Advanced Features: Double Star Orbits
[Update: This will be a regular feature in V2.2, not an “advanced” feature]
Double (or multiple) star systems are not static. The primary and secondary stars typically move with respect to each other. These relative movements can be extremely small (i.e. they don’t appear to move over a period of decades or longer), or they might move relatively rapidly (with noticeable relative motion over a period of months or years). The double star catalogues, and in particular the Washington Double Star (WDS) catalogue, typically only show a single instance or snapshot of relative position (separation and position angle) for each object, which might be quite out-of-date (some even 100+ years out of date).
Many such star systems have been observed and the relative positions of the stars noted over an extended period, allowing the actual orbits of the stars to be estimated. A new feature in the Advanced version of AstroPlanner allows you to see those orbits and estimate the current relative positions of the stars.