  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.

Solar and Stellar Astrophysics