Our home planet Earth is the third planet out from the Sun and is the largest and densest of the four inner planets. Earth is also the first planet out from the Sun that has a Moon and is the only planet in our Solar System where liquid waters exists in large quantities on the surface. It is generally agreed that the existence of liquid water is the main reason why Earth is the only place in the known universe where life exists.

The structure of Earths interior consists of rock and metal arraigned if layers. At the center is the solid inner core which is composed of nickel and iron and is 1200 kilometers in diameter. Remarkably, the inner core of the Earth is actually hotter than the surface of the Sun. The next layer out is the outer core and it is compose of liquid nickel and iron. The mantle sits on top of the outer core and is made mostly of dense, solid silicate rock. The crust of Earth rests on top of the mantle and is thin, solid layer of mainly silicate rock.

Earth orbits the Sun at a distance of about 149.6 million kilometers and its axis is tilted 23.45 degrees away from the plane of Earths orbit. The result of this tilt is the change in the climatic seasons here on Earth with the Moons gravity providing the ocean tides. The Earth has an average diameter of approximately 12,742 kilometers with a slight bulge around the equator created by the centrifugal force caused by the Earths rotation on its axis. This bulge makes the Earth about 43 kilometers great in diameter at the equator than if you measured pole to pole. The highest point on Earth is Mount Everest at 8,848 meters above sea level but due to the equatorial bulge the summit of Mount Chimborazo in Ecuador is actually farther away from the Earths center.

The atmosphere surrounding planet Earth is composed of 77 percent nitrogen, 21 percent oxygen and 2 percent traces of other gases including argon and carbon dioxide. Earths atmosphere is divided into five layers starting with the troposphere which begins at sea level and rises up to an altitude of about 16 kilometers. The troposphere is where the weather patterns form and the temperature in the troposphere is relatively mild with a global average temperature at the surface of 15 degrees centigrade. At the top of the troposphere is the tropopause which is the boundary layer between the troposphere and the stratosphere.

The stratosphere starts at the tropopause and extends up to an altitude of about 50 kilometers. Unlike the troposphere which is warm at the bottom and gets cooler as you go up, the stratosphere is cool at the bottom and gets warmer higher up. Within the stratosphere is the ozone layer that absorbs ultraviolet radiation from the Sun. Without this ozone layer life would not be able to exist here on Earth. At the top of the stratosphere is the stratopause which has a temperature of about 3 degrees centigrade, just above the freezing point of water.

Above the stratopause is the mesosphere which starts at 50 kilometers up from the surface of the Earth to an altitude of 80 to 90 kilometers. The mesosphere is too high to be reached by aircraft and much of what we know about it was gained by launching sounding rockets up into it. The data from these rockets tells us that the temperature in the upper mesosphere can fall as low as -100 degrees centigrade. We also know that countless meteors burn up in the mesosphere every day and at night we see them as meteoroids. The mesosphere ends at the mesopause.

The thermosphere rests atop the mesopause and rises up to as much as 1000 kilometers above the surface of the Earth. The thermosphere is also where the ionosphere resides, which allows some radio waves to propagate far over the Earth by reflecting them back down. The thermosphere has a very low density and the International Space Station orbits right through it at an altitude of about 320 to 380 kilometers. Beyond the thermosphere is the exosphere that simply blends into outer space.