Parsecs

Parsec stands for Parallax Arc Second, and is a measure of astronomical distances approximately equal to 3.26163626 light years. The parsec is a useful unit of measurement because it directly translates a measurable stellar phenomenon into a distance measurement.

Parallax is the name given to the apparent movement of an object against its background as the observer's viewing angle changes.

Objects and distances in the sky can not be measured in centimeters or inches. Instead, distances in the sky are measured in terms of how much of the sky's arc they cover. For most objects, this is on the order of arcseconds — 1/3600 of a degree. The Sun, for example is about 1,900 arcseconds in diameter, and the Moon is only slightly smaller at 1,800 arcseconds. One arcsecond is just over one mile on the Moon as seen from Earth.

The closest star to the Sun, Proxima Centauri, has a measurable parallax of about 0.77 arcseconds, so it is about 1.3 parsecs, or 4.2 light years, away.

The idea of stellar parallax was first proposed by the Greek astronomer Hipparchus (190–120 BC), most famous for developing the apparent magnitude classification for the brightness of stars. The heliocentric and geocentric theories of the solar system were being debated at the time, and Hipparchus realized that if the Earth did indeed orbit around the Sun, there should be observable parallax in the stars, assuming the stars are not infinitely far away. Unfortunately, without the aid of telescopes, Hipparchus was not able to measure any parallax in the stars and concluded the geocentric model was correct. 

The unaided human eye has a resolution of about 60 arcseconds (opinions vary widely), far too large to detect stellar parallax. By comparison, the Hubble Space Telescope has a resolution of about 0.05 arcseconds (20 parsecs, or 65 light years) 

(to put that in perspective, there are about 1,000 stars within 50 light years, 133 of which are visible to the naked eye).

t was not until 1838 that the German astronomer Friedrich Wilhelm Bessel was the first person to be able to measure stellar parallax.