Passage: Eta Carinae was brighter still on January 2, 1838, equivalent to Alpha Centauri, before fading slightly over the following three months. Herschel did not observe the star after this, but received correspondence from the Reverend W.S. Mackay in Calcutta, who wrote in 1843, "To my great surprise I observed this March last (1843), that the star Eta Argus had become a star of the first magnitude fully as bright as Canopus, and in colour and size very like Arcturus." Observations at the Cape of Good Hope indicated it peaked in brightness, surpassing Canopus, over March 11 to 14, 1843 before beginning to fade, then brightened to between the brightness of Alpha Centauri and Canopus between March 24 and 28 before fading once again. For much of 1844 the brightness was midway between Alpha Centauri and Beta Centauri, around magnitude +0.2, before brightening again at the end of the year. At its brightest in 1843 it likely reached an apparent magnitude of −0.8, then −1.0 in 1845. The peaks in 1827, 1838, and 1843 are likely to have occurred at the periastron passage—the point the two stars are closest together—of the Binary star orbit. From 1845 to 1856, the brightness decreased by around 0.1 magnitudes per year, but with possible rapid and large fluctuations.
How many days did Eta Carinae surpass Canopus in brightness? How many years apart was the first peak of brightness from the very last peak of brightness? How many years did the brightness decrease by around 0.1 magnitudes per year? Which lasted less, when Eta Carinae peaked in brightness or when it brightened to between the brightness of Alpha Centauri and Canopus?