1 light year is equal to Earth years. How is the distance to stars measured and what is a light year? Light year - how many Earth years is it?
Surely, having heard in some science fiction action movie an expression a la “twenty 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 scales in units that a radio wave (which light, in particular, is) travels over certain time intervals.
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.
Of course, the exact meaning light years slightly different from what we calculated. But when describing distances to stars in popular science articles, the highest accuracy is, in principle, 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.
And to the big one closest to us galaxies, spaceship will arrive only after 36 billion years...
These are the pies. But in theory, even Universe arose only 16 billion years ago...
And finally...
One can begin to marvel at the cosmic scale even without going beyond solar system, because it itself is very large. This was demonstrated very well and clearly, for example, by the creators of the project If the Moon wereonly 1 pixel (If the Moon Were Just One Pixel): http://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html.
With this, I think I’ll end today’s article. I am glad to welcome all your questions, comments and wishes in the comments below.
The distances between stars are so great that measuring them in kilometers or miles is an exercise with endless zeros. The usual measurement system is used to designate distances in one system. For example, they say that the minimum distance from Earth to Mars is 55.76 million kilometers. With stars, everything is more complicated, and here the concepts of light year and parsec are usually used.
Astronomical unit- a unit of measurement adopted in astronomy for objects of the Solar System and the objects of the Universe closest to it. An astronomical unit is equal to 149,598,100 km (+- ~750 km), which is approximately equal to the average distance of the Earth from the Sun. Modern observations have recorded a gradual increase in value by 15 cm annually, which is explained by the possible loss of mass by the Sun, the consequences of the solar wind.
Light year- the distance that light travels in one year, in meters it is 9,460,730,472,580,800. In fact, the light of the stars that we see on a cloudless night traveled to our planet for many centuries, and some of them no longer exist at all.
Parsec, also known as “arcsecond parallax,” is the distance from which the average radius of the Earth’s orbit (perpendicular to the line of sight) is visible at an angle of one arcsecond. Quite simply, a parsec = 3.26 light years.
It is interesting that in popular science and fantasy literature it is customary to use the concept of a light year, and parsecs are usually used only in professional works and research.
(Galaxy UDFj-39546284 is the farthest galaxy from Earth (13.3 billion light years from Earth), in the image taken by the Hubble telescope it looks like a red dot)
The closest star to us is Alpha Centauri, which is located 4.37 light years from Earth. But the most distant galaxy (as of December 2012) is as much as 13.3 billion light years from Earth! It turns out that when the sun of this very galaxy (known under the symbol UDFj-39546284) goes out, humanity will not know about it soon.
Do you know why astronomers don't use light years to calculate distances to distant objects in space?
A light year is a non-systemic unit of measurement of distances in outer space. It is widely used in popular books and textbooks on astronomy. However, in professional astrophysics this figure is used extremely rarely and is often used 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 turn out to be so huge that it will be impractical and inconvenient to use it for physical and mathematical calculations. Therefore, instead of the light year in professional astronomy, a unit of measurement is used, 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 a ray of light travels in one Earth year. Such a definition may satisfy an amateur, but a cosmologist will find it incomplete. He will note that a light year is not just the distance that light travels in a year, but the distance that a ray of light travels in a vacuum in 365.25 Earth days, unaffected by magnetic fields.
A light year is equal to 9.46 trillion kilometers. This is exactly the distance a ray of light travels in a year. But how did astronomers achieve such precise determination of the ray path? We'll talk about this below.
How was the speed of light determined?
In ancient times, it was believed that light travels throughout the Universe instantly. However, starting in the seventeenth century, scientists began to doubt this. Galileo was the first to doubt the above proposed statement. It was he who tried to determine the time it takes a ray of light to travel a distance of 8 km. But due to the fact that such a distance was negligibly small for such a quantity as the speed of light, the experiment ended in failure.
The first major shift in this matter was the observation of the famous Danish astronomer Olaf Roemer. In 1676, he noticed a difference in the time of eclipses depending on the approach and distance of the Earth to them in outer space. Roemer successfully connected this observation with the fact that the further the Earth moves away from, the longer it takes the light reflected from them to travel the distance to our planet.
Roemer grasped the essence of this fact accurately, but he failed to calculate the reliable value of the speed of light. His calculations were incorrect because in the seventeenth century he could not have accurate data on the distance from the Earth to the other planets of the solar system. These data were determined a little later.
Further advances in research and the definition of the light year
In 1728, the English astronomer James Bradley, who discovered the effect of aberration in stars, was the first to calculate the approximate speed of light. He determined its value to be 301 thousand km/s. But this value was inaccurate. More advanced methods for calculating the speed of light were made without regard to cosmic bodies - on Earth.
