the change in position has been greatly exaggerated. Although he found a small change in its position over the course of the year, the position shift was different from what it would have been if parallax were the cause.įor clarity. However, Bradley found that the position of Eltanin did not change as predicted. The distance to star (Dss) is equal to the distance between the Earth and the Sun (Des) divided by the tangent of the parallax angle (A). However, it had not yet been accepted by the Catholic church and many thinkers outside the field of astronomy still believed the older geocentric theory Finding the parallax of a star would confirm that the heliocentric theory was correct and would also allow the distance to the star to be calculated. In Bradley’s time the heliocentric theory, in which the Earth and all the planets orbit the Sun, was generally accepted by astronomers. The expected shift in the position of a star during the year due to parallax James Bradley (1693 – 1762) – image from Wikimedia Commonsīradley was looking for a small shift in the position of the star, known as the parallax. In 1727 the English astronomer James Bradley had been taking measurements of the position of Eltanin, the brightest star in the constellation Draco. Observations were made to test a scientific theory, but unexpected results were found, which in turn led to new discoveries. It is an interesting case of the application of the scientific method. This post is a bit more technical than usual and discusses stellar aberration, the apparent movement of stars as a result of the motion of the Earth.
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