Calculating the Cosmos by Ian Stewart

Author:Ian Stewart
Language: eng
Format: epub, azw3
Publisher: Basic Books
Published: 2016-09-29T00:00:00+00:00

Observed light curve of Delta Cephei.

Cepheids were the long-sought standard candle. Together with the associated yardstick – the formula describing how a star’s apparent brightness varies with distance – they allowed us to take another step up the cosmic distance ladder. Each step involved a mixture of observations, theory, and mathematical inference: numbers, geometry, statistics, optics, astrophysics. But the final step – a truly giant one – was yet to come.

12

Great Sky River

See yonder, lo, the Galaxy,

Which men call the Milky Way,

For it is white.

Geoffrey Chaucer, The House of Fame

IN ANCIENT TIMES there was no street lighting beyond the occasional torch or fire, and it was virtually impossible not to notice one amazing feature of the heavens. Today the only way to see it is to live in, or visit, a region where there’s little or no artificial lighting. Most of the night sky is dusted with bright pinpoints of stars, but across the whole thing runs a broad, irregular band of light, more like a river than a scatter of glowing points. Indeed, to the ancient Egyptians it was a river, the celestial analogue of the Nile. Today we still call it the Milky Way, a name that reflects its puzzling form. Astronomers call the cosmic structure that creates it the Galaxy, a word derived from the ancient Greek names galaxias (milky one) and kyklos galaktikos (milky circle).

It took millennia for astronomers to realise that this milky smear across the sky is, despite appearances, a gigantic band of stars, so distant that the eye can’t resolve them into individual points. This band is actually a lens-shaped disc, seen edge-on, and we’re inside it.

As astronomers surveyed the heavens with ever more powerful telescopes, they noticed other faint smudges, quite unlike stars. A few are visible to a keen eye: the tenth-century Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy as a small cloud, and in 964 he included the Large Magellanic Cloud in his Book of Fixed Stars. Originally, western astronomers called these faint, fuzzy wisps of light ‘nebulas’.

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