|Artist's Rendition of Double Star System Albireo in Cygnus|
Due to the almost endless selections of these star systems and the fact that the availability of these specimens can be seen on almost any night of the year makes this type of observing possible on a bright night with small telescopes. Some double stars are known as "fixed" double stars meaning their components have not moved since their discovery. However, the majority of these star systems do show orbital motion over periods of time. The explanation is that the secondary star appears at a different position as it orbits the primary star. Some stars show orbital motion over a span of years while others may take decades to change their relative positions.
Double star observers use a unit of measurement called a position angle. Determining position angles of double stars is a project that any amateur astronomer or student could undertake with any size telescope. The position angle measured between the two stars is from 0 to 360 degrees in an easterly direction from the north. The difficult part of determining position angles is knowing exactly what orientation your telescope's field is. For example, a reflecting telescope has a field of view south at the top and east is to the right. An easy way to remember position angles for this field is to picture an imaginary clock face with the primary star at the center...
North (0°) is at 6 o'clock, East (90°) at 3 o'clock, South (180°) at 12 O'clock, and West (270°) at 9 o'clock. For example, the position angle for the double Albireo, a double star in the constellation Cygnus the swan is 55° with a separation of 34.6".
An article posted on Sky and Telescope Magazine's website has a list of the best double stars to observe as well as more information on observing double stars.