Question

One parsec is equal to how many light years?

Answer 1

The concept behind parsec and its relation with light years needs to be probed upon. The answer relates to the experimental value that was determined to measure the light years in terms of parsec. The application of the concept of parsec is required in order to determine why the convention between them was made and why it was named so.

Complete answer:
In order to determine the relationship between a light year and a parsec we first need to know what they are. The above problem, the concept of light years and parsecs, comes under a branch of physics called astronomy. A light year is said to be the distance light travels in one year or in a single Earth year, that is, the time the Earth takes to complete one revolution around the sun in its own axis.

Knowing this we will now look at the concept of parsecs. Parsec is a short term or an acronym for the words parallax seconds. We have seen the use of various instruments like telescopes which help in stars, planets and other interstellar objects that are far away more closely. Similarly, the concept of parsec comes into picture when viewing these interstellar objects which are far apart from our planet.

Our planet Earth as we know is in constant motion and the Earth is said to revolve around the sun. Due to this motion which constantly varies with time, a sort of error occurs when viewing these objects from space. There occurs a parallax error wherein the nearby objects are said to be appearing more distant from what it actually is.

The effect is like an illusion where our viewpoint keeps changing over a period of some months due to the constant revolution of the Earth and its corresponding change in position of the Earth.For example when we see a particular star in the sky the Earth will be at one position, that is, it will be viewed from one side of the Sun’s orbit and when viewing the star again after a certain time period the same star could be appearing elsewhere since the Earth continuously orbits the sun.

The difference in position of the apparent image from its original location is said to be measured through an angle. This angle is created when we consider two different lines of sight drawn from the Earth’s orbits which seem to make a triangle and thus this angle which is measured is known as the parsec or the parallax angle.

Therefore parsec is defined as half the angle of the triangle’s apex .In other words, as per the concept discussed above a single parsec is said to be the distance to a star whose apparent position shifts by one arc second from one side of the Earth’s orbit to another. The term arc second refers to nothing but the measurement of the angle formed and arc come into play since the Earth moves around the Sun in orbits.

Since the distance of far objects in space are measured through light years there is said to be a relation between a parsec and a light year. Since the parsec is said to be the distance to the star which is to be viewed, the distance can be measured in terms of light years. For a single parsec to occur, that is, for a parallax angle of one parsec the star must be at a distance of $3.26$ light years approximately from the sun.

Hence one parsec is said to be equal to $3.2615$ or $3.26$ light years approximately.

Note: Parsec is a measure of distance and not time which is often mistaken since light years are a measure of time as the name suggests and since these two are related parsec is also mistaken to be a measure of time. Parsecs are preferred over light years since astronomers find the concept of parsecs much easier to handle since they give the distance between the interstellar objects in an easier manner which was useful in astronomy.