How simpler words, the more there are. I warned you - now don't complain!
The Earth has an elliptical orbit. An ellipse, unlike a circle, does not have a “radius”, but has two “semi-axes” of different lengths - major and minor. Accordingly, there are two points in the earth's orbit that lie on the major axis and are the most distant from each other compared to any other pair of orbital points. Exactly in the middle of the segment between these points we draw a perpendicular to the plane in which the orbit lies (the ecliptic plane). An observer moving along a perpendicular will see the Earth's orbit under different angles. That is, if we draw rays from the observer’s location to the two previously mentioned points in the Earth’s orbit, the angle between the rays will depend on the distance to the ecliptic plane. Very close to the plane the rays form very obtuse angle(almost 180°). Very far - very sharp (almost 0°). And there is a distance at which this angle will be equal to exactly 2" (two arc seconds; one second is equal to 1°/3600). This is a parsec.
For a stationary alien sitting on the above-described perpendicular one parsec from the Earth and able to somehow see it (this is quite difficult, since the Earth is not bright enough for such a distant observer), the Earth will change its apparent location slightly due to its orbital movement. The displacement angle between the two extreme visible positions of the Earth will be exactly 2" (we specifically placed the alien at exactly this distance to obtain such a displacement angle). And relative to a certain "average" visible location, the Earth will move a maximum of 1" (half from 2"). An alien may say that the "annual trigonometric parallax" of the Earth is 1" (one arcsecond). And call the distance to Earth a “parsec” (PARALLAX - SECOND).
The parsec was needed, of course, not by aliens, enthusiastically observing the Earth from perpendicular to the ecliptic, but by terrestrial astronomers. The stars are so far away from us that they own movement does not lead to a change in position in the sky even within a year. But they seem to “rotate” in the sky in a circle due to the rotation of the Earth around its axis (one revolution per day). In addition to this, the stars ADDITIONALLY “move” across the sky due to the movement of the Earth in orbit, although this is hardly noticeable (for complete happiness, another influence will be added earth's atmosphere and hesitation earth's axis, but let’s say we took this into account and overcame it). If you try really hard, you can identify this subtle (against the background of daily “rotation” and other interference) movement and measure the annual trigonometric parallax of the star. And if the star were located near the above-described perpendicular to the ecliptic and had an annual parallax of 1", then it would be (damm!) exactly one parsec from us. After all, in the reference frame associated with the Earth, it is not the Earth moving in an elliptical orbit , and for some reason the rest of the world makes a similar movement, but in reverse side. Accordingly, for an earthly astronomer watching the above-described alien (or the star next to it), this alien (or the star next to it): 1) for some reason rotates around the Earth at wild speed (with full turn per 1 day) and 2) additionally moves along an elliptical orbit (with a full revolution of one year and semi-axes, like the earth’s), parallel to the ecliptic plane.
The distance to the remaining stars can also be easily calculated (only geometry with trigonometry and nothing more) in parsecs, if you can measure their annual parallax and (additionally) take into account their position in the sky. The parsec itself is equal (by definition and from trigonometry) to the cotangent of 1", multiplied by the semimajor axis earth's orbit(per "astronomical unit"). Small angle cotangent equal to one, divided by the angle itself in radians. 180° is pi radians, 1° is pi/180 radians, 1"=1°/3600=pi/(180×3600). Cotangent 1" is 180×3600/pi≈206.000. Accordingly, a parsec is approximately equal to (slightly more) 206 thousand “astronomical units” (semi-major axes of the earth’s orbit). And since we know the parameters of the earth’s orbit (including its semimajor axis), we can express the parsec itself in other distance units (meters, light years, etc.) - this is approximately 3.2 light years. The stars closest to us have an annual trigonometric parallax of less than (but on the order of) 1" and, accordingly, are located at a distance of more than (but on the order of) one parsec.
Image source: mattbodnar.com
Because of its uniqueness, every person who watched this cartoon remembered this word.
“It’s not far here, a hundred parsecs!” - thus Gromozeka, one of the heroes of “The Secret of the Third Planet,” reported the distance to the planet to which he recommended flying to Prof. Seleznev and his team.
However, few people know what exactly parsec means, what distance we're talking about and how far the characters of the popular cartoon were forced to fly.
Meaning of the term "parsec"
This term was derived from the words "parallax" And "second", which here represents not a unit of time, but an arc second - an extra-system astronomical unit, which is identical to a plane angle second.
