EARTH

Apollo 17 image of Earth/NASAEarth is the third closest planet to our Sun at nearly 93 million miles.  It revolves around the Sun in slightly more than 365 days and rotates every 23.93 hours.  Earth is the only celestial body in our solar system with liquid water able to sustain life.  In fact, 70 percent of the Earth's surface is water, either in liquid form or ice.  

Atmosphere and Weather:   Earth is primarily an oxygen, nitrogen atmosphere.  Oxygen makes up 21 percent of the atmosphere, with nitrogen taking up nearly the rest.  No other planet in the solar system even comes close to having a 21 percent oxygen content.  The atmosphere is thick, extending upward around 100 miles, although becomes less dense rapidly above 20,000 feet.  The atmosphere, along with Earth's very strong magnetic field and ozone layer is able to protect life by reducing incoming ultraviolet radiation from the Sun and by vaporizing most meteors through intense heat as they enter our atmosphere.  

The Earth is characterized by complex weather systems carried by high winds aloft.  The winds aloft can exceed 150 mph, mainly north of the tropics in what is referred to as the jet stream.  Making the weather even more complex is the interaction of our oceans and other large bodies of water on short and long term events.  Clouds are made of water vapor and/or ice.  This balance of water and ice in the clouds is what is able to create precipitation.  Precipitation is formed primarily in two processes.  In the tropics, where ice in the clouds is non-existent or extremely limited, precipitation is formed by the coalescence of cloud water droplets.  After coalescence, the drops will finally become heavy enough to fall through the atmosphere and reach the surface.  The other main precipitation process is through the interaction of water vapor and ice in clouds north of the tropics.  In general, an ice particle has a lower saturation vapor pressure than water.  The sequence of events can literally "draw" the water droplet toward the ice particle where the two accrete.  Finally, the particle will become heavy enough to fall through the atmosphere as rain or hail.  

Diurnal temperature swings on Earth can range from very large in dry climates to minimal in humid regions.  Overall, the warmest recorded temperature on Earth is 135.9 degrees F. in Libya and the coldest recorded temperature is -128.6 degrees F. in Antarctica.  Although the greenhouse effect has received a lot of bad attention as of late, without it, life on Earth would cease.  The very small amount of carbon dioxide that is in the Earth's atmosphere produces just enough greenhouse effect to raise the Earth's temperature from -6 degrees F. to an average of 59 degrees F.  As one can see, without the greenhouse effect, water would freeze and Earth would become just another lifeless planet.  

QUICK FACTS
(Data is from NASA Goddard)

Average distance from Sun 93,000,000 miles
Perihelion 91,400,000 miles
Aphelion 94,500,000 miles
Sidereal Rotation 23 hours 57 minutes
Length of Day 24.0 hours
Sidereal Revolution 365.242 Earth days
Diameter at Equator 7,926 miles 
Tilt of axis 23.45 degrees
Moons 1
Atmosphere Nitrogen 78.1%, Oxygen 20.1%, Argon 0.93%, Carbon Dioxide 0.04%, other trace elements

DEFINITIONS:

Average distance from Sun:  Average distance from the center of a planet to the center of the Sun. 
Perihelion:  The point in a planet's orbit closest to the Sun.
Aphelion:  The point in a planet's orbit furthest from the Sun. 
Sidereal Rotation:  The time for a body to complete one rotation on its axis relative to the fixed stars such as our Sun.  
Length of Day:  The average time for the Sun to move from the Noon position in the sky at a point on the equator back to the same position.  
Sidereal Revolution:  The time it takes to make one complete revolution around the Sun.
Axis tilt:  Imagining that a body's orbital plane is perfectly horizontal, the axis tilt is the amount of tilt of the body's equator relative to the body's orbital plane.  


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