2016ApJ...822L..34S

KIC 8462852 is a completely ordinary F3 main-sequence star, except that the light curve from Kepler shows episodes of unique and inexplicable day-long dips with up to 20% dimming. Here, I provide a light curve of 1338 Johnson B-band magnitudes from 1890 to 1989 taken from archival photographic plates at Harvard. KIC 8462852 displays a secular dimming at an average rate of 0.164±0.013 mag per century. From the early-1890s to the late-1980s, KIC 8462852 faded by 0.193±0.030 mag. The decline is not an artifact because nearby check stars have closely flat light curves. This century-long dimming is unprecedented for any F-type main-sequence star. Thus, the Harvard light curve provides the first confirmation (past the several dips seen in the Kepler light curve alone) that KIC 8462852 has anything unusual. The century-long dimming and the day-long dips are both just extreme ends of a spectrum of timescales for unique dimming events. By Ockham's Razor, two such unique and similar effects are very likely produced by one physical mechanism. This one mechanism does not appear as any isolated catastrophic event in the last century, but rather must be some ongoing process with continuous effects. Within the context of dust-occultation models, the century-long dimming trend requires 10⁴-10⁷ times as much dust as for the deepest Kepler dip. Within the context of the comet-family idea, the century-long dimming trend requires an estimated 648,000 giant comets (each with 200 km diameter) all orchestrated to pass in front of the star within the last century.