Parallax - changing location celestial body depending on where the observer is located.
Modern astronomy distinguishes the following types of parallax:
Daily– the difference in directions to a certain star in both the geocentric and topocentric directions. This angle directly depends on the height of the celestial body above the horizon.
At annual parallax changes in direction to a certain object directly depend on the rotation of the Earth around the Sun.
Concerning secular parallax, then it makes it possible to determine the difference in the direction of a celestial body depending on its movements in the Galaxy.
Parsec - meaning of the term
If we talk accessible language, then “parsec” is a unit of change in the distance between celestial bodies located outside solar system. Typically parsec is used to calculate distances within Milky Way. These are basically multiple units: kiloparsecs, megaparsecs And gigapersecs. Submultiple units are usually not used because it is more convenient to use standard astronomical units instead.
Parsec greatly simplifies calculations for astronomers, because it is much easier to say that the distance from the Sun to a certain star is one and a half parsecs than it is more than 46 trillion km.
Who invented parsec?
in 1838, the German Friedrich Bessel was the first to achieve success in measuring the distances to objects in space. He was the first to produce accurate calculations Cygnus stars 61 annual parallax. To calculate the distance from this star, Bessel used the old method, calculating the difference in angles resulting from two measurements.
Determining the distance to stars using the parallax method. Image source: bigslide.ru
First, measurements were taken with the Earth facing the Sun on one side, and six months later repeated measurements were taken (with the Earth facing the Sun on the other side).
However, the term “parsec” itself appeared only in 1913 thanks to the English astronomer Herbert Turner.
How is parsec calculated and what is it equal to?
Schematic representation of a parsec (not to scale) Image source: wikipedia.org
One parsec is defined as the distance at which one astronomical unit (the average distance between the Earth and the Sun) represents the angle of one arc-second.
The annual parallax is used to calculate the parsec. When using an imaginary triangle with right angles, parsec is the distance to the star, provided that its parallax is 1 arcsecond.
A parsec is 3.26 light years or about 30 trillion km. It represents one of the first methods for determining distances to stars and is designated as "pc"
The essence of parsec is to use the principle of parallax to determine the distance to celestial bodies in space due to their tiny shift as the Earth moves around the Sun.
Some distances to space objects in parsecs:
The distance to the star closest to the Sun, Proxima Centauri, is 1.3 parsecs.
The distance from the Sun to the center of the Milky Way is about 8 kiloparsecs.
The distance from the Sun to the Andromeda nebula is 0.77 megaparsecs.
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In the section on the question What is 1 parsec equal to? given by the author chevron the best answer is 3.2616 light years
Source: wikipedia.org
Answer from Lysander[newbie]
1 light year. I don’t remember exactly, but one light second is the distance from the Earth to the Moon, so you can look it up in a reference book and calculate how much it will be per year))
Answer from Plane[newbie]
The distance a beam of light travels at a speed of 300,000 km/sec in one year.
Answer from AB[guru]
Parsec (abbreviated pc) is a non-systemic unit of distance measurement common in astronomy. The name comes from parallax arcsecond and denotes the distance to an object whose annual trigonometric parallax is equal to one arcsecond. According to another equivalent definition, a parsec is the distance from which average radius Earth's orbit (equal to 1 AU), perpendicular to the line of sight, visible at an angle of one arcsecond (1″).
Answer from Help[guru]
A light year is the distance that light travels in a year.
Light travels from the Earth to the Moon in a little more than a second.
A parsec is the distance at which the Earth is visible at an angle of one second (1/3600 of a degree). I don’t remember exactly, it’s a little more than 3 light years.
Answer from Larisa Krushelnitskaya[guru]
A parsec is the distance from which the semimajor axis of the Earth's orbit appears at an angle of 1 arcsecond. That is
1 parsec = 1 astronomical unit / sin 1”
sin 1” = π/(180 60 60) = 1/206264.806
1 parsec = 206264.806 astronomical units =
= 206264.806 149 597 870.691 km = 3.08567758 10^13 km
Answer from Dmitry(C.)[guru]
1 parsec (parallax/second) is the distance at which an object has a parallax of 1 arcsecond. There are 3.26 light years in one parsec, or 206,265 astronomical units (the distance from the Earth to the Sun), or 31 trillion kilometers (3.1 * 10 to the thirteenth power).
Parsec is a cosmic unit of measurement with which astronomers determine the distance to particularly distant objects in the Universe.
