# The units of measurement that we use in astronomy

The distances in the universe are so big, that not even the biggest measures that we use in the International System, are useful to us so that we can express distances with numbers that are reasonably easy to handle. To make ourselves an idea of these enormous distances we have to appeal to new measures.

#### The space is very big

The distances in the universe are very big. For example, the average distance between the Earth and the Moon is 380 000 kilometres. The distance of the Earth and the Sun is 149,5 million kilometres. The average distance that separates us from Mars is about 225 million kilometres. The average distance to Jupiter is 780 million kilometres.

All these numbers are gigantic, and we even haven’t arrived at the edges of our solar system. The nearest star from us, apart from the Sun, is Proxima Centauri, and it is situated at 39.900.000.000.000 (39,9 billion) kilometres from us. These are so big distances that they are difficult to imagine. For this reason, we have to appeal for new measures.

#### Different measures for different distances

For this reason, we have different measures to refer to different distances. For example, normally, the distances inside the solar system are expressed in millions of kilometres. However, sometimes we talk about the distance in which an asteroid will pass near to our planet in terms of lunar distances. A lunar distance is 384 000 kilometres, which is the medium distance that separates us from our natural satellite.

When we go out from the Earth-Moon system, we also use another measure: the astronomical unit (AU). An AU is the distance that separates the Sun and the Earth: 149,9 million kilometres. In this way, we say that the Earth is at 1 AU from the Sun, and that Mars is at a distance of 1,53 AU. Inside the solar system, the AU serves us very well. We get around well with this measure. The Oort cloud is at a distance of 50 000 AU from the Sun, that to say, 50 000 times the distance between the Earth and the Sun.

However, we get out of the solar system we need to appeal to another distance as the astronomical unit one gets already short.

To talk about distances between stars, planets outside the solar system, or even of galaxies, we use another measure: the light year. A light-year is the distance that covers the light during a period of one year. The light moves at a speed of approximately 300 000 km/s, therefore, in one year it goes over a distance of, exactly, 9.460.730.472.580,8 kilometres (almost 9,5 billion kilometres). With this measure, we say that the distance that separates us from Proxima Centauri is 4,24 light-years.

The light-year is used in the cosmic scale indistinguishably in the big majority of scientific articles and texts. Even so, there is another unit that is also very used: the parsec.

In distance terms, a parsec is equal to 3,26 light-years. Using the parsec as the measuring unit, Proxima Centauri is at a distance of 1,3 parsecs. Under the same scale, our galaxy’s centre is situated at a distance of 7 600 parsecs (which equivalent to 28 000 light-years).

Moreover, when we refer to very distant galaxies, we introduce the kilo-parsec (1 000 parsecs, or 3 262 light-years), the mega-parsec (1 million parsecs, or 2,26 millions of light-years), and the gigaparsec (a thousand of millions of parsecs). With this scale, we would say that the Andromeda galaxy, which is situated at 2,5 million light-years, is at 0,7 mega-parsecs.

#### Gigantic numbers

In conclusion, the space is very big, and we need to appeal for equally big measures to attempt to express with comprehensible terms the enormous distances that separate us from the planets, stars and galaxies. Even in galaxies or galaxy cumulus very distant to us, some of the mentioned measures get short. The distances in the universe are enormous.

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