Cosmic distances are difficult to measure in ordinary meters and kilometers, so astronomers use other physical units in their work. One of them is called a light year.


Many sci-fi fans are familiar with this concept, as it appears frequently in films and books. But not everyone knows what a light year is equal to, and some even think that it is similar to the usual annual calculation of time.

What is a light year?

In fact, the light year is not a unit of time, as one might assume, but a unit of length used in astronomy. It is understood as the distance traveled by light in one year.

It is commonly used in astronomy textbooks or popular science fiction to determine lengths within the solar system. For more accurate mathematical calculations or measurements of distances in the Universe, another unit is taken as a basis - .

The appearance of the light year in astronomy was associated with the development of stellar sciences and the need to use parameters comparable to the scale of space. The concept was introduced a few years after the first successful measurement of the distance from the Sun to the star 61 Cygni in 1838.


Originally, a light year was called the distance traveled by light in one tropical year, that is, for a period of time equal to the full cycle of the seasons. However, since 1984, the Julian year (365.25 days) has been taken as the basis, as a result of which the measurements have become more accurate.

How is the speed of light determined?

To calculate the light year, the researchers had to first determine the speed of light. Once upon a time, astronomers believed that the propagation of rays in space occurs instantly, but in the 17th century, such a conclusion began to be questioned.

The first attempts to make calculations were made by Galileo Gallilei, who decided to calculate the time during which light travels 8 km. His research was unsuccessful. James Bradley managed to calculate the approximate value in 1728, who determined the value of the speed at 301 thousand km / s.

What is the speed of light?

Despite the fact that Bradley made fairly accurate calculations, it was only in the 20th century that they could determine the exact speed using modern laser technology. Perfect equipment made it possible to make calculations adjusted for the refractive index of the rays, as a result of which this value was 299,792.458 kilometers per second.


Astronomers operate with these figures to this day. In the future, simple calculations helped to accurately determine the time that the rays need to fly around the orbit of the globe without being affected by gravitational fields.

Although the speed of light is not comparable to earthly distances, its use in calculations is explained by the fact that people are used to thinking in "earthly" categories.

What is a light year?

If we take into account that a light second is equal to 299,792,458 meters, it is easy to calculate that light travels 17,987,547,480 meters in a minute. As a rule, astrophysicists use these data to measure distances within planetary systems.

To study celestial bodies on the scale of the Universe, it is much more convenient to take as a basis a light year, which is equal to 9.460 trillion kilometers or 0.306 parsecs. Observation of cosmic bodies is the only case when a person can see the past with his own eyes.

It takes many years for the light emitted by some distant star to reach Earth. For this reason, when observing space objects, you see them not as they are at the moment, but as they were at the moment of light emission.

Examples of distances in light years

Thanks to the ability to calculate the speed of the rays, astronomers were able to calculate the distance in light years to many celestial bodies. So, the distance from our planet to the Moon is 1.3 light seconds, to Proxima Centauri - 4.2 light years, to the Andromeda Nebula - 2.5 million light years.


The distance between the Sun and the center of our galaxy is about 26 thousand light years, and between the Sun and the planet Pluto - 5 light hours.

Off-system unit of length used in astronomy; 1 S. g. is equal to the distance traveled by light in 1 year. 1 S. g. \u003d 0.3068 parsec \u003d 9.4605 1015 m. Physical encyclopedic dictionary. Moscow: Soviet Encyclopedia. Editor-in-chief A. M. Prokhorov. ... ... Physical Encyclopedia

LIGHT YEAR, a unit of astronomical distance, equal to the distance that light travels in open space or in VACUUM in one tropical year. One light year is equal to 9.46071012 km... Scientific and technical encyclopedic dictionary

LIGHTYEAR, a unit of length used in astronomy: the path traveled by light in 1 year, i.e. 9.466?1012 km. The distance to the nearest star (Proxima Centauri) is approximately 4.3 light years. The most distant stars in the Galaxy are located on ... ... Modern Encyclopedia

Unit of interstellar distances; the path that light travels in a year, i.e. 9.46 × 1012 km ... Big Encyclopedic Dictionary

Light year- LIGHTYEAR, a unit of length used in astronomy: the path traveled by light in 1 year, i.e. 9.466´1012 km. The distance to the nearest star (Proxima Centauri) is approximately 4.3 light years. The most distant stars in the Galaxy are located on ... ... Illustrated Encyclopedic Dictionary

