I'm sure you've seen it. It's impossible to miss. Partway up the western sky in evening twilight, it looks like a star but shines far more brightly than any other. Do you know what it is? It's the planet Venus, the most dazzling object in the heavens except for the Sun and Moon.
Venus has been gleaming in the west after sunset since the beginning of January, heralding the start of the International Year of Astronomy 2009. It's fitting that this planet should dominate the sky as our celebration of 400 years of telescopic astronomy gets under way, because Galileo Galilei's observations of Venus led to one of his most important discoveries.
Studying Venus in his telescope beginning in late 1610, Galileo noticed that the planet goes through a complete series of phases, just like the Moon. Although Nicolaus Copernicus claimed in 1543 that all the planets, including Earth, orbit the Sun, most scientists of Galileo's time still adhered to the ancient idea that the Sun, Moon, and planets all circle Earth. If that were true, then if Venus revolves between Earth and the Sun it can never show a fully illuminated disk. Alternatively, if Venus orbits beyond the Sun it can never show a thin crescent. Yet Galileo saw Venus go from crescent to full and back again. He realized that it's sometimes closer than the Sun and sometimes farther away — in other words, it orbits the Sun. Copernicus was right!
Or was he? Many people mistakenly think that Galileo's discovery of the phases of Venus clinched the heliocentric, or Sun-centered, cosmology and relegated geocentrism — the idea that everything orbits Earth — to the ash-heap of history. Yes, Galileo's observations proved that Venus orbits the Sun, but they left open the possibility that the outer planets orbit Earth. After all, Mars, Jupiter, and Saturn don't exhibit a full range of phases like the Moon and Venus (and Mercury). Despite Galileo's findings, many scientists favored either of two alternatives to the Copernican hypothesis: (1) Mercury and Venus go around the Sun but everything else goes around Earth, or (2) the Moon and Sun go around Earth but everything else goes around the Sun.
Galileo's other telescopic discoveries dealt body blows to geocentrism, but like his discovery of the phases of Venus, they weren't fatal. For example, his discovery of four moons circling Jupiter showed that neither the Earth nor the Sun is the only center of motion in the universe, and that a planet could move through space without losing its satellites. (One of the strongest objections to the Sun-centered cosmology was the idea that if Earth moved around the Sun, it would leave the Moon behind.) But Galileo never made any observations that proved Jupiter orbits the Sun.
It took the better part of another century, especially the work of Isaac Newton in the late 1600s, to finally put the nail in geocentrism's coffin. I suppose you could say Galileo's telescope was the hammer.
My friend and colleague Sean Walker of Sky & Telescope assembled the following composite showing Venus going through half an orbit, and half a complete set of phases, over several months. He shot the images in ultraviolet light, which reveals dark and light markings in the planet's clouds. To the human eye, Venus's clouds are essentially featureless and pure, brilliant white.
If you own a telescope or know someone who does, you can see the phases of Venus for yourself. As February gives way to March and Venus hurdles toward its "inferior conjunction" between Earth and the Sun, the planet becomes an ever larger and thinner crescent. It's one of the prettiest sights you can see in a small telescope, and I can't urge you strongly enough to take advantage of this opportunity to see it. By 100 Hours of Astronomy in early April, Venus will shine in the predawn sky. It'll be no less spectacular, just a lot less convenient unless you like getting out of bed before sunrise!
Blogs 
