Finding the size of everything

How far away is the Moon? You cannot pace it out. The Greeks did it with geometry. Around 129 BC an astronomer called Hipparchus compared how an eclipse of the Sun looked from two towns a known distance apart, and worked out that the Moon is between 62 and 73 times as far away as the Earth’s centre is from its surface. The true figure is 60. This drawing is to scale. You could fit thirty Earths in a row into the gap, and light takes one and a quarter seconds to cross it.

The Sun was much harder. The trick, when it came, was to wait for the planet Venus to pass across the face of the Sun, and to time it from places far apart on the Earth. In 1769 expeditions sailed and trekked round the world to do so. One of them took James Cook to Tahiti. The answer came out at 153 million km, only about two per cent too big. This real photograph shows Venus as a black dot on the edge of the Sun in June 2012. It will not happen again until 2117.

With the size of the Earth’s path known, the stars came within reach. As the Earth goes from one side of the Sun to the other, a near star seems to shift a little against the far ones, just as your thumb jumps against the wall when you look at it with one eye and then the other. The shift is tiny. In 1838 Friedrich Bessel measured it at last for a faint star in the Swan. His answer put the star 10.4 light years away: about a hundred million million km.

That method works only for stars close by. For anything further a new idea was needed, and it came from Henrietta Leavitt, who measured stars on photographs at Harvard for 30 cents an hour. She studied a kind of star that grows brighter and dimmer in a steady beat, and in 1912 she showed that the slower the beat, the more light the star really gives out. Time the beat and you know how bright the star truly is. See how faint it looks and you know how far away it is.

In 1920 astronomers were still arguing over whether the Milky Way was the whole universe. Some thought the faint spirals seen in telescopes were small swirls of gas inside it. In 1923 Edwin Hubble found one of Leavitt’s beating stars in the largest spiral, Andromeda. It was far too faint to be inside the Milky Way. Andromeda was another galaxy, and so were all the other spirals. It is 2.5 million light years away. With that one star the universe grew beyond anything anyone had measured.

Look at how it went. Each step used the one before, and each made everything far bigger: the Sun 390 times further than the Moon, the first star 700,000 times further than the Sun, the first galaxy 200,000 times further than that star. The furthest things that can be seen are about 19,000 times further again. No one has ever been further than the Moon. All the rest was measured from here, by thinking.