What is parsec? Observing a star from different points of the earth's orbit, we will notice that its position changes against the background of other, more distant stars. This unit of measurement is based on this effect.
Since astronomy operates with huge distances, the usual units of measurement, for example, kilometers, are not suitable for expressing them. Scientists were forced to invent others. One of them is parsec.
What is the essence of the method?
Look at your outstretched finger with one eye, then close it and look with the other. You will easily notice that its position has changed relative to the distant background. This phenomenon is called parallax. In the same way, you can look at a distant astronomical object (for example, a star) from different points of the globe or from different points of the Earth's orbit as it revolves around the Sun. For the observer, the position of this star will change against the background of other, more distant stars. Next comes geometry. Knowing the distance between the points of our observation and the angle of displacement of the object, you can calculate the distance to it.
Astronomers have agreed that they will call a parsec the distance from which the average radius of the earth's orbit is visible at an angle of one second of arc. We are talking about a second, by which angles are measured, not time: it is equal to 1/60 of an arc minute, and one minute is 1/60 of a degree.
I will explain it again. Imagine the radius of the earth's orbit (roughly speaking, the distance between the Earth and the Sun) as a bright line. Imagine that this segment moves away from us until it becomes visible to us at an angle of 1 second. When this happens, the distance to it will be 1 parsec.
If translated into the usual units of measurement, then 1 parsec is equal to 31 trillion kilometers, or 3.259 light years. The latter value means that light from an object located at a distance of 1 parsec will travel 3.259 years to us.
Here are some examples. The closest star to the Sun, Proxima Centauri, is 1.3 parsecs away from it. The distance to the nearest galaxy Andromeda is 770 thousand parsecs (or 0.77 megaparsecs). The diameter of our Milky Way galaxy is 30 thousand parsecs (or 30 kiloparsecs).