Observations of the speed of light in a 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.
Exact speed of light
Scientists were able 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 speed of light as 299,792.458 km/s. Astronomers still use this quantity. Further determining the daylight hours, month and year was already a matter of technology. Through simple calculations, scientists arrived at a figure of 9.46 trillion kilometers—that’s exactly how long it would take a beam of light to travel the length of the Earth’s orbit.
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 fantasy fans are very familiar with this concept, as it often appears in films and books. But not everyone knows what a light year is, and some even think that it is similar to the usual annual calculation of time.
What is a light year?
In reality, a light year is not a unit of time, as one might assume, but a unit of length used in astronomy. It refers to the distance traveled by light in one year.
It is usually used in astronomy textbooks or popular science fiction to determine lengths within the solar system. For more accurate mathematical calculations or measuring 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 several years after the first successful measurement of the distance from the Sun to the star 61 Cygni in 1838. 
Initially, a light year was the distance traveled by light in one tropical year, that is, in a period of time equal to the full cycle of seasons. However, since 1984, the Julian year (365.25 days) began to be used as a basis, as a result of which the measurements became more accurate.
How is the speed of light determined?
To calculate a light year, researchers had to first determine the speed of light. Astronomers once believed that the propagation of rays in space was instantaneous, but in the 17th century this conclusion began to be questioned.
The first attempts to make calculations were made by Galileo Gallilei, who decided to calculate the time it takes light to travel 8 km. His research was unsuccessful. James Bradley managed to calculate the approximate value in 1728, who determined the speed at 301 thousand km/s.
What is the speed of light?
Despite the fact that Bradley made fairly accurate calculations, they were able to determine the exact speed only in the 20th century, using modern laser technologies. Advanced equipment made it possible to make calculations corrected for the refractive index of rays, resulting in this value being 299,792.458 kilometers per second. 
Astronomers operate with these figures to this day. Subsequently, simple calculations helped to accurately determine the time that the rays needed to fly around the orbit of the globe without the influence of gravitational fields on them.
Although the speed of light is not comparable to earthly distances, its use in calculations is explained by the fact that people are accustomed to thinking in “earthly” categories.
What is a light year equal to?
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 this data to measure distances inside 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. Observing cosmic bodies is the only case when a person can see the past with his own eyes.
It takes many years for light emitted by a distant star to reach Earth. For this reason, when observing cosmic 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 movement of rays, astronomers were able to calculate the distance in light years to many celestial bodies. Thus, 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 takes rays approximately 26 thousand light years, and between the Sun and the planet Pluto - 5 light hours.
One way or another, in our daily lives we measure distances: to the nearest supermarket, to a relative’s house in another city, to, and so on. However, when it comes to the vastness of outer space, it turns out that using familiar values like kilometers is extremely irrational. And the point here is not only in the difficulty of perceiving the resulting gigantic values, but in the number of numbers in them. Even writing so many zeros will become a problem. For example, the shortest distance from Mars to Earth is 55.7 million kilometers. Six zeros! But the red planet is one of our closest neighbors in the sky. How to use the cumbersome numbers that result when calculating the distance even to the nearest stars? And right now we need such a value as a light year. How much is it equal? Let's figure it out now.
The concept of a light year is also closely related to relativistic physics, in which the close connection and mutual dependence of space and time was established at the beginning of the 20th century, when the postulates of Newtonian mechanics collapsed. Before this distance value, larger scale units in the system
were formed quite simply: each subsequent one was a collection of units of a smaller order (centimeters, meters, kilometers, and so on). In the case of a light year, distance was tied to time. Modern science knows that the speed of light propagation in a vacuum is constant. Moreover, it is the maximum speed in nature admissible in modern relativistic physics. It was these ideas that formed the basis of the new meaning. A light year is equal to the distance a ray of light travels in one Earth calendar year. In kilometers it is approximately 9.46 * 10 15 kilometers. Interestingly, a photon travels the distance to the nearest Moon in 1.3 seconds. It's about eight minutes to the sun. But the next closest stars, Alpha, are already about four light years away.
Just a fantastic distance. There is an even larger measure of space in astrophysics. A light year is equal to about one-third of a parsec, an even larger unit of measurement of interstellar distances.
Speed of light propagation under different conditions
By the way, there is also such a feature that photons can propagate at different speeds in different environments. We already know how fast they fly in a vacuum. And when they say that a light year is equal to the distance covered by light in a year, they mean empty outer space. However, it is interesting to note that under other conditions the speed of light may be lower. For example, in air, photons scatter at a slightly lower speed than in vacuum. Which one depends on the specific state of the atmosphere. Thus, in a gas-filled environment, the light year would be somewhat smaller. However, it would not differ significantly from the accepted one.