In our daily life, the unit of distance often used is meters, kilometers, for example, my height is a few meters, how many kilometers are between two cities, the earth's equator is more than 40,000 kilometers, etc., however, when this scale is enlarged to the entire solar system and even outer galaxies, this length unit is very pale and weak, so in 1976 the International Astronomical Union established the concept of astronomical units, the astronomical unit is about 149.6 million kilometers, which is equivalent to the distance between the sun and the earth.
However, with the improvement of observation technology, the astronomical distance is not enough when measuring celestial bodies outside the solar system, so the distance unit of light year is used, and the speed of light has actually been calculated very early, but the early understanding of it is generally imperfect, such as the Bessel era, when the speed of time has not been listed as a constant, until the eighties of the last century was recorded into the astronomical constant system began to use. A light-year is the distance that light travels in a year, the speed of light is about 300,000 kilometers per second, and 1 light-year is equivalent to 63,240 astronomical units.
The light year is the largest unit of length in people's minds, but on the one hand, because light travels for different times in different mediums, and the universe continues to expand, there will be errors when calculating the distance between two celestial bodies in light years, and moreover, the light year and the universe ratio are still too small, and the light emitted by the stars in the universe is all light produced a long time ago, so at the same time the stars may no longer exist, but you will not be able to observe it until a long time later, or you will not be able to observe it in your lifetime. The Milky Way alone is more than 100,000 light-years in diameter, and the Laniakea Supercluster spans 520 million light-years.
As a result, light-years are often featured in popular science articles, science fiction films, and novels, but what most people don't know is that professional astronomers use parsecs as a unit of distance, and parsecs are set according to the parallax angle between celestial bodies.
The parallax angle is the difference between the position of a foreground object relative to the background object seen by the left and right eyes, and the distance between the foreground object and you can be calculated theoretically, because the distance between your two eyes is known, and the apparent displacement of the object is known. This is the same principle as seeing the position of a nearby object change relative to the background object during the Earth's revolution.
Parallax describes the change in the position of an foreground object and can be used astronomically to determine the distance of an foreground object. Astronomers stipulate that a 1 parsec is that if the parallax of the distance between the Sun and Earth at a star is exactly one arc second, then the distance between the star and the Earth is 1 second, and the distance can be calculated using the angle formula of a right triangle, which is equivalent to 3.26 light-years. However, it should be noted here that the 2-parsec gap is not mechanically equal to the addition of 2 1-parsec gaps, but has a trigonometric relationship.
Of course, this is true in theory, but in practice, because the angle is very small, the tangent or sine of the angle can be directly replaced by radians, so that the size of the angle is inversely proportional to the size of the distance, and the 2-second difference (the angle is 1/2 second) can be approximately seen as 2 times the 1-second difference. In practical application, astronomers will also add thousands (K), millions (M), billions (G), etc. in front of the parsecs (pc), which can be used to describe the distance between different galaxies or celestial bodies, which is indeed a higher-order existence compared to light years.