Giuseppe Piazzi and the Discovery of the Proper Motion of 61-Cygni

Fr. Giuseppe Piazzi
Fr. Giuseppe Piazzi
  • Article
  • 8 pages
  • Level: university

Fr. Giuseppe Piazza discovered the proper motion of the star 61 Cygni at the end of the eighteenth century. A star’s proper motion is its motion across the sky compared to other stars. The position of 61 Cygni, a star that is faint but visible to the naked eye, changes measurably over the course of a year. A star whose position changes like this must be a relatively nearby star. In this 1990 article in the Journal for the History of Astronomy, Georgia Fodera Serio of the Palermo Observatory discusses the story of Fr. Piazzi and his “flying star”, and writes on the importance of Piazzi’s discovery. Serio notes that Piazzi “abandoned the old paradigm that ‘brightness implies nearness'”. This was an important step for astronomers, who previously had supposed that those stars that appear brighter in our night sky are nearer to us. In the 1830s the German astronomer Friedrich Wilhelm Bessel would measure the distance to 61 Cygni using annual parallax, finding that it is about 10 light years away. This was the first direct measurement of the distance of any star other than the sun.

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