The radius of the earth's orbit is often used in astronomy as a unit of length, called astronomical unit, to measure the distance between celestial bodies and the sun in the solar system

The radius of the earth's orbit is often used in astronomy as a unit of length, called astronomical unit, to measure the distance between celestial bodies and the sun in the solar system

Light year, a unit of length, is generally used to calculate the distance between stars. Light year refers to the distance that light travels in a vacuum for one Julian year. It is calculated by time and speed
brief introduction
The distance between the universe is very large, so it can only be measured in light years. The distance that light travels in a year is called a light year. The speed of light is 300000 kilometers per second, so a light year is 9460 billion kilometers
Edit the basic introduction of this paragraph
"Year" is a unit of time, but "light year" has the word "year", but it is not a unit of time, but a unit of astronomy to measure the distance between celestial bodies. In the universe, the distance between celestial bodies is very long. If we use the meter and kilometer (kilometer) as our daily measurement unit, the number to measure the distance between celestial bodies is often more than ten or dozens, It's very inconvenient. So astronomers created a unit of measurement light year, that is, the distance light travels in a vacuum in a year. Distance = speed × time. The speed of light is about 300000 kilometers per second (299792458 meters per second), and one light year is about 9460.5 billion kilometers, Although the earth is large, it is a drop in the ocean at most in the solar system. The average distance between the earth and the nearest celestial body, the moon, is 384400 kilometers, which is almost 30 times the diameter of the earth. However, the distance between the earth and the nearest planet, Venus, is only a drop in the ocean, The distance between the earth and the sun is 149.6 billion meters, and the distance between the earth and Pluto is more than 4 billion kilometers. Such a number is too large. For convenience, people take the average distance between the earth and the sun as 1. It is called "astronomical unit". It is more convenient to measure the distance of the solar system with this unit. The distance between the sun and the earth is 1 astronomical unit, The solar system is large, but it is very small in the galaxy and the universe, far less than a drop in the ocean. The nearest star to the sun, Centaurus A, is 43 trillion kilometers away from us, It is more than 3 billion times of this number. Such a number is too large to be expressed in astronomical units, so people use a new unit light year (one light year is equal to 64230 astronomical units). That is to measure the distance between stars by using the distance of light walking for one year as 1. As we all know, light travels 300000 kilometers in one second, In this way, we can say that the distance from the sun to Centaurus A is 4.3 light years, 8.7 light years from Sirius, 16.63 and 26.3 light years from Altair and Vega, and 850 light years from the famous star Rigel, The Milky way has a span of 100000 light-years. To Andromeda, it is 2.3 million light-years. At present, the farthest star known by human beings is 15 billion light-years away from us. That is to say, if this kind of star was born in the big bang 15 billion years ago, then what human beings see now is the light it emitted when it was just born
Edit the length unit of this paragraph
The distance of light walking for one year is called "one light year". One light year is about 9.46x10 ^ 12km. It is 500 million light years away from human beings
A more formal definition is: in the time of a Julian year (365.25 days, and a day is equal to 86400 seconds, a year is 31557600 seconds), in free space and infinitely far away from any gravitational field or magnetic field, the distance a photon travels. Because the speed of light in vacuum is 299792458 meters per second (accurate value), a light year is 94607304725800 meters, or 5878897915, 661.709 miles, or 5108794455193.632 nautical miles, or about 9.46 × 10 ^ 15m = 9.46 petmeters. (Note: 1 km (km) = 0.6214 miles = 0.540 nautical miles; 1 Julian year = 31557600 seconds.) light years are generally used to measure large distances, such as the distance between the solar system and another star, One second gap = 3.26 light years. Compared with other commonly used length units in astronomy, one second gap is about 3.26 light years, and one light year is about 63240 astronomical units. It takes about eight minutes for an example of light to reach the earth from the sun (that is, the distance between the earth and the sun is eight light minutes), The distance is about 4.22 light-years. The diameter of our galaxy, the Milky way, is about 100000 light-years. Assuming that there is a near light speed spaceship from one end of the Milky way to the other, it will take more than 100000 years, The journey experienced by the crew on board is actually only a few minutes. This is due to the time expansion of the mobile clock in special relativity. At present, the astronomical observation range has expanded to more than 10 billion light-years (called the total Galaxy). As mentioned earlier, "light-year" is a distance unit. But because it has the word "year", it is often mistaken for a time unit, For example, in some popular songs, it may be misused to describe the long time, such as "in a twinkling of an eye, in a moment, in a light year." or "through a few light years."
Edit the theoretical basis of this paragraph
The planet is about 190 trillion kilometers (about 20 light years) away from the earth. It is orbiting a red dwarf star (code named "581") which is smaller than the sun and lower than the sun. Its mass is about six times that of the earth, and its surface temperature is estimated to be between 0 ℃ and 40 ℃, which is equivalent to the surface temperature of the earth, Very similar to the earth, there may be rivers and oceans on it; if it is composed of "big ice balls", then its surface may be a "water world" without a piece of land. Because the planet "581c" is very close to its "sun" red dwarf star, so its revolution is very fast, only 13 days a year, "It's an earth like planet with potentially habitable conditions," NASA Astrobiology expert Chris Mackay said
Since this planet is about 20 light-years away from the earth, it is an unimaginable question whether human beings will be able to explore there or colonize this planet in the future. Even if human beings can invent a spaceship that can fly close to the speed of light in the future, it will take at least 20 light-years to get there, It seems that only through the "wormhole" or "time tunnel" described in science fiction movies. Light year refers to the distance that light travels in a year, that is, about 9460 billion kilometers (or 5880 billion miles). More formally, it is defined as: in a Julian year (365.25 days, and each day is equal to 86400 seconds), Because the speed of light in vacuum is 299792458 meters per second (accurate), one light year is equal to 94607304725800 meters (accurate), or about 9.46 × 1015 M = 9.46 beat meters, In my opinion, the report finally said that "even if human beings can invent a spaceship that can fly close to the speed of light in the future, it will take at least about 20 light years to get there." there is a suspicion that light years are regarded as a unit of time, and the relationship between contexts in the report is questionable. It is about 11 billion from the earth
It should be pointed out that light year is generally used to measure a great distance (one light year is equal to 63240 astronomical units). For example, the distance between the solar system and another star (known as the nearest star to the solar system is nearby Centaurus, which is 4.22 light years away). It takes about eight minutes for light to reach the earth from the sun. We say that the distance between the earth and the sun is eight "light minutes", The second gap is a very common unit. One second gap is equal to 3.26 light years. The diameter of our galaxy, the Milky way, is about 100000 light years, The journey experienced by the people on board is actually only a few minutes. This is due to the time expansion of the mobile clock in special relativity