measurements of the Martian orbit taken by the Danish astronomer Tycho Brahe led Kepler to conclude in 1609 that the planets follow elliptical paths around the Sun. At approximately that time, images from the first astronomical telescope inspired Galileo to propose that the planets are worlds in their own right and that the stars are distant suns. These findings, in turn, led to the Newtonian portrait of a vast, possibly infinite, universe—home to myriad celestial objects interacting with one another according to the law of gravity.

Telescopes became larger and larger, revealing deeper layers of cosmic order. As they demonstrated, in the race across the celestial plains, stars are hardly lone rangers. Rather, they ride like horses on grand merry-go-rounds called galaxies. Galaxies belong to clusters— assembled, in turn, into even greater superclusters. In 1929, Edwin Hubble, using a colossal device on Mount Wilson in California, discovered that all distant galaxies are receding from each other. This finding led to the standard Big Bang model of an expanding universe—the crown jewel of 20th-century cosmology.

Just as it was enhanced observations that led science to abandon the Ptolemaic model and usher in the modern age, it is the dramatically improved equipment and techniques that have resulted in a rethinking of the standard cosmological approach. In the 1990s and early 2000s astronomy leapt above the clouds with extraordinary new orbiting instruments. Circling high above Earth at distances ranging from hundreds to hundreds of thousands of miles, these telescopic satellites have spanned the spectrum with their light-gathering power. Joining the Hubble Space Telescope, equipped to collect optical light, are infrared instruments, X-ray probes, and several microwave detectors—including the Cosmic Background Explorer and, most recently, the Wilkinson Microwave Anisotropy Probe (WMAP). WMAP has yielded the most precise estimate to date for the age of the universe: 13.7 billion years.

Space-based imaging has been accompanied by other astronomical breakthroughs. Digital cameras, able to absorb and record every single photon (particle of light) streaming down from space, have



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