section_path: Angles in geography and astronomy
section_name: Angles in geography and astronomy
paragraph_text: Angles in geography and astronomy
  
  In geography, the location of any point on the Earth can be identified using a geographic coordinate system. This system specifies the latitude and longitude of any location in terms of angles subtended at the center of the Earth, using the equator and (usually) the Greenwich meridian as references.
  
  In astronomy, a given point on the celestial sphere (that is, the apparent position of an astronomical object) can be identified using any of several astronomical coordinate systems, where the references vary according to the particular system. Astronomers measure the angular separation of two stars by imagining two lines through the center of the Earth, each intersecting one of the stars. The angle between those lines and the angular separation between the two stars can be measured.
  
  In both geography and astronomy, a sighting direction can be specified in terms of a vertical angle such as altitude / elevation with respect to the horizon as well as the azimuth with respect to north.
  
  Astronomers also measure objects' apparent size as an angular diameter. For example, the full moon has an angular diameter of approximately 0.5° when viewed from Earth. One could say, "The Moon's diameter subtends an angle of half a degree." The small-angle formula can convert such an angular measurement into a distance/size ratio.
  
  Other astronomical approximations include:
  
  These measurements depend on the individual subject, and the above should be treated as rough rule of thumb approximations only.
  
  In astronomy, right ascension and declination are usually measured in angular units, expressed in terms of time, based on a 24-hour day.