Distance to Sirius

Sirius dominates the night sky at apparent magnitude −1.46, and this distance is the reason: at 8.6 light-years it is the seventh-nearest stellar system, and its true luminosity of 25 Suns — respectable, not remarkable — is flattered by the inverse-square law. Put Rigel at Sirius's distance and it would cast shadows; put Sirius at Rigel's and it would vanish among thousands. Distance is the first correction every stellar comparison must make, and Sirius is its textbook case.

The star carries deep history: its heliacal rising fixed the Egyptian calendar and foretold the Nile flood, and in 1718 Halley compared its position against Ptolemy's catalogue and discovered that the "fixed" stars move — proper motion, revealed over sixteen centuries of drift. Huygens even estimated its distance around 1698 by comparing its light to the Sun's through a pinhole, arriving at 27 664 au: twenty times short, yet the first defensible measure of how far away a star is.