Distances to stars comparatively near to our Sun can be measured using trigonometric parallax. This can only be done if the star is close enough to show an apparent shift in position relative to much farther background stars, as seen at upper right. This requires viewing the star on two occasions, when Earth is at opposite sides of the Sun (seen at lower left). The Hubble Space Telescope can then precisely measure the very small angular displacement of the star between observations. When the offset value is combined using geometry with the value for Earth's orbital diameter, a precise distance can be calculated. Land surveyors commonly use this triangulation technique.
