
© Idel Waisberg & Boaz Katz, arXiv (2026). DOI: https://doi.org/10.48550/arXiv.2606.30748Photo of the southern part of the Eridanus constellation as seen from Jacutinga, MG, Brazil on 2026-03-22 UTC 23:06 shows Theta (θ) Eridani.
For more than a century, astronomers have puzzled over a strange discrepancy in the historical record. Theta Eridani, a modest star in the constellation Eridanus, is the third-brightest star in its constellation, with a magnitude of 2.9. But ancient astronomers described it very differently. A new study, posted to the
arXiv preprint server on June 29, has proposed a plausible explanation for this mystery.
The 'forgotten' bright starAncient astronomers, including Hipparchus, Ptolemy and al-Sufi, all described Theta Eridani as
one of the brightest stars in the night sky. Hipparchus, writing around 129 B.C., called it "the brightest and preceding and southernmost of all in the River." Ptolemy's Almagest, compiled in 137 A.D., ranked it among the 13 brightest stars in the entire sky. And nearly 800 years later, in 964 A.D., the Persian astronomer al-Sufi still listed it as having magnitude 1.
Then, in 1603 A.D., one of the members of the Dutch expeditions, Frederick de Houtman, published a star catalog of the southern sky in which the star was assigned a magnitude of 3, consistent with its modern, ordinary brightness. Whether this sudden drop in brightness was real or a mistake in the historical record has been debated for centuries.
Interestingly, Theta Eridani, long thought to be a single star, is actually a system of three stars. Modern studies have shown that it consists of a close binary pair of stars, plus a third, more distant companion.
Comment: Space Daily adds: And from EarthSky: