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.