A light year is not a unit of time, as one might think from its name. A light year is a unit of distance used in astronomy.

Heavenly distances are difficult to measure in meters and kilometers familiar to us - they are so huge! A light year is the distance that a sunbeam travels in a year, that is, in 365 days. But the speed of a beam of light is almost 300 thousand kilometers per second! So, a light year is 9,460,800,000,000 km, that is, about 10 trillion kilometers. Therefore, it is much easier, for example, to say: the brightest star in the sky, Sirius, is 8 light years away from us.

At first glance, such a unit of measurement may seem inconvenient. But in fact, it is very convenient for its visibility. For example, distance, measured in light years, shows which stars radio or other electromagnetic messages can be sent to in order to receive a response in real time, and not in hundreds or thousands of years.

And did you know that...

  • The average distance to the Moon is approximately 376,300 km. This means that a beam of light sent from the surface of the Earth will take 1.2-1.3 seconds to reach the surface of our natural satellite.
  • The nearest star to us, not counting the Sun, Proxima Centauri, is located at a distance of 4.22 light years from us.
  • The diameter of our galaxy - the Milky Way - is 100,000 light years.
  • The nearest spiral galaxy to us - the famous Andromeda galaxy - is 2.5 million light-years away from us.

On February 22, 2017, NASA announced that 7 exoplanets have been found around the single star TRAPPIST-1. Three of them are in the range of distances from the star where the planet can have liquid water, and water is a key condition for life. It is also reported that this star system is located at a distance of 40 light years from Earth.

This message made a lot of noise in the media, it even seemed to some that humanity was one step away from building new settlements near a new star, but this is not so. But 40 light-years is a lot, it's a LOT, it's too many kilometers, that is, this is a monstrously colossal distance!

From the course of physics, the third is known space velocity- this is the speed that a body must have at the surface of the Earth in order to go beyond solar system. The value of this speed is 16.65 km/s. conventional orbital spaceships start at a speed of 7.9 km / s, and revolve around the Earth. In principle, a speed of 16-20 km/s is quite affordable for modern earthly technologies, but no more!

Mankind has not yet learned how to accelerate spaceships faster than 20 km/sec.

Let's calculate how many years it will take for a starship flying at a speed of 20 km/sec to overcome 40 light years and reach the star TRAPPIST-1.
One light year is the distance that a beam of light travels in a vacuum, and the speed of light is approximately 300,000 km/sec.

A human-made spacecraft flies at a speed of 20 km/sec, that is, 15,000 times slower than the speed of light. Such a ship will overcome 40 light years in a time equal to 40*15000=600000 years!

An earth ship (with the current level of technology) will fly to the star TRAPPIST-1 in about 600 thousand years! Homo sapiens exists on Earth (according to scientists) only 35-40 thousand years, and here as much as 600 thousand years!

In the near future, technology will not allow a person to reach the star TRAPPIST-1. Even promising engines (ion, photon, space sails, etc.), which are not in earthly reality, can be estimated to accelerate the ship to a speed of 10,000 km / s, which means that the flight time to the TRAPPIST-1 system will be reduced to 120 years . This is already a more or less acceptable time for flying with the help of suspended animation or for several generations of migrants, but today all these engines are fantastic.

Even the nearest stars are still too far from people, too far, not to mention the stars of our Galaxy or other galaxies.

The diameter of our galaxy Milky Way is approximately 100 thousand light years, that is, the path from end to end for a modern earthly ship will be 1.5 billion years! Science suggests that our Earth is 4.5 billion years old, and multicellular life is about 2 billion years old. The distance to the nearest galaxy to us - the Andromeda Nebula - is 2.5 million light years from Earth - what monstrous distances!

As you can see, of all people living today, no one will ever set foot on the earth of a planet near another star.

Cosmic distances are difficult to measure in ordinary meters and kilometers, so astronomers use others in their work. physical units. 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 in this moment, and what they were at the moment of emission of light.

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.

