As you know, to measure the distances from the Sun to the planets, as well as between the planets, scientists came up with an astronomical unit. What is it light year?

First of all, it should be noted that the light year is also a unit of measurement adopted in astronomy, but not of time (as it might seem, judging by the meaning of the word “year”), but of distance.

What is a light year equal to?

When scientists managed to calculate the distances to the nearest stars, it became obvious that the astronomical unit was inconvenient for use in the stellar world. Let's say for starters that the distance from the Sun to the nearest star is approximately 4.5 light years. This means that light from our Sun to the nearest star (by the way, it is called Proxima Centauri) takes 4.5 years to travel! How far is this distance? Let’s not bore anyone with mathematics, let’s just note that in a second, particles of light fly 300,000 kilometers. That is, if you send a signal with a flashlight towards the Moon, this light will be seen there in less than a second and a half. Light travels from the Sun to Earth in 8.5 minutes. How long then do the rays of light travel in a year?

Let's say right away: a light year is approximately 10 trillion kilometers(a trillion is one followed by twelve zeros). More precisely, 9,460,730,472,581 kilometers. If recalculated in astronomical units, it will be approximately 67,000. And this is only for the nearest star!

It is clear that in the world of stars and galaxies the astronomical unit is not suitable for measurements. It is easier to operate in calculations with light years.

Applicability in the stellar world

For example, the distance from Earth to the brightest star in the sky, Sirius, is 8 light years. And the distance from the Sun to the North Star is about 600 light years. That is, light from us gets there in 600 years. This would be approximately 40 million astronomical units. For comparison, we point out that the size (diameter) of our Galaxy - the Milky Way - is about 100,000 light years. Our closest neighbor, a spiral galaxy called the Andromeda Nebula, is 2.52 million light years away from Earth. It is very inconvenient to indicate this in astronomical units. But there are objects in the Universe that are generally 15 billion light years away from us. Thus, the radius of the observable Universe is 13.77 billion light years. And the complete Universe, as is known, extends beyond the observable part.

By the way, the diameter of the observable Universe is not at all 2 times larger than the radius, as you might think. The thing is that over time, space expands. Those distant objects that emitted light 13.77 billion years ago have flown even further away from us. Today they are more than 46.5 billion light years away. Doubling this gives us 93 billion light years. This is the true diameter of the observable Universe. So the size of the part of space that is being observed (and which is also called the Metagalaxy) is increasing all the time.

Measuring such distances in kilometers or astronomical units makes no sense. To be honest, light years don't quite fit here. But nothing better people haven't figured it out yet. The numbers are so huge that only a computer can handle them.

Definition and essence of light year

Thus, light year (light year) is a unit of length, not time, that represents the distance traveled sunbeam per year, that is, 365 days. This unit of measurement is very convenient for its clarity. It allows you to answer the question, after what period of time you can expect a response if you send an electromagnetic message to a certain star. And if this period is too long (for example, a thousand years), then there is no point in such actions.

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Light year

During the exploration of their planet, people needed various measures for measuring distances and segments. Initially, length measures were inaccurate because different nations there were their own ways of measuring. It was only in 1791 that 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 space exploration. And the fact that the Universe has incredible distances, already exists 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, or even stars, the number will contain an incredible number of zeros.
And then scientists decided to introduce the term “light years”.

How many light years is that?

In just one second, photons of light travel a distance of 300 thousand 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 much?

We assume that spaceship flies from third escape velocity(about 16.8 kilometers per second), then in 18 thousand years the ship will fly one light year. And the ship will fly through our Milky Way galaxy, which is about one hundred thousand light years in diameter, 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 you calculate it in kilometers, the figure turns out to be very large.
But if we compare the distance from Proxima Centauri to the nearest galaxy, the Andromeda Nebula, the star turns out to be very close, since Andromeda is located two and a half million light years from the Milky Way. The spacecraft will be able to get there in 35 billion years.

What else are light years useful for?

Using light years helps us understand where in the universe we can try to find intelligent civilizations. This is how 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 a sent signal that will last at least one human life.

Surely, having heard in some science fiction action movie an expression a la “twenty minutes to Tatooine” light years", many asked legitimate questions. I'll mention some of them:

Isn't a year a time?

Then what is it light year?

How many kilometers is it?

How long will it take to overcome light year spaceship with Earth?

I decided to devote today’s article to explaining the meaning of this unit of measurement, comparing it with our usual kilometers and demonstrating the scale that it operates Universe.

Virtual racer.

Let's imagine a person, in violation of all the rules, rushing along a highway at a speed of 250 km/h. In two hours it will cover 500 km, and in four – as much as 1000. Unless, of course, it crashes in the process...

