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Thanks to Newton's law of universal gravitation, a sun not only spins its planets round in their orbits, but is itself being spun in relatively minute circles. Astronomer Robert Noyes compares this phenomenon to a square dance in which "a big heavy guy swings a tiny partner. But what's really happening is that, to stay stable, they're both swinging around the center of gravity between the two of them." Our sun, for example, weighs a thousand times as much as Jupiter, which it swings around in its orbit. During that same 11-year period, "the sun is going around an orbit that's 1/1000 as big as Jupiter's orbit"--a small "wobble" that gives rise to a tiny shift in the wavelength of the sun's spectral lines. Earth, on the other hand, has only 1/300,000 the mass of the sun, "so the size of the sun's additional wobble due to Earth is extremely tiny--in fact, that wobble is deep within the sun itself," Noyes explains.
To see such small gravitational wobbles, Noyes and his colleagues designed and built a spectrograph that measures the wavelengths of a star's spectral lines--"and more importantly, their change"--with great precision, he says, referring to the Doppler shift in wavelength that reveals the speed at which a distance is increasing or decreasing. "If a star's speed toward us changes by 12 meters a second, which is the speed of the sun's wobble due to Jupiter's motion, we can measure it by a very tiny change in the wavelength or color of the light."