A non-systemic unit of length used in astronomy. 1 light year is the distance that light travels in 1 year. 1 light year is equal to 9.4605E+12 km = 0.307 pc... Astronomical dictionary

Unit of interstellar distances; the path that light travels in a year, that is, 9.46 1012 km. * * * LIGHTYEAR LIGHTYEAR, a unit of interstellar distances; the path that light travels in a year, i.e. 9.46x1012 km ... encyclopedic Dictionary

Light year- a unit of distance equal to the path traveled by light in one year. A light year is 0.3 parsec... Concepts of modern natural science. Glossary of basic terms

light year- šviesmetis statusas T sritis Standartizacija ir metrologija apibrėžtis Astronominis ilgio matavimo vienetas, lygus nuotoliui, kurį vakuume nusklinda šviesa per 1 atogrąžinius metus. Žymimas šm: 1 šm = 9.46073 10¹² km. atitikmenys: engl. light… … Penkiakalbis aiskinamasis metrologijos terminų žodynas

light year- šviesmetis statusas T sritis fizika atitikmenys: angl. light year vok. Lichtjahr, n rus. light year, m pranc. année lumière, f … Fizikos terminų žodynas

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Despite the fact that its name is similar to the time period of the year, the year does not measure time at all, but distance. This unit is designed to measure huge.

A light year is a non-systemic unit of length. This is the distance that light travels in a vacuum in one year (365.25 days or 31,557,600 seconds).

The comparison of a light year with a calendar year began to be used after 1984. Before that, a light year is the distance traveled by light in one tropical year.

The length of the tropical year does not have an exact value, since its calculations are related to the angular velocity of the Sun, and there are variations for it. For a light year, an average value was taken.

The difference in calculation between a tropical light year and a Julian light year is 0.02 percent. And since this unit is not used for high-precision measurements, there is no practical difference between them.

The light year as a length is used in popular science literature. In astronomy, there is another off-system unit for measuring large distances - the parsec. The calculation of the parsec is based on the average radius of the earth's orbit. 1 parsec is equal to 3.2616 light years.

Calculations and distances

The calculation of a light year is directly related to the speed of light. For calculations in physics, it is usually taken to be 300,000,000 m/s. The exact value of the speed of light is 299,792,458 m/s. That is, 299,792,458 meters is just one light second!

The distance to the moon is approximately 384,400,000 meters, which means that the light beam will reach the surface of the moon in approximately 1.28 seconds.

The distance from the Sun to the Earth is 149,600,000,000. Therefore, a sunbeam hits the Earth in a little less than 7 minutes.

So, there are 31,557,600 seconds in a year. Multiplying this number by a distance equal to one light second, we get that one light year is equal to 9,460,730,472,580,800 meters.

1 million light years, respectively, will be equal to 9,460,730,472,580,800,000,000 meters.

According to approximate calculations of astronomers, the diameter of our galaxy is about 100,000 light years. That is, within our Galaxy there cannot be distances measured in millions of light years. Such numbers are applicable for measuring distances between galaxies.

The Andromeda galaxy closest to Earth is 2.5 million light-years away.

To date, the largest cosmic distance from Earth that can be measured is the distance to the edge of the observable universe. It is about 45 billion light years.

Do you know why astronomers don't use the light year to calculate distances to distant objects in space?

A light year is a non-systemic unit for measuring distances in outer space. It is ubiquitous in popular books and textbooks on astronomy. However, in professional astrophysics, this figure is used extremely rarely and often to determine distances to nearby objects in space. The reason for this is simple: if you determine the distance in light years to distant objects in the Universe, the number will be so huge that it will be impractical and inconvenient to use it for physical and mathematical calculations. Therefore, instead of a light year, professional astronomy uses such a unit of measurement as , which is much more convenient to operate when performing complex mathematical calculations.

Definition of the term

We can find the definition of the term "light year" in any astronomy textbook. A light year is the distance that a ray of light travels in one Earth year. Such a definition may satisfy the amateur, but the cosmologist will find it incomplete. He will notice that a light year is not just the distance that light travels in a year, but the distance that a beam of light travels in 365.25 Earth days in vacuum, without being affected by magnetic fields.

A light year is 9.46 trillion kilometers. This is the distance a ray of light travels in a year. But how did astronomers achieve such an accurate determination of the ray path? We will talk about this below.

How is the speed of light determined?

