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Parsec
 
 
 
Standard:  astronomical units 
Quantity:  length 
Symbol:  pc 
Expressed in:  1 pc = 
SI units  3.0857×10^{16} m 
imperial & US units  1.9174×10^{13} mi 
other astronomical  2.0626×10^{5} AU 
units  3.26156 ly 
A parsec is a unit of distance used in astronomy. It is the distance that light travels in 3.26 years, or just under 31 trillion kilometres (about 19 trillion miles). This is from the definition: a parsec is the distance from the Sun to an object which has a parallax angle of one arcsecond.
Distances measured in fractions of a parsec usually involve objects within the same star system.
Surveying provides an everyday example of parallax. Go outside and look at a distant object. Hold your hand out with one finger raised and close one eye. Line up your finger so it covers the object from view. Then close that eye and open the other without moving your arm. Now, move your finger to block that eye's line of sight. The two positions your finger was at can be used to find the distance from the object. This is what a survey crew does to figure out land plot boundaries. The mathematical technique used in the calculation is trigonometry. A parsec is so large that if you could see that far that you would be looking in to the past. You are looking 3.26 years in the past because it takes 3.26 years for the light to come from a parsec away.
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