It would seem that this is speed! But in order to circumnavigate the entire globe (≈ 40,000 km), our racer will need 40 times more time. And this is already 4 x 40 = 160 hours. Or almost a whole week of continuous driving!

In the end, however, we will not say that he covered 40,000,000 meters. Because laziness has always forced us to invent and use shorter alternative units of measurement.

Limit.

From a school physics course, everyone should know that the fastest rider in Universe- light. In one second, its beam covers a distance of approximately 300,000 km, and thus it will circle the globe in 0.134 seconds. That's 4,298,507 times faster than our virtual racer!

From Earth before Moon the light reaches on average 1.25 s, up to Sun its beam will reach in a little more than 8 minutes.

Colossal, isn't it? But the existence of speeds greater than the speed of light has not yet been proven. Therefore, the scientific world decided that it would be logical to measure cosmic scale in units that a radio wave passes over certain time intervals (which is what light, in particular, is).

Distances.

Thus, light year- nothing more than the distance that a ray of light travels in one year. On interstellar scales, using distance units smaller than this does not make much sense. And yet they are there. Here are their approximate values:

1 light second ≈ 300,000 km;

1 light minute ≈ 18,000,000 km;

1 light hour ≈ 1,080,000,000 km;

1 light day ≈ 26,000,000,000 km;

1 light week ≈ 181,000,000,000 km;

1 light month ≈ 790,000,000,000 km.

Now, so that you understand where the numbers come from, let’s calculate what one is equal to light year.

There are 365 days in a year, 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute. Thus, a year consists of 365 x 24 x 60 x 60 = 31,536,000 seconds. In one second, light travels 300,000 km. Therefore, in a year its beam will cover a distance of 31,536,000 x 300,000 = 9,460,800,000,000 km.

This number reads like this: NINE TRILLION, FOUR HUNDRED AND SIXTY BILLION AND EIGHT HUNDRED MILLION kilometers.

Certainly, exact value light years slightly different from what we calculated. But when describing distances to stars in popular science articles highest precision in principle, it is not needed, and a hundred or two million kilometers will not play a special role here.

Now let's continue our thought experiments...

Scale.

Let's assume that modern spaceship leaves solar system with the third escape velocity (≈ 16.7 km/s). First light year he will overcome it in 18,000 years!

4,36 light years to the closest star system to us ( Alpha Centauri, see the image at the beginning) it will overcome in about 78 thousand years!

Our Milky Way galaxy, having a diameter of approximately 100,000 light years, it will cross in 1 billion 780 million years.

In order to understand the meaning of the concept of “light year”, you first need to remember school course physics, especially that section that concerns the speed of light. So, the speed of light in a vacuum, where it is not affected by various factors such as gravitational and magnetic fields, suspended particles, refraction of a transparent medium, etc., is 299,792.5 kilometers per second. You need to understand that in this case, by light we mean perceived human vision.

Lesser-known units of distance are the light month, week, day, hour, minute and second.

For quite a long time, light was considered an infinite quantity, and the first person to calculate the approximate speed of light rays in a vacuum was the astronomer Olaf Roemer in the middle of the 17th century. Of course, his data were very approximate, but the very fact of determining the final speed value is important. In 1970, the speed of light was determined to within one meter per second. More accurate results have not yet been achieved, as problems have arisen with the error of the meter standard.

Light year and other distances

Since the distances are enormous, measuring them in conventional units would be irrational and inconvenient. Based on these considerations, a special one was introduced - the light year, that is, the distance that light travels in the so-called Julian year (equal to 365.25 days). Considering that every day contains 86,400 seconds, it can be calculated that in a year a ray of light travels a distance of slightly more than 9.4 kilometers. This value seems enormous, however, for example, the distance to the closest star to Earth, Proxima Centauri, is 4.2 years, and the diameter of the Milky Way galaxy exceeds 100,000 light years, that is, the visual observations that can be made now reflect a picture that existed about hundreds of thousands of years ago.

A ray of light travels the distance from the Earth to the Moon in about a second, but sunlight takes more than eight minutes to reach our planet.

In professional astrophysics, the concept of a light year is rarely used. Scientists primarily use units such as parsec and astronomical unit. A parsec is the distance to the imaginary point from which the radius of the Earth's orbit is seen at an angle of one arcsecond (1/3600 of a degree). The average radius of the orbit, that is, the distance from the Earth to the Sun, is called the astronomical unit. A parsec is equal to approximately three light years or 30.8 trillion kilometers. An astronomical unit is approximately equal to 149.6 million kilometers.