For their calculations, astronomers use special units of measurement that are not always clear. ordinary people. It is understandable, because if cosmic distances were measured in kilometers, then the number of zeros would ripple in the eyes. Therefore, to measure cosmic distances, it is customary to use much larger quantities: an astronomical unit, a light year, and a parsec.

Quite often used to indicate distances within our own solar system. If you can still express it in kilometers (384,000 km), then the closest way to Pluto is about 4,250 million km, and this will already be difficult to understand. For such distances, it is time to use the astronomical unit (AU), equal to the average distance from earth's surface to the sun. In other words, 1 a.u. corresponds to the length of the semi-major axis of the orbit of our Earth (150 million km.). Now, if you write that the shortest distance to Pluto is 28 AU, and the longest path can be 50 AU, this is much easier to imagine.

The next largest is the light year. Although the word "year" is present, one should not think that we are talking about the time. One light year is 63,240 AU. This is the path that a ray of light travels in 1 year. Astronomers have calculated that it takes more than 10 billion years for a beam of light to reach us from the farthest corners of the universe. To imagine this gigantic distance, let's write it down in kilometers: 950000000000000000000000. Ninety-five billion trillion habitual kilometers.

The fact that light does not propagate instantly, but at a certain speed, scientists began to guess since 1676. It was at this time that a Danish astronomer named Ole Römer noticed that the eclipses of one of Jupiter's moons began to lag, and this happened precisely when the Earth was heading in its orbit towards the opposite side of the Sun, the opposite of where Jupiter was. Some time passed, the Earth began to return back, and the eclipses again began to approach the previous schedule.

Thus, about 17 minutes of time difference was noted. From this observation, it was concluded that it took 17 minutes for light to travel a distance the length of the diameter of the Earth's orbit. Since the diameter of the orbit was proved to be approximately 186 million miles (now this constant is 939,120,000 km), it turned out that a beam of light traveled at a speed of about 186,000 miles per second.

Already in our time, thanks to Professor Albert Michelson, who set out to determine as accurately as possible what a light year is, using a different method, the final result was obtained: 186,284 miles in 1 second (about 300 km / s). Now, if we calculate the number of seconds in a year and multiply by this number, we get that a light year is 5,880,000,000,000 miles long, which corresponds to 9,460,730,472,580.8 km.

For practical purposes, astronomers often use the unit of distance known as the parsec. It is equal to the displacement of the star against the background of other celestial bodies by 1 "" when the observer is displaced by 1 radius

A light year is the distance that light travels in one year.. The International Astronomical Union gave its explanation for the light year - this is the distance that light travels in a vacuum, without the participation of gravity, in a Julian year. The Julian year is equal to 365 days. It is this interpretation that is used in the scientific literature.

If we take professional literature, then here the distance is calculated in parsecs or kilo- and megaparsecs.

There are specific figures that determined the distance of light hours, minutes, days, etc.

  • A light year is 9,460,800,000,000 km,
  • month- 788 333 million km.,
  • a week- 197,083 million km.,
  • day- 26,277 million km,
  • hour- 1,094 million km.,
  • minute- about 18 million km.,
  • second- about 300 thousand km.

It is interesting! From the Earth to the Moon, light reaches an average of 1.25 s, while its beam reaches the Sun in a little more than 8 minutes.

The star Betelgeuse in the constellation Orion should explode in the foreseeable future (in fact - within a few centuries).

Betelgeuse is located at a distance of 495 to 640 light years from us.
If it explodes right now, then the inhabitants of the Earth will see this explosion only in 500-600 years.

And if you see an explosion today, then remember that in fact the explosion occurred around the time of Ivan the Terrible ...

earth year

An Earth year is the distance traveled by the earth in one year. If we take into account all the calculations, then one light year is equal to 63242 earth years. This figure refers specifically to the planet Earth, to others, such as Mars or Jupiter, they will be completely different. A light year calculates the distance from a single object celestial body to another. The numbers for light years and earth years are so different, even though they mean distance.

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Quick answer: not at all.

Often we are asked very interesting questions, the answers to which are very non-standard.

