The Cosmic Distance Scale

We all are familiar with some of the most common units of length that are meter(m) and kilometer(km).

But every branch of science has its own order of work field, and so the units of measurement which are convenient to the scientists or researchers or any person who is studying that branch. For example, the material scientist is more convenient with Micron (10^-6 m), atomic physicist deals easily with Angstrom (10^-10 m), a nuclear physicist deals with Fermi (10^-15 m), and so on. Although there is an SI unit of length i.e. meter, it would be so absurd to state the size of an atom in meters and kilometers.

The space beyond the Earth is so incredibly large that the units of measurement that are convenient for us in everyday life can become gigantic. The distances between planets, and especially between stars, can become so big in miles and kilometers that they become difficult to manage.

When we talk about the enormity of the universe, it's easy to come up with big numbers - but it's much harder to understand the magnitude, the distance, and the multitude of celestial bodies. Our own galaxy is a  collection of stars bound together by gravity, spinning in a spiral through space. Based on the deepest images obtained to date, it is one of about 2,000 billion galaxies in the observable universe. Groups of them are linked in galaxy clusters, and these are in superclusters;  superclusters are arranged in vast sheets that stretch across the universe, interspersed with dark voids and giving the whole a sort of spider web structure. Our galaxy may contain 100 to 400 billion stars and is about 100,000 light-years long. It seems huge, and it is, at least until we start comparing it with other galaxies. For example, our neighboring galaxy Andromeda is about 220,000 light-years across. Another galaxy, IC 1101, spans up to 4 million light-years.

So, just as in the case, Astronomy also has its own special units of measuring distances between heavenly bodies in the universe. 

There are three major units of distance in Astronomy which are: 

• Astronomical Unit (AU) 

• Light Year (LY)

• Parsec (pc)

Let us discuss their concepts and importance.

Astronomical Unit (AU)

Astronomical Unit, abbreviated as AU, is the average distance between the Sun and the Earth and their exact value is 149, 597, 870, 700 meters, or about 150 million kilometers (93 million miles).

Now some of you might be thinking that the orbit of revolution of all the planets of our solar system, around the Sun is elliptical instead of circular and so the distance between the Earth and Sun keeps on changing over its revolutionary period. Yes, you are right!! The AU was originally defined as the semi-major axis of the Earth's elliptical orbit. But in 1976, this definition was revised by the International Astronomical Union (IAU) for greater precision.

Now the AU is defined as the distance from the center of the sun to the point at which a particle with almost negligible mass