To ancient astronomers, Theta Eridani was brighter for 1,000 years-now we know why

There's a bit of a historical mystery surrounding the star Theta Eridani. Ptolemy in the second century A.D. and al-Sufi in A.D. 964 both recorded Theta Eridani as one of the 13 brightest stars in the sky. Hipparchus may have said the same. But there's a problem. For it to be one of the 13 brightest, it had to be much more luminous than it is today.