EarthThe Blue Marble: This photo is of Africa, Antarctica, and the Arabian Peninsula as taken en route to the Moon by Apollo 17 on December 7, 1972.
View of the Earth as seen by the Apollo 17 crew traveling toward the moon. This translunar coast photograph extends from the Mediterranean Sea area to the Antarctica south polar ice cap. This is the first time the Apollo trajectory made it possible to photograph the south polar ice cap. Note the heavy cloud cover in the Southern Hemisphere. Almost the entire coastline of Africa is clearly visible. The Arabian Peninsula can be seen at the northeastern edge of Africa. The large island off the coast of Africa is Madagascar. The Asian mainland is on the horizon toward the northeast.

Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center. File Name AS17-148-22727 

Earth is the third planet from the Sun and is the largest of the four solar terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity and the strongest magnetic field.
Home to millions of species, Earth for now, is the only place in the universe known to harbor life.
In one orbit around the sun, Earth rotates about 366 times around its axis. This length of time is a sidereal year, which is equal to 365 solar days. The number of solar days is one less than the number of sidereal days because the orbital motion of the Earth around the Sun results in one additional rotation of the planet around its axis.
Earth's axis of rotation is tilted, producing seasonal variations on the planet's surface with a period of one tropical year.  Scientists have been able to reconstruct detailed information about the planet's past. As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent.
Earth's biosphere has significantly altered its atmosphere. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth's include transporting water vapor, providing useful gases, causing small meteors to burn up before they hit the surface.
 Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation.
Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. Each of these layers has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere.
The magnetosphere is formed by Earth's magnetic field, wich is generated within Earth's molten outer core region. Here heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field, which can deflect particles in the solar wind.
An important part of the atmosphere for life on Earth is the ozone layer, a component of the stratosphere that partially shields the surface from ultraviolet light. The Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.
Earth orbits the Sun at an average distance of about 150 million kilometers. Earth's only satellite is the Moon, the Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth's.
The gravitational attraction between the Earth and Moon cause tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet.
Viewed from Earth, the Moon's size looks just the same as the size of the Sun. The size of these two bodies match because the Sun's diameter is about 400 times as large as the Moon's and it is also 400 times more distant. This makes total and annular eclipses possible.

Earth: Facts & Figures  
Discovered By
  Known by the Ancients
Date of Discovery
Average Distance from the Sun
  Metric: 149,597,890 km
English: 92,955,820 miles
Scientific Notation: 1.4959789 x 108 km (1.000 A.U.) 
Perihelion (closest)
  Metric: 147,100,000 km
English: 91,400,000 miles
Scientific Notation: 1.471 x 108 km (0.983 A.U.) 
Aphelion (farthest)
  Metric: 152,100,000 km
English: 94,500,000 miles
Scientific Notation: 1.521 x 108 km (1.017 A.U.) 
Equatorial Radius
  Metric: 6,378.14 km
English: 3,963.19 miles
Scientific Notation: 6.37814 x 103 km
By Comparison: 1 x Earth's
Equatorial Circumference
  Metric: 40,075 km
English: 24,901 miles
Scientific Notation: 4.0075 x 104 km
  Metric: 1,083,200,000,000 km3
English: 259,900,000 mi3
Scientific Notation: 1.0832 x 1012 km3
By Comparison: 1 x Earth's
  Metric: 5,973,700,000,000,000,000,000,000 kg
Scientific Notation: 5.9737 x 1024 kg
  Metric: 5.515 g/cm3
Surface Area
  Metric: 510,065,700 km2
English: 196,937,500 square miles
Scientific Notation: 5.100657 x 108 km2
Equatorial Surface Gravity
  Metric: 9.766 m/s2
English: 32.041 ft/s2
Escape Velocity
  Metric: 40,248 km/h
English: 25,009 mph
Scientific Notation: 11,180 m/s
Sidereal Rotation Period (Length of Day)
  0.99726968 Earth days 
23.934 hours 
Sidereal Orbit Period (Length of Year)
  1.0000174 Earth years 
365.24 Earth days 
Mean Orbit Velocity
  Metric: 107,229 km/h
English: 66,629 mph
Scientific Notation: 29,785.9 m/s
Orbital Eccentricity
Orbital Inclination to Ecliptic
  0.00005 degrees
Equatorial Inclination to Orbit
  23.45 degrees
Orbital Circumference
  Metric: 924,375,700 km
English: 574,380,400 miles
Scientific Notation: 9.243757 x 108 km
Minimum/Maximum Surface Temperature
  Metric: -88/58 (min/max) °C
English: -126/136 (min/max) °F
Scientific Notation: 185/331 (min/max) K
Atmospheric Constituents
  Nitrogen, Oxygen
Scientific Notation: N2, O2
By Comparison: N2 is 80% of Earth's air and is a crucial element in DNA.

