Modern Physics

astronomical units

Astronomy uses the units of measurement according to its need. These units are called astronomical units.

Distance measurements are made in our daily lives according to our need. They are mainly expressed in centimeters, meters and kilometers. The distance between two cities, for example, is conveniently measured in kilometers. Imagine that, instead of saying that from city A to city B there are 150 km distance, we say that there are 15,000,000 cm. The number would be too big, wouldn’t it?

In the same way that we use measurement units in the most convenient way for us, Astronomy also uses measurement units according to its need. As the universe is immense, measurements made in kilometers generate, in most cases, very large numbers that are difficult to use. To resolve the issue, proper units were established for Astronomy, the so-called astronomical units.

The following are the three units of distance used in Astronomy:


The light year (al) is generally viewed as a unit of time, but the definition shows that this is a unit of distance. A light year is the distance traveled by light in a vacuum in one year. Knowing that the speed of light in a vacuum corresponds to 300,000 km/s and that a year is exactly 31,536,000 s, we can define the size of 1 light year from the average speed equation :

M = Δ s

Δs = V M . Δt

Δs = 300,000 km/s. 31,536,000 s

Δs ≈ 9.5 x 10 12 km ≈ 1 light year

This unit is generally used for distances from bodies that are outside the solar system. The table below shows the distance of some bodies in the universe from the Earth.


The so-called astronomical unit (au) is used for distances within the solar system and corresponds to the average distance from the Earth to the Sun. The definition of this unit had as parameters concepts of Newtonian gravitation and was determined from experiments and reference systems. However, in August 2012, the International Astronomical Union defined a more exact value for this measurement, thus 1 AU = 149,597,870,700 m.


Parallax is the apparent displacement of an object seen by two different observers. This displacement is proportional to the angle that exists between the lines that connect the observed object to the observers. One parsec corresponds to the distance referring to the parallax of 1 second.

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