Why the distance in space is measured not in kilometers, but in light years and parrseca?
Kilometer – too small unit for cosmic distances. Even from Earth to the Sun, almost 150 million km, and to the nearest star – proxims in the constellation of the Centaur (Centaurus) – 40 260 billion km. Write down the numbers: 40 260,000,000,000 km. Such long numbers are hard to compare. Much clearly expresses the distance to stars and galaxies in the duration of traveling their light to us.
Looking at the nebula of Andromeda (and it can be seen even with the naked eye on the Dark Country Sky), we are transferred to the era when there were no mammoths on Earth!
For example, from the Sun, the ray of light gets to the Earth for 500 seconds, or 8.3 minutes. And from proxy – for 4.2 years. True, it is easy to compare one school change and half of school life. From the center of our galaxy, the light gets to us for 25,000 years. When he tried on the road, Mammoths were still walking along the ground! And to the neighboring galaxy – Andromeda nebula – 2.5 million light years. Therefore, looking at the nebula of Andromeda (and it can be seen even with a naked eye on the dark countryside), we are transferred to the era when the mammoths on Earth have not yet been! As you can see, light years – quite a suitable scale for cosmic distances.
Parseci (PCs) are the units of the length of the same scale as light years: 1 parseca is equal to 3.26 light year. But for astronomical calculations they are more convenient. The distance to the not too far astronomer objects are measured by their visible angular displacement when the Earth moves in orbit. This shift is called pararallax. The further the object, the smaller his parallax. If parallax is 1 angular second, then the distance is 1 parse. This very word is just formed from Pararallax and "Secund". If parallax is 0.1 corner second, then the distance is 10 parses. For astronomers of the Dokomputer era, it was very convenient. Measured Pararallax, divided the unit to its value – and you can immediately write the distance in Parsek. It really is a good scale. The distance to the proxima of the Centaur (Centaurus), for example, 1.3 parses. Between the stars surrounding us, too, about 1 parses. Express such distances in kilometers are not very convenient, because 1 parsec is equal to 30,857 billion km.