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  Krishna Kalki December 24, 2013 at 8:49pm

  The sun is at the heart of the solar system. All of the bodies in the solar system — planets, asteroids, comets, etc. — revolve around it. The distance from Earth to the sun is called an astronomical unit, or AU, which is used to measure distances throughout the solar system. The AU has been defined as 149,597,870,700 meters (92,955,807 miles).
  
  Astronomers use the AU for measuring distances throughout the solar system. Jupiter, for example, is 5.2 AU from the sun. Neptune is 30.07 AU from the sun. On the outer edges of the solar system, the Oort Cloud, where comets are thought to originate, is 100,000 AU from the sun. The distance to the nearest star, Proxima Centauri, is about 250,000 AU. However, to measure longer distances, astronomers use light-years, or the distance that light travels in a single Earth year, which is equal to 63,239 AU. So Proxima Centauri is about 4.2 light-years away.
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  Krishna Kalki December 24, 2013 at 8:46pm

  The nearest that Mars has ever been to the Earth is 56 million km or about 34.8 million miles. On the opposite end of the scale, Mars and Earth can be a whopping 401 million km (249 million miles) apart when they are in opposition and both are at aphelion. The average distance between the two is 225 million km (140 million miles).
  
  We figure that any trip to mars would be undertaken at a point when the two were closest, so we've used that distance to make our calculations!
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  Krishna Kalki December 24, 2013 at 8:42pm

  The moon's orbit around Earth is elliptical. At perigee — its closest approach — the moon comes as close as 225,623 miles (363,104 kilometers). At apogee — the farthest away it gets — the moon is 252,088 miles (405,696 km) from Earth. On average, the distance from Earth to the moon is about 238,855 miles (384,400 km). However, the moon is moving away from Earth at a rate of about 1.5 inches (4 cm) per year.