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When accurately recorded historical observations do not agree with what we what we see today it is often very rewarding to find out why. In modern telescopes we see Theta Eridani to a be pair separated by about 425 times our distance from the Sun orbiting a common center of gravity. Detailed observations of the spectrum of one of these show it to be itself a pair of stars orbiting each other every 4.1 days. One of this very close pair, as it is aging, dumps matter onto its companion causing it to brightened significantly. In the past this mass transfer rate was much higher than it is today and the naked eye brightness of Theta Eridani is fainter to us than it was long ago.
