I'm trying to map out local space using Astrosynthesis and have been looking up stellar data online. When I come to companion stars, rather than use the RA and Dec for the star - only its position at the time of observation - I use the orbital data relative to the primary. One of the measurements used is the Semi-major Axis, 'a.' Unfortunately, this measurement is not always given in a standard unit of length. In fact, most of the time it is given in " (seconds of arc) and, occasionally, in ' (minutes of arc). My problem is - what's the arc? I get that a minute is 1/60th and that a second is 1/3600th, but of what? What is the baseline that I use to determine the size of the semi-major axis? Thanx in advance for any help!