To measure the distance within the solar system was created astronomical unit - a measure of the distance, which is equal to the average distance between the Sun and Earth.However, even for the solar system, this unit does not appear to be the best that can be shown on the visual example.If we imagine that the center of a small table corresponds to the Sun and the astronomical unit to take up to 1 cm, to designate the Oort cloud - the "external border" of the solar system - will have to move away from the table at 0.5 km.

If the astronomical unit was not sufficiently large for even the solar system, the more other units needed to measure distances between stars and galaxies.

## Lightyear

Unit distances across
the universe was based on some absolute value.Such is the speed of light.The most accurate measurement of it was made in 1975 - the speed of light is 299,792,458 m / s or 1,079,252 848.8 km / h.
the unit of measurement was taken the distance that light, moving at a rate runs for the earth is not a leap year - 365 Earth days.This unit was called the light year.

Currently light-years away in most states in the popular science books and science fiction novels than in scientific works.Astronomers often use larger units - Parsec.

## Parsec and its derivatives

name "parsec" stands for "parallax arcsecond".Arcseconds - a unit of measurement of the angle: a circle divided into 360 degrees, degrees - 60 minutes, a minute - 60 seconds.Parallax is called a change of the observed position of the object depending on the location of the observer.According to the annual parallax of stars calculated the distance to them.If we imagine a right-angled triangle, one of the legs of which - half the Earth's orbit, and the hypotenuse - the distance between the Sun and other stars, the size of the angle in it - an annual parallax of a given star.

At a certain distance of an annual parallax is equal to 1 arc second, that it was taken away, and the unit of measurement called parsecs.The international designation of this unit - Pc, Russian - PC.

Parsec is 30,856,800,000,000 km or 3.2616 light years.However, for the cosmic scale, and it was not enough.Astronomers are derived units: kpc is 1000 PC, Mpc - 1 million PCs and gigaparsek - 1 billion PCs.