Ibn Rushd travelled to Morocco to see a star hidden from Córdoba
In 1153 the philosopher Ibn Rushd journeyed from Córdoba to Marrakesh to watch Canopus, a brilliant star that never climbed above his home horizon. Seeing it appear as he moved south, he used the change to argue, following Aristotle, that Earth must be a fairly small sphere. The same star now helps steer spacecraft.
Canopus is the second-brightest star in the night sky, beaten only by Sirius. Yet it lies around 310 light years away, far beyond Sirius, and shines so brightly because it is a giant: more than 10,000 times as luminous as the Sun, nine to ten times as massive and swollen to 71 times its radius. Having used up the hydrogen in its core, it is now burning helium, in a stage known as the blue loop. Though sometimes described as yellowish, it is essentially white; any tint seen from northern sites comes from viewing it through thick air near the horizon.
Its low southern position shaped its lore. Theoretically it cannot be seen north of about 37 degrees latitude, just south of Athens, San Francisco and Seoul, so for Middle Eastern observers it appeared only fleetingly, and its Arabic name, Suhayl, came to imply rarity. The Bedouin of the Negev and Sinai navigated by it and by Polaris, contrasting the fickle, vanishing southern star with the ever-present northern one. The southeastern wall of the Kaaba in Mecca aligns with its rising point. Its Greek name honours the pilot of Menelaus's ship; in India it was named after the sage Agastya, and Chinese astronomers called it the Old Man of the South Pole.
Canopus earns its brightness through sheer power rather than proximity, which gives it staying power. Over the past four million years it has been the brightest star in our sky during three separate epochs, displaced only when another star swings temporarily close. Sirius took over about 90,000 years ago, but in roughly 480,000 years Canopus should reclaim the title and hold it for about half a million years.
Because it is bright and lies far from the plane of the planets, Canopus is a favourite reference for space navigation. Many spacecraft carry a dedicated Canopus star tracker, and in 1964 Mariner 4 became the first probe to stabilise itself using a star, locking onto Canopus after first finding the Sun.
Source: Canopus