Parsec (abbreviated “parallax second”) is a non-systemic unit of measurement used in cosmology to measure distances to particularly distant objects outer space. This unit not only performs practical function– helps to calculate the distance to a particular object in the Universe, but also creates a kind of comfort for astronomers. Judge for yourself, it is much easier to say that the distance from the Sun to the nearest star is 1.3 parsecs than to say that it is 40.7 trillion kilometers. A person who would regularly operate with numbers like this a huge amount zeros, sooner or later he would go crazy. Thus, by inventing the parsec, scientists significantly simplified computational processes in astronomy.
Parsec is a popular unit of measurement in astrophysics. Fans of this science are well aware that it is equal to 3.2616 light years. Many of them can freely name the distance to one or another distant object in parsecs. But, unfortunately, not everyone understands how this unit of measurement was born and how to calculate it correctly.
History of discovery
If distances to close objects in space can be measured using a radio telescope with an accuracy of a few centimeters, then measuring distances to distant corners of the Universe is much more difficult. However, scientists needed to find a way to calculate this value and they decided to use the horizontal parallax method, which is well known in geometry.
The essence of the horizontal parallax method is simple: if you look at a distant object from different places, then against the background of other, more distant objects it will change its position. Knowing the distance between the places from which observation is carried out, as well as the angle of displacement of the object against the background of distant objects, you can calculate the distance to it by geometric calculations. Astronomers decided to take advantage of this axiom; it served as the basis for the discovery new unit measurements - parsec.
How to determine parsec
Let's say you're looking at a star and want to determine its distance in parsecs. But to do this, you need to know what a distance of 1 parsec is. This distance represents the displacement of a celestial body against the background of other, more distant objects by an angle equal to one arc second when the observer moves half the diameter of the earth's orbit.
Some may find this definition difficult to understand. In fact, the essence of the definition of parsec is not that difficult to understand. Returning to our star, the distance in parsecs to which we want to determine, we will have to make two observations of this object with different points earth's orbit. This can be done without any space devices, but simply by waiting for the Earth itself to pass half of its annual path and stand on the opposite side Sun.
Knowing the length between the points from which observations were made (it is equal to 1 astronomical unit– the distance of the Earth from the Sun or the radius of the Earth’s orbit), as well as the displacement of the star against the background of more distant stars and galaxies, we can calculate the distance to it. If in the observed range the star has moved by 1 arc second, the distance to it is one parsec, but if it has moved by half a second, it is two parsecs. Contrary to guesses, the smaller the parallax (displacement) of a celestial body, the more parsecs there are to it.
Distance between space objects are not comparable to those on Earth, and one could “drown in zeros” by measuring them in kilometers. That’s why astronomers needed special units for measuring distances, and one of them is the parsec.
What does this word mean
The word “parsec” is made up of two words: parallax and.
Second in in this context– this is not about time, but about angle. As you know, angles are measured in degrees, each of which is divided into 60 parts, called, and each is subdivided into 60 seconds.
Parallax is the displacement of an object relative to the background, determined by the position of the observer. Astronomers deal with three types of parallax - daily, annual and secular. In relation to the parsec, it is the annual one that is of interest.
By determining the annual parallax of a star, astronomers calculate the distance from Earth to it. To do this you need to build an imaginary right triangle. The hypotenuse in it will be the distance from this star to the Sun, and one of the legs will be the semimajor axis of the Earth's orbit. The size of the angle in this triangle corresponding to the star is the annual parallax.
The distance to the star at which this angle is one second is called a parsec. The international unit of this unit is pc, and in Russian it is designated as pk.
What parsec
When they talk about long distances V cosmic scale, they are often measured in . This unit of measurement corresponds to the distance that a light ray will travel in a year, and it is equal to 9,460,730,472,580.8 km. An impressive size, but the parsec is even larger!
A parsec is 3.2616 light years, which is 30.8568 trillion km. It is this unit of measurement, and not the light year, that professional astronomers usually use. Distance in light years is often indicated in popular science publications or fantasy novels and films.
But even this unit of measurement turned out to be insufficient for the needs of space exploration. We had to introduce units equal to a million parsecs - kiloparsecs (kpc) and megaparsecs (Mpc).
Thus, the distance that the heroes of “The Secret of the Third Planet” were asked to overcome turns out to be very impressive. 100 pc is more than 326 light years! However, modern astronomy knows even greater distances. For example, the distance to the Virgo cluster, the closest cluster of galaxies to Earth, is 18 Mpc.
