PREDICTING THEαCOMAE BERENICES TIME OF ECLIPSE: HOW 3 AMBIGUOUS MEASUREMENTS OUT OF 609 CAUSED A 26 YEAR BINARY’S ECLIPSE TO BE MISSED

Abstract

The dwarf stars in the 26 year period binary alpha Com were predicted to eclipse each other in early 2015. That prediction was based on an orbit model made with over 600 astrometric observations using micrometers, speckle interferometry, and long baseline optical interferometry. Unfortunately, it has been realized recently that the position angle measurements for three of the observations from ~100 years ago were in error by 180 degrees, which skewed the orbital fit. The eclipse was likely 2 months earlier than predicted, at which point the system was low on the horizon at sunrise.