For their calculations, astronomers use special units of measurement that are not always clear to ordinary people. It is understandable, because if cosmic distances were measured in kilometers, then the number of zeros would ripple in the eyes. Therefore, to measure cosmic distances, it is customary to use much larger values: an astronomical unit, a light year, and a parsec.
The astronomical unit is quite often used to indicate distances within our own solar system. If the distance to the Moon can still be expressed in kilometers (384,000 km), then the closest way to Pluto is about 4,250 million km, and this will already be difficult to understand. For such distances, it is time to use the astronomical unit (AU), equal to the average distance from the earth's surface to the Sun. In other words, 1 a.u. corresponds to the length of the semi-major axis of the orbit of our Earth (150 million km.). Now, if we write that the shortest distance to Pluto is 28 AU, and the longestthe path might be 50 AU, which is much easier to imagine.
The next largest is the light year. Although the word “year” is present, you should not think that it is about time. One light year is 63,240 AU. This is the path that a ray of light travels in 1 year. Astronomers have calculated that it takes more than 10 billion years for a ray of light to reach us from the farthest corners of the universe. To imagine this gigantic distance, let's write it down in kilometers: 950000000000000000000000. Ninety-five billion trillion familiar kilometers.
The fact that light does not propagate instantly, but at a certain speed, scientists began to guess since 1676. It was at this time that a Danish astronomer named Ole Römer noticed that the eclipses of one of Jupiter's moons began to lag, and this happened precisely when the Earth was heading in its orbit towards the opposite side of the Sun, the opposite of where Jupiter was. Some time passed, the Earth began to return back, and the eclipses again began to approach the previous schedule.
Thus, about 17 minutes of time difference was noted. From this observation, it was concluded that it took 17 minutes for light to travel a distance the length of the diameter of the Earth's orbit. Since it has been proven that the diameter of the orbit is approximately 186 million miles (now this constant is 939,120,000 km), it turned out that a beam of light moves at a speed of about 186 thousand miles per second.
Already in our time, thanks to Professor Albert Michelson, who set out to determine as accurately as possible what a light year is, using a different method, the final result was obtained: 186,284 miles in 1 second (about 300 km / s). Now, if we count the number of seconds in a year and multiply by that number, we get that a light year is 5,880,000,000,000 miles long, which is 9,460,730,472,580.8 km.
For practical purposes, astronomers often use the unit of distance known as the parsec. It is equal to the displacement of the star against the background of other celestial bodies by 1'' when the observer is displaced by 1 radius of the Earth's orbit. From the Sun to the nearest star (this is Proxima Centauri in the Alpha Centauri system) 1.3 parsecs. One parsec is equal to 3.2612 sv. years or 3.08567758 × 1013 km. Thus, a light year is slightly less than a third of a parsec.
