Diagram showing how astronomers measure the distances to nearby stars using trigonometric parallax. When you hold your finger up in front of your face and close one eye and then the other, its position relative to the background changes â a phenomenon known as parallax. It occurs because each eye sees the finger from a slightly different position. Similarly, the nearest stars seem to move relative to the more distant background stars, as the Earth orbits the Sun. When the Earth in on one side of its orbit, say in January, a nearby star will have a certain position relative to the background stars. But six months later, the star's location will change by a tiny amount, an angle known as the trigonometric parallax. Knowing this angle and the diameter of Earth's orbit (300 million km), astronomers can calculate the distances to these nearby stars with a fairly high accuracy.
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