[Enchilada]

Originally Posted by mozzie

when chasing double stars the p.a does seem to be the same as they are saying
is that because of the northen hem and insteed of doing our angle from the right i should be starting left any help would be greatful
peter

Your question you ask is not very clear. So below is some general details about position angles for you...

POSITION ANGLE (PA - forget the intervening dots) is actually defined as the observed angle of the primary through the secondary, as measured in the angle deviating from NORTH as it increases towards the EAST.

A 0 degrees position angle is celestial north, the 90 deg. PA is EAST, 180 deg is SOUTH, 270 deg is WEST through to 360 deg that is again NORTH.

(See attached Figure on Position Angle

It is independent of what hemisphere you are in, as the north direction is just the same anywhere in the world, (as is east.)

- North in the field can be found by physically moving the telescope northwards. The stars arriving in the field is north. (The movement is usually opposite in the eyepiece as the telescopic field in inverted. If you are using a star diagonal - the field may be also be a mirrored field, and the direction just seem as the reverse of this - but the stars still enter field the same way regardless of the orientation of the field.)

- East can be determined by letting the star drift across the field - where the stars arrive into the field is east.

Position angles of doubles do not favour any particular direction or some position angle.

For many double stars - well most they change little over the years. Most true binary stars do tend to show some orbital motion, but those that exceed periods of several centuries but these will show just minor differences or changes in PA over even several decades. Others may change rapidly over shorter periods. I.e. Alpha Centauri is changing rather rapidly at the moment. Most pairs are are less than 5 degrees over many decades. Some of the differences might be due to earlier measures in say the last century - though I doubt it. (If I can recall, the values quoted are from the Washington Double Star Catalogue 2001 (WDS01), and for most of the wider pairs, they give the Hipparcos measures of 1991.)

Select some easy bright double stars to try. I.e. Acrux, Gamma Crucis, Mu Crucis. Find north and east as suggested above, and see if the PA's roughly matches Sissy's book. Sadly, Stephen Saber's Double Star List in the Ice in Space section, "Projects, Articles, DIY Guides and How-To's" - 110 Southern Sky Doubles does not give any PA's at all - very sad, because it only makes identification of similar pairs sometimes difficult - especially if you only want to look at them.

Note position angle is influenced by the Precession of the Equinoxes, so all values must refer to a certain epoch. Ie. Epoch 1950.0 or 2000.0. It is important to note that these quoted values are easily converted, if necessary, which often applies when using older star or double star catalogues. If you are using Sissy's book, this does not really apply.

Small suggestion

Perhaps you should also just download some planetarium software - like free Stellarium ( http://www.stellarium.org/ ) or even the current trial version of AstroPlanner 1.6.2 ( http://www.ilangainc.com/astroplanner/ ), which has versions that runs on both Mac and PC.

AstroPlanner is probably better, as it has an astrometric tool allowing you to draw a line between two points and measure its PA and separation directly. To measure angles, either holding down the Option key (Macintosh) or Alt key (Windows) and put the mouse arrow on the object in question then dragging the mouse cursor to the location you are measuring. A small box displays the measured PA and separation for you, whilst the field shows north, south, east, and west.