Relatively nearby objects beyond the solar system appear to shift position relative to more distant objects as the Earth moves from one side of the Sun to the other—a phenomenon called parallax. You can use parallax to determine the distance to stars, as long as these stellar objects are within a few dozen light years of Earth. (More distant objects in the sky do not change their position enough as the Earth orbits from one side of the Sun to the other.) First, measure the position of the star in the sky; then, measure it again in six months when the Earth is on the opposite side of its orbit. If the distance a and the angle ac are known (as seen in the diagram below), using trigonometry, c can be determined as a / cos (ac).