Many systems in the Washington Double Star Catalog have shown significant relative motion since their discovery.
The Catalog of Rectilinear Elements provides linear fits for those systems whose motion does not appear to be Keplerian.
While a few of these may in fact be very long-period physical pairs whose orbital motion is not yet apparent, most are probably optical pairs (i.e., chance alignments of unrelated stars). These linear fits, then, just describe the relative proper motions between these pairs of stars.
The purposes of this catalog are threefold:
- The very well-defined motions of some of these systems means they may prove useful for scale calibration for imaging systems such as CCDs or photographic cameras
- These differential proper motions may allow us to improve upon proper motions of individual components
- Finally, these linear fits, especially in comparison with Hipparcos proper motions, may be useful in searches for submotions due to closer components