Cassiopeia

Cassiopeia's five bright stars form the sky's most recognisable W, but the constellation's most consequential contribution to astronomy was a star that appeared where no star had been seen before, and then vanished again within a couple of years.

In November 1572, the Danish astronomer Tycho Brahe observed a new point of light in Cassiopeia bright enough to be seen in daylight, brighter for a time than Venus, where his own careful star charts showed nothing at all. Tycho measured its position repeatedly over months and could detect no parallax shift against the background stars, the test of the day for distinguishing something nearby in the atmosphere from something genuinely among the fixed stars, and concluded it lay far beyond the Moon, in the region Aristotelian cosmology held to be permanent and unchanging.

The star, now known as Tycho's Supernova, SN 1572, faded from naked-eye visibility within about eighteen months, but the damage to the old cosmology was already done: a change in the supposedly changeless heavens had been measured, not merely observed, by an astronomer whose reputation for precision made the result hard to dismiss.

The queen's throne, in Greek myth a punishment for vanity, briefly held the brightest new light seen in Europe's sky in living memory, and helped end an idea about the sky that had stood for close to two thousand years.

Cassiopeia is a prominent northern constellation, one of Ptolemy's forty-eight, representing the vain queen of the Perseus myth, mother of Andromeda, set in the sky seated on her throne. Her five bright stars form an unmistakable W or M shape, opposite the Big Dipper across the pole, and because she is circumpolar from northern latitudes she never sets but wheels endlessly around Polaris. In the myth her boast that she or her daughter surpassed the sea-nymphs in beauty brought the wrath that sent the monster against Andromeda, and she was placed among the stars to circle the pole, at times hanging upside down as a humbling. In astronomical tradition the constellation is a pattern of stars rather than a zodiac sign. These are the features of Cassiopeia as the classical and modern traditions describe it, distinct from any astrological reading.

Cassiopeia, like her husband Cepheus, borders the early seventeenth-century Camelopardalis, one of several constellations introduced in this period specifically to give a name to the large, faint stretches of circumpolar sky between the ancient, brighter figures that the Ptolemaic tradition had judged not worth outlining on its own. Cassiopeia's distinctive W shape has made it, alongside the Big Dipper on the opposite side of the pole, one of the most universally taught naked-eye landmarks for beginning stargazers learning to navigate the northern circumpolar sky.