Mentor

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TL;DR

Theta Eridani is magnitude 2.9 today, hardly remarkable. Multiple ancient astronomers described it as one of the brightest stars. Recent research found a possible explanation of the discrepancy.

Hipparchus (129 BC), Ptolemy (137 AD) and al-Sufi (964) all described Theta Eridani as one of the brightest stars in the sky. Today it is still visible to the naked eye but unremarkable at magnitude 2.9. A new publication explains the larger historic brightness as mass transfer between two stars in the system, leading to a temporary brightness increase for something like 1000 years:
The forgotten bright star: Theta Eridani as a millenary stellar transient observed by Hipparchus, Ptolemy and al-Sufi
Article discussing the publication

This shows the long-term value of surveys - even if they were done with the naked eye! Many interesting effects only show up if you can compare systems at different times.