In space of distance are so big that if to measure them by standard system units, numbers will turn out very inspiring. Light year – just that unit of length which allows to measure huge distances, applying at the same time the smaller number of figures.

## Light year

The light year is known to much of fantastic novels. In spite of the fact that its name is similar to a period of time year, the light year measures not time at all, and distance. This unit is intended for measurement of huge space distances.

Light year – stand-alone unit of length. This distance which in one year (365.25 days or 31,557,600 seconds) passes light in a vacuum.

Comparison of light year with calendar began to be applied after 1984. Before light year was called the distance passed by light in one tropical year. Duration of tropical year has no exact value as its calculations are connected with the angular speed of the Sun, and for it there are variations. For light year the average value was taken. The difference in calculations between the tropical light year and light year correlated to the Julian calendar makes 0.02 percent. And as this unit for high-precision measurements is not used, there is no practical difference between them. The light year as unit of measure of length is applied in scientifically popular literature. In astronomy there is other stand-alone unit for measurement of long distances – a parsec. Calculation of a parsec makes a start from the average radius of a terrestrial orbit. 1 parsec is equal 3.2616 light years.

## Calculations and distances

Calculation of light year is directly connected with the speed of light. For calculations in physics it usually undertakes equal 300,000,000 m/s. Exact value of the speed of light of 299,792,458 m/s. That is, 299,792,458 meters are only one light second! The distance to the Moon approximately equally in 384,400,000 meters, so of the surface of the moon will reach a light beam approximately in 1.28 seconds. Distance from the Sun to Earth 149,600,000,000. Therefore, the ray of sunlight gets to Earth slightly less than in 7 minutes. So, in a year of 31,557,600 seconds. Having increased this number by the distance equal to one light second, we will receive that one light year will be equal 9,460,730,472,580,800 meters.1 one million light years respectively are equal 9,460,730,472,580,800,000,000 meters. By approximate calculations of astronomers, diameter of our Galaxy about 100,000 light years. That is within our Galaxy there cannot be distances measured by millions of light years. Such numbers are applicable for measurement of distances between galaxies.

Andromeda's galaxy, next to Earth, is at distance 2.5 million light years.

Today the longest space distance from Earth which is possible for measuring is a distance to edge of the observed Universe. It makes about 45 billion light years.