As a planet orbits its star, it appears at first glance that gravity is only pulling on the planet, while the star remains stationary. However, the planet is also producing a gravitational field which pulls on the star as well, causing the star to 'wobble' slightly as the planet orbits. This small wobble is detectable by telescopes from Earth as a shift in the star's colour, due to the Doppler Effect. As the star wobbles, at certain times it is pulled towards the Earth, and other times it is pulled away. When it is being pulled toward the earth, its light waves stack, causing a fequency shift in its emitted light towards the blue end of the spectrum (i.e. the light wavelengths shorten). As the planet is pulled away from the Earth, the lightwaves are spread out, causing a shift towards the red (i.e. the light wavelengths lengthen). By measuring these spectral fluctuations, astronomers can detect planets. This method is best for detecting large planets that orbit close to their stars.