# Light year is how much?

Often, in a science fiction movie on a space theme, you can hear: Alpha Centauri is located at a distance of 4500 light years from Earth. Or: we have 200 parsecs left to fly. The question immediately arises: light year, how much is this? And is he equal to parsek?

## What is a light year?

According to the definition of the IAC (International Astronomical Union), a light year is equal to the distance that a photon of light travels in a vacuum for one Julian year (365.25 days in which 86,400 seconds are in SI) when it is not affected by the gravitational fields of the planets.

This definition has been true since 1984. Prior to this, the calculations used the value of the so-called tropical or solar year. The new value differs from the old by 0.002%. But, due to the fact that this unit is not used in high-precision calculations, the difference of 0.002% is not of practical importance.

## Numeric values of light year measurement

- In meters - 9 460 730 472 580 800 m, or 9.46 petameters.
- In astronomical units - 63,241.1 a.e. A.e.- This is the average distance between the Sun and the Earth.
- In parsecs - 0.306601 pc.

In popular publications, it is common practice to use related units of measurement that show the distance traveled by a photon of light per unit of time. One light-second is 299,792.458 km.

Knowing the distance between the Earth and the Sun with the Moon, one can calculate that a ray of light reaches the surface of the Moon in 1.3 seconds. And the astronomical unit is 500 seconds.