Re:The Largest Black Hole in the Known Universe
by throwaway_yy2Di on Sunday June 15, 2014 @04:13PM
(#7896167)
(Incidentally that's what defines a "parsec" -- a unit of distance of about 3 light years. If you see a parallax of one arcsecond ("par-sec"), from opposite sides of the earth's orbit around the sun (2 AU apart), that something is 1 parsec away. If it's 10 parsecs away, the parallax will be 0.1 arcsec -- the more distant the object, the less it appears to shift. This works to about 0.001 arcsec of precision (distance limit of ~1,000 parsecs), and soon down to 20 microarcseconds (~50,000 parsecs).
https://en.wikipedia.org/wiki/Parsec
https://en.wikipedia.org/wiki/Hipparcos
https://en.wikipedia.org/wiki/Gaia_%28spacecraft%29
This is the lowest rung of the "cosmic distance ladder":
https://en.wikipedia.org/wiki/Cosmic_distance_ladder
https://terrytao.wordpress.com/2010/10/10/the-cosmic-distanc...
In case anyone's curious: The double-quote is an arc-second. When measuring angles, a single quote is an arc-minute, which is 1/60th of a degree. An arc-second is, of course, 1/60th of an arc-minute. Rifle accuracy is measured in minutes of arc, with 1' being about an inch diameter circle at 100 meters. Proxima Centauri has a parallax of about 0.8".
To give you an idea of how far away Proxima Centauri is, we can scale everything down. A light-year is about 63,240 AU. A mile is 63,360 inches. So if Terra was an inch from Sol, Proxima Centauri would be about 4 miles away. The parallax from Proxima Centauri is the same as if you looked at an object 4 miles away, then moved your head 2 inches laterally. That's 0.8 arc-seconds. That's the second-closest star to us.