Posted 29 May 2013 - 08:06 PM
The WDS lists both the first measure of a double star, and the most recent. That appears to be where you've got confused. Porrima is a binary that's been changing quickly in recent years, so the numbers depend a lot on the date. Porrima was at 4.0" separation in 1980, 0.4" (!!) in 2004, and is now widening again.
The first good measure of it was by FGW Struve back in the early 1800s, and since then it's gone through more than a complete orbital revolution - the orbital period is listed at 169 years. And the orbit, as we see it from Earth, gives a huge variation in the separation of the stars.
The 2012 measure in the WDS is a separation of 1.7 arcseconds in PA 013 - just east of north. Because the pair are in the faster-moving part of their mutual orbit the numbers are changing quickly, and if I remember correctly the separation is now close to 2.0".
With this double as with others in the WDS, the summary line gives two time-slices - two dates of measures - which don't always provide a clear picture of what's happening. If you want more information on Porrima, the 6th Orbital Catalog, available on the USNO website along with the current WDS and other lists, has a diagram showing the measures superimposed on the calculated orbit - click on "P" on the far right of the data lines in the Orbit Catalog. There's also an ephemeris - presently, 2011-2015 - under "E" beside that.