Thursday, May 5, 2022


In everyday life we usually use the metre and all its multiples and submultiples, such as cm and km, as measurements of length. In astronomy, given the large distances and dimensions of the objects studied, these units of measurement are relatively little used. Think for example that the distance of Proxima Centauri (the closest star to the Sun) expressed in meters is 4e18 meters, which is equivalent to a 4 followed by 18 zeros! These large numbers are inconvenient to use and for this reason astronomers in the past have introduced three new units of measurement: the astronomical unit (AU), the light year (ly) and the parsec (pc). 

One or the other of these units is used depending on the context in which you are: for example for distances within the Solar System is used the astronomical unit, for interstellar distances is used the light year or the parsec, while for the distances between galaxies we prefer to use the megaparsec (Mpc, one million parsecs).

The astronomical unit is defined as the average distance that separates the Earth from the Sun and is equivalent to approximately 150 million km. Consequently our planet is 1 AU away from the Sun, while for example Mars moves on an orbit with an average radius of 1,52 AU and Neptune, the outermost planet in the Solar System, is 30 AU from the Sun.

Instead, the light year is defined as the distance travelled by light in a vacuum (in the absence of gravitational fields) in a year and is equivalent to 9 461 billion kilometers (more than 63 thousand AU). Since its definition is easy to understand, the light year is usually used in the field of popularization.

Finally, the parsec is a unit of measurement used mainly in science that is equivalent to 3.26 light years. The definition of parsec is based on the trigonometric parallax. To understand what we are talking about, let’s look at the drawing of the post. Consider the Earth’s orbit around the Sun and a point P at a certain distance from the Solar System. The parsec is defined as the distance of a star at point P that has an annual parallax of one arcsecond. This is equivalent to saying that a star located at a distance of 1 pc sees the semi-major axis of the Earth’s orbit subtended under an angle of one arcsecond. 

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