One of these questions you see in the title. And really, how many Earth years are there in one light year? You may be disappointed, but there is no real answer.

The fact is that a light year is not a measure of time, but a distance measure. To be more precise, a light year is the distance distance in vacuum without gravity fields, one effect of the Julian year (equivalent to 365.25 standard days per 86,400 SI seconds or 31,557,600 seconds) by the International Astronomical Federation,

To do this, we take the mark of 300 thousand kilometers per second (which is exactly the speed of light) and multiply with 31.56 million seconds (many seconds per year), and we get a huge number - 9460800000 000 km (or 9.46 million kilometers). This fantastic number means a distance equal to a light year.

  • 1 light month ~ 788,333,000,000 km
  • 1 easy week ~ 197,083,000 km
  • 1 light day ~ 26,277 million km
  • 1 light hour ~ 1,094 million km
  • 1 light minute ~ about 18 million km
  • 1 light second ~ 300 thousand km

To find out how many kilometers in a light year you need to use a simple web calculator.

In the left box, enter the number of light years of interest you want to convert. In the field on the right you will see the result of the calculation. Just click on the appropriate link to convert light years or miles to other units.

What is "bright summer"

The light year of a one-way system (St., ly) is equal to the distance traveled by light in vacuum in one July year (365.25 days).

This term is mainly used in science and fiction, and in the professional environment, the term "parsecs" was adopted with the prefix "kilo" and "mega".

not before 1984, according to the bright year, to understand the distance with the light traveled in the tropical year, now the value has changed by 0.002%, and the practical value of this difference, because very accurate measurements are not made in light years. The speed of light is about 300,000.

km per second and the luminous year is about 10 trillion kilometers (9460.8800 million km). As for the distances, for example, that Sirius has 8 light years from the closest proximity to the star Proxima Centauri - 4.22 light years, and the diameter of the Roman road - our galaxy, which is 100,000 light years.

What is "kilometer"

The kilometer kilometer (km, km) is a plural unit of reference distances and is widely used throughout the world.

One kilometer 1000 meters, 0.621 miles, 0.9374 miles, 1094 yards, 3281 meters, 1.057 x 10 - 13 light years, 6.67 x 10 - 9 astronomical units.

Easy years

For hundreds of years people have been inventing their own planet to invent more and more remote sensing systems. Therefore, it was decided to take into account a universal unit of one meter in length and a long measurement path in kilometers.

But in the next twentieth century, this created a new problem for mankind. People began to carefully study the universe - and it turned out that the size of the universe is so large that miles are simply not suitable here.

In ordinary units, you can express the distance from the Earth to the Moon or from the Earth to Mars. But if you're trying to figure out how far the nearest star is from our planet, the number "grows" with an imperceptible number of characters per decimal point.

What is 1 light year?

It was obvious that a new unit of space exploration was needed - and it's been a bright year.

In a second, light travels 300,000 kilometers. Easy years — this is the distance whose light will travel exactly one year, and when translated into a more familiar system of numbers, this distance is 9,460,730,472,580.8 kilometers. It is clear that the use of a concise "simple flight" is much more convenient than the use of each huge number in the calculations.

Of all the stars closest to us, Proxima Centauri - this was removed only "4.2 light years". Of course, based on kilometer data, there is an unimaginable amount. However, everything is relative - given that the nearest Andromeda galaxy is separated from the Roman road by as much as 2.5 million light years, the star and truth begin to seem very close neighbors.

By the way, the use of light years helps scientists understand in which corners of space it is reasonable to find intelligent life and where it is completely useless to send radio signals.

After all, the speed of a radio signal is similar to the speed of light, so a greeting sent towards a distant galaxy will reach its destination in millions of years. It makes sense to expect a response from neighboring "neighbors" - objects whose hypothetical response signals will reach terrestrial devices even during a person's life.

1 light year - how many earth years?

There is a common misconception that the light year is a unit of time.

Actually it is not. This term has nothing to do with Earth years, it does not refer to them and represents only the distance that light travels in one Earth year.