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A natural starting point for astronomical distances is the average distance from the Earth to the Sun. As planets move in ellipses around the sun, the average distance is taken to be the semi-major axis of the earth's orbit. However the unit is only appropriate on interplanetary scales as the distances to other stars are so great as to render it too small a unit to be useful. Owing to parallax, the nearby star appears to shift position relative to the more distant stars. A parsec is the distance at which the observed parallax of the star is equal to 1 second of arc.