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, without being influenced 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 for 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 was never able to calculate a reliable value for 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 produced 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.
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.
Whatever lifestyle we lead, whatever we do, one way or another, we use some units of measurement every day. We ask for a glass of water, heat our own breakfast to a certain temperature, visually estimate how far we need to walk to the nearest post office, arrange a meeting at a certain time, and so on. All these actions require
Not just calculations, but also a certain measurement of various numerical categories: distance, quantity, weight, time and others. We use numbers regularly in our daily lives. And we have long been accustomed to these numbers, as if to some kind of instruments. But what happens when we step out of our everyday comfort zone and encounter numerical values that are unusual for us? In this article we will talk about the fantastic figures of the Universe.
Universal spaces
The situation with cosmic distances is even more surprising. We are fully aware of the kilometers to the neighboring city and even from Moscow to New York. But it is difficult to visualize distances when it comes to the scale of star clusters. It is now that we will need the so-called light year. After all, the distances even between neighboring stars are extremely large, and measuring them in kilometers or miles is simply irrational. And here the matter is not only in the difficulty of perceiving the huge resulting numbers, but in the number of their zeros. It becomes a problem to write the number. For example, the distance from Earth to Mars during the period of closest approach is 55.7 million kilometers. A value with six zeros. But Mars is one of our closest cosmic neighbors! The distance to the nearest star other than the Sun will be millions of times greater. And then, whether we measured it in kilometers or miles, astronomers would have to spend hours of their time just recording these gigantic quantities. A light year solved this problem. The solution was quite ingenious.
What is a light year equal to?
Instead of inventing a new unit of measurement, which is the sum of units of a smaller order (as happens with millimeters, centimeters, meters, kilometers), it was decided to tie distance to time. Actually, the fact that time is also a physical field influencing events is more
Moreover, interconnected and convertible with space, it was discovered by Albert Einstein and proven through his theory of relativity. The speed of light became constant. And the passage of a certain distance by a light beam per unit of time gave new physical spatial quantities: light second, light minute, light day, light month, light year. For example, per second a beam of light (in space conditions - vacuum) travels a distance of approximately 300 thousand kilometers. It is easy to calculate that one light year is equal to approximately 9.46 * 10 15. Thus, the distance from the Earth to the nearest cosmic body, the Moon, is a little more than one light second, and to the Sun is about eight light minutes. According to modern ideas, the outer bodies of the Solar System rotate in orbit at a distance of one light year. The next closest star to us, or rather, a system of double stars, Alpha and Proxima Centauri, is so far away that even the light from them reaches our telescopes only four years after its launch. And these are still the celestial bodies closest to us. Light from the other end of the Milky Way takes more than a hundred thousand years to reach us.
Material prepared by Alexander Zanin
The light second is a unit of length used in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is exactly 299,792,458 meters. This is just over 186,282 miles and almost 9.84 x 10 8 feet.
Also, the speed of light serves as the basis for other units of time and units of length, ranging from the nanosecond of light (just under one US or British foot) to the light minute, light hour and light day, which is sometimes used in popular science publications. The more commonly used light year is currently defined to be exactly 31,557,600 light seconds, since the definition of a year is based on the Julian definition of the length of the year (not the Gregorian) of 365.25 days, each of exactly 86,400 SI seconds.
Definition of meter
A meter is the distance that light travels in a vacuum in a time interval of 1/299792458 of a second.
This definition fixes the speed of light in a vacuum as 299792458 m/s, and therefore a light second corresponds to 299792458 m.
Use in telecommunications
Communications signals to Earth rarely travel exactly at the speed of light in free space, but distances of fractions of light seconds are still useful for planning telecommunications networks because they indicate the lowest possible delay between sender and receiver.
One nanosecond of light is almost 300 millimeters (299.8 mm, 5 mm less than one foot), which limits the speed at which data can be transferred between different parts of a large computer.
One microsecond of light corresponds to a distance of about 300 meters.
The average distance, if the signal travels along the perimeter of the Earth, between opposite sides of the Earth is 66.8 light milliseconds.
Communications satellites are typically located at altitudes between 1.337 light milliseconds (low Earth orbits) and 119.4 light milliseconds (geostationary orbits) from the Earth's surface. Therefore there will always be a delay of at least a quarter of a second in communications via geostationary satellite systems (119.4 ms *2 times); this delay is barely noticeable in transoceanic telephone conversations routed via satellite.
Use in astronomy
The light second is a convenient unit for measuring distances in the inner Solar System because it matches very closely the radiometric data used to determine them (the match is not exact for a terrestrial observer due to very little correction for the effects of relativity). The value of an astronomical unit (such as the distance from the Earth to the Sun) in light seconds is one of the basic measurements for calculating modern ephemeris (tables of planetary coordinates): this is usually called "light unit time per unit distance" in tables of astronomical constants, and in Currently, its value (distance from the Earth to the Sun) is 499.004786385 (20) s.
- The average diameter of the Earth is about 0.0425 light seconds.
- The average distance from the Earth to the Moon is about 1.282 light seconds.
- The diameter of the Sun is about 4.643 light seconds.
- The average distance from the Earth to the Sun is 499.0 light seconds.
- The multiple of the light second can be defined, although, in addition to the light year, it is more often used in popular science publications than in scientific research works. For example, a light minute is equal to 60 light seconds, and the average distance from the Earth to the Sun is 8.317 light minutes.
- Light second: 2.997924580 × 10^8 m 2.998*10^5 km 1.863 × 10^5 miles. The average distance from the Earth to the Moon is about 1.282 light seconds.
- Light minute = 60 light seconds - 1.798754748 × 10^10 m = 1.799 × 10^7 km = 1.118 × 107 miles. The average distance from the Earth to the Sun is 8.317 light minutes.
- light hour= 60 light minutes
- = 3600 light seconds 1.079252849 × 10^12 m = 1.079 × 10^9 km = 6.706 × 108 miles. The semimajor axis of Pluto's orbit is about 5,473 light hours.
- daylight hours =24 daylight hours=
- = 86400 light seconds - 2.590206837 × 10^13m = 2.590 *10^10 km = 1.609 × 10^10 miles. The most distant planet from the Sun, Sedna, is currently 0.52 light-days from the Sun in an orbit that ranges from 0.44 light-days at perigee to 5.41 light-days at apogee.
- Light week= 7 light days
- = 604800 light seconds -1.813144786 × 10^14 m = 1.813 × 10^11 km = 1.127 × 10^11 miles. The Oort Nebula is believed to occupy an area between 41 and 82 light weeks from the Sun.
- Light year =365.25 light days=
- = 31557600 light seconds 9.460730473 × 1015 m = 9.461 × 1012 km = 5.879 × 1012 miles. Proxima Centauri is the closest star to the Sun, located at a distance of about 4.24 light years from Earth.
Exploring their own planet, over hundreds of years, people invented more and more new systems for measuring distance segments. As a result, it was decided to consider one meter as the universal unit of length, and measure the long distance in kilometers.
But the advent of the twentieth century presented humanity with a new problem. People began to carefully study space - and it turned out that the vastness of the Universe is so vast that kilometers are simply not suitable here. In conventional units you can still express the distance from the Earth to the Moon or from the Earth to Mars. But if you try to determine how many kilometers the nearest star is from our planet, the number “overgrows” with an unimaginable number of decimal places.
What is 1 light year equal to?
It became obvious that a new unit of measurement was needed to explore the spaces of space - and the light year became it. In one second, light travels 300,000 kilometers. Light year - this is the distance that light will travel in exactly one year - and translated into a more familiar number system, this distance is equal to 9,460,730,472,580.8 kilometers. It is clear that using the laconic “one light year” is much more convenient than using this huge figure in calculations every time.
Of all the stars, Proxima Centauri is closest to us - it is “only” 4.22 light years away. Of course, in terms of kilometers the figure will be unimaginably huge. However, everything is learned in comparison - if you consider that the nearest galaxy called Andromeda is as much as 2.5 million light years away from the Milky Way, the above-mentioned star really begins to seem like a very close neighbor.
By the way, using light years helps scientists understand in which corners of the Universe it makes sense to look for intelligent life, and where sending radio signals is completely useless. After all, the speed of a radio signal is similar to the speed of light - accordingly, a greeting sent towards a distant galaxy will reach its destination only after millions of years. It is more reasonable to expect an answer from closer “neighbors” - objects whose hypothetical response signals will reach earthly devices at least during a person’s lifetime.
1 light year is how many Earth years?
There is a widespread misconception that the light year is a unit of time. In fact, this is not true. The term has nothing to do with earthly years, does not correlate with them in any way and refers solely to the distance that light travels in one earthly year.
