The term "light year" occurs in many scientific articles, popular telecasts, textbooks and even in news from the world of science. However some people are sure that the light year is a certain unit of measure of time though actually advanced in years it is possible to measure also distance.
How many kilometers in a year
To realize the meaning of the concept "light year", at first it is necessary to remember a school course of physics, especially that its section which concerns the speed of light. So, the speed of light in a vacuum where it is not influenced by various factors, such as gravitational and magnetic fields, the weighed particles, refraction of the transparent environment and other, is 299,792.5 kilometers per second. It is necessary to understand that in this case light is meant as the electromagnetic waves perceived by human sight.
Less known units of measure of distance are light month, week, day, hour, minute and second.
Rather long time the speed of light was considered as infinite size, and the astronomer Olaf Remer in the middle of the 17th century became the first person who calculated the approximate speed of light beams in a vacuum. Of course, its data were very approximate, but the fact of determination of final value of speed is important. In 1970 the speed of light was determined with an accuracy of one meter per second. More exact results did not manage to achieve still as there were problems with a margin error of a meter standard.
Light year and other distances
As distances in space are huge, their measurement in habitual units would be irrational and inconvenient. Proceeding from these reasons, the special unit of measure – light year, that is distance which light passes for so-called Julian year (equal 365.25 days) was entered. Considering that every day comprises 86,400 seconds, it is possible to calculate that in a year the ray of light overcomes distance of several more than 9.4 trillion kilometers. This size seems huge, however, for example, the distance to the star of Proxima of the Centaur, next to Earth, makes 4.2 years, and diameter of the galaxy the Milky Way exceeds 100,000 light years, that is those visual observations which can be made now display the picture existing about one hundred thousands years ago.
The ray of light overcomes distance from Earth to the Moon approximately in a second, and here sunlight reaches our planet more than eight minutes.
In professional astrophysics the concept of light year is used seldom. Scientists mainly operate with such units as a parsec and astronomical unit. The parsec is a distance to the imagined point with which the radius of an orbit of Earth is visible at an angle in one angular second (1/3600 degrees). Average radius of an orbit, that is distance from Earth to the Sun, is called astronomical unit. The parsec is equal to about three light years or 30.8 trillion kilometers. Astronomical unit is approximately equal 149.6 million kilometers.