Sander Pool wrote: > I noticed that when I have two stars, A and B and let CdC calculate the > distance and Position Angle (PA) between them the order in which I > select them matters. I would expect the PA from A to be plus the PA from > B to A to be 360 but it's not. One particular example is this one: > > From "GSC 4048-102" to "BSC HR860" > Separation: +00°30'51.0" PA:297° > Offset: -0h03m50.4s +00°13'55" > > and > > From "BSC HR860" to "GSC 4048-102" > Separation: +00°30'51.0" PA:116° > Offset: 0h03m50.4s -00°13'55" > > The distance is the same (thankfully :) but together the PAs don't make > 360. I'm probably not understanding PA properly. Can someone explain it > please or send me a link to an article that I should read? I've googled > the subject a bit and I'm not finding fault with my reasoning yet.
The two position angles don't always add to 360 degrees because you are measuring the arcs from due north on two different centers. When you draw the two different angles on two pieces of paper and put one arc on top of the other you will see that they overlap. If you subtract the smaller angle from 180 degrees and add the difference to the larger angle you will get 360 degrees allowing for round-off errors in the original calculations of the position angles. Bud