Earth's Moon: Facts & Figures  
Discovered By
  Known by the Ancients
Date of Discovery
Average Distance from Earth
  Metric: 384,400 km
English: 238,855 miles
Scientific Notation: 3.84400 x 105 km (0.00257 A.U.) 
By Comparison: 0.00257 x Earth's Distance from the Sun
Perigee (closest)
  Metric: 363,300 km
English: 225,700 miles
Scientific Notation: 3.633 x 105 km (0.00243 A.U.) 
By Comparison: 0.00247 x Earth's Distance from the Sun
Apogee (farthest)
  Metric: 405,500 km
English: 252,000 miles
Scientific Notation: 4.055 x 105 km (0.00271 A.U.) 
By Comparison: 0.00267 x Earth's Distance from the Sun
Equatorial Radius
  Metric: 1737.4 km
English: 1079.6 miles
Scientific Notation: 1.734 x 103 km
By Comparison: 0.2724 x Earth
Equatorial Circumference
  Metric: 10,916 km
English: 6,783 miles
Scientific Notation: 1.0916 x 104 km
  Metric: 21,970,000,000 km3
Scientific Notation: 2.197 x 1010 km3
By Comparison: 0.020 x Earth
  Metric: 73,483,000,000,000,000,000,000 kg
Scientific Notation: 7.3483 x 1022 kg
By Comparison: 0.0123 x Earth
  Metric: 3.341 g/cm3
By Comparison: 0.606 x Earth
Surface Area
  Metric: 37,932,330 km2
English: 14,645,750 square miles
Scientific Notation: 3.793233 x 107 km2
By Comparison: 0.074 x Earth
Equatorial Surface Gravity
  Metric: 1.622 m/s2
English: 5.322 ft/s2
By Comparison: 0.166 x Earth
Escape Velocity
  Metric: 8,568 km/h
English: 5,324 mph
Scientific Notation: 2,380 m/s
By Comparison: 0.213 x Earth
Sidereal Rotation Period (Length of Day)
  27.321661 Earth days 
655.72 hours 
By Comparison: Synchronous With Earth
Sidereal Orbit Period (Length of Year)
  0.075 Earth years 
27.321661 Earth days 
By Comparison: Orbit Period = Rotation Period
Mean Orbit Velocity
  Metric: 3,682.8 km/h
English: 2,288.4 mph
Scientific Notation: 1,023 m/s
By Comparison: 0.034 x Earth
Orbital Eccentricity
By Comparison: 3.285 x Earth
Orbital Inclination to Ecliptic
  5.145 degrees
By Comparison: Oscillates roughly 0.15 degrees in 173 days.
Equatorial Inclination to Orbit
  6.68 degrees
Orbital Circumference
  Metric: 2,290,000 km
English: 1,423,000 miles
Scientific Notation: 2.290 x 106 km
Minimum/Maximum Surface Temperature
  Metric: -233/123 °C
English: -387/253 °F
Scientific Notation: 40/396 K

 Additional Information:

Because of the gravitational pull of the Sun, the extreme ranges of the Moon from the Earth are from 356,400 km to 406,700 km. 

A Planet


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