In ancient times, it was believed that light propagates in the universe instantly. However, beginning in the seventeenth century, scholars began to doubt this. Galileo was the first to doubt the above proposed statement. It was he who tried to determine the time during which a ray of light travels a distance of 8 km. But due to the fact that such a distance was negligible for such a value as the speed of light, the experiment ended in failure.

The first major shift in this issue was the observation of the famous Danish astronomer Olaf Römer. In 1676, he noticed the difference in the time of an eclipse depending on the approach and removal of the Earth to them in outer space. Roemer successfully connected this observation with the fact that the farther the Earth moves away from, the more time it takes for the light reflected from them to travel the distance to our planet.

Roemer caught the essence of this fact exactly, but he did not succeed in calculating the reliable value of the speed of light. His calculations were wrong, because in the seventeenth century he could not have accurate data on the distance from the Earth to other planets in the solar system. These data were determined somewhat later.

Further advances in research and determination of the light year

In 1728, the English astronomer James Bradley, who discovered the effect of stellar aberration, was the first to calculate the approximate speed of light. He determined its value at 301 thousand km / s. But this value was inaccurate. More advanced methods for calculating the speed of light were produced independently of cosmic bodies - on Earth.

Observations of the speed of light in vacuum using a rotating wheel and a mirror were made by A. Fizeau and L. Foucault, respectively. With their help, physicists managed to get closer to the real value of this quantity.

Accurate speed of light

Scientists managed to determine the exact speed of light only in the last century. Based on Maxwell's theory of electromagnetism, using modern laser technology and calculations, corrected for the refractive index of the ray flux in air, scientists were able to calculate the exact value of the speed of light 299,792.458 km/s. This value is still used by astronomers. Further, to determine the light day, month and year was already a matter of technology. By simple calculations, scientists got the figure of 9.46 trillion kilometers - that is how much time it would take for a beam of light to fly around the length of the earth's orbit.

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Light year

During the exploration of their planet, people needed various measures of measuring distances and segments. Initially, length measures were inaccurate, because different peoples had their own ways of measuring. Only in 1791, scientists from France introduced a measure that is still used today - the meter (from the Greek - "measure").
But at the beginning of the twentieth century, people began to turn their attention to the study of space. And the fact that the Universe has incredible distances, the already existing metric system turned out to be unsuitable for measuring such large distances. It is possible to measure in kilometers the distance from our planet to the Moon or to Mars, but if you measure the distances to other planets, and even stars, then there will be an incredible number of zeros in the figure.
And then scientists decided to introduce the term "light years".

How many light years?

In just one second, photons of light cover a length of 300,000 km. A light year is the number of kilometers that light travels in 12 months. In kilometers it will be - 9,460,730,472,580.8 kilometers ≈ 9.46 1015.
Of course, using the term "light years" is more convenient than using huge kilometers. But, of course, there are approximate values:
1 light second ≈ 300 thousand kilometers.
1 light minute ≈ 18 million kilometers.
1 light hour ≈ 1,080,000,000 kilometers.
1 light day ≈ 26,000,000,000 kilometers.
1 light week ≈ 181,000,000,000 kilometers.
1 light month ≈ 790,000,000,000 kilometers.

How many?

We assume that the spacecraft flies at the third cosmic speed (about 16.8 kilometers per second), then in 18 thousand years the ship will fly by one light year. And our galaxy the Milky Way, which is about a hundred thousand light years in diameter, the ship will fly by in almost 2 billion years!
The closest star to the Sun is Proxima Centauri. It is located at a distance of about four light years. If converted to kilometers, the figure is very large.
But if we compare the distance from Proxima Centauri to the nearest galaxy, the Andromeda Nebula, then the star is very close, since Andromeda is two and a half million light-years from the Milky Way. The spacecraft will be able to fly there in 35 billion years.

What else are light years useful for?

Using light years helps to understand where in the universe you can try to find intelligent civilizations. So scientists determine where it makes sense to send radio signals, and where not.
How it works: the speed of light is equal to the speed of a radio signal, and it turns out that sending messages to where they will reach in thousands, or even billions of years, is completely useless. It makes sense to look for "neighbors" through the sent signal, which will go on for at least one human life.

How many earth years are there in light years?

It is a fundamentally wrong belief that this term measures time. The light year has nothing to do with Earth time and is not related to it in any way. It denotes only a measure of the distances that light travels in 1 year on Earth.