rich in nitrogen and oxygen and has an abundance of water in solid, liquid and vapor forms. Nearly three-quarters of the earth is covered with water (71%). The rest of the earth's surface varies from flat lands to dry desserts to towering mountain peaks. Earth is covered with life. There are over 175 million forms of life identified on earth. Because of Earth's geographical location in the Solar System, it is in a special place well suited for life and appears to be the only place in our Solar System where life exists. If the planet is too close to the Sun, it is too hot for life-giving molecules to form and if it is too far away, it will be too close for molecules to join and too cold for liquid water to exist.
Our daily experience is that of a fixed Earth at the center of the Universe. It's always a bit of a shock to remind students that we live on a ball of rock and water that spins once a day around the glow of a star (our Sun). Earth revolves around the Sun once every 365 days, which we call a year. Earth and all the other planets are held in orbit around the Sun by the Sun's gravity. Earth also rotates on its axis once every 24 hours. It is night on Earth when we are on the side of Earth in shadow, facing away from the Sun. When Earth's rotation brings us out of shadow and back into Sunlight again, it is daytime. In the course of a year, regular climate changes occur regularly on Earth. These changes are called seasons and they occur because Earth's rotation axis does not stand upright relative to our planet's orbit but tilts at the angle of 23.5 degrees. Earth revolves around the Sun. When the side we live on is tilted down toward the Sun, we have summer because we receive lots of the Sun's heat. Six months later our part of Earth is tilted up away from the Sun at it is winter because we then receive less of the Sun's heat.
Internally, Earth consists of three parts: core, mantle, and crust. The Earth has a partly molten core of nickel-iron that is about 4,400 miles (7,000km) in diameter. Earth's mantle contains high-density basaltic rocks, rich in iron and magnesium. It extends from the top of the core almost to the surface. The mantle's top layers lie beneath the crust (the part we live upon). The thickness of the crust varies, being 5 to 6 miles (10km) thick in the ocean basins, but reaching a depth of 50 to 60 miles (100 km) under the continents.
Other factors in creating "our world" are the atmosphere and water in all its forms. Wind and rain work slowly and steadily, removing material from the land and returning it to the sea. By attacking the softest rocks and soil first, erosion shapes our landscape here on Earth. The air and the seas also serve to moderate Earth's climate, carrying heat into the arctic regions and bringing cool currents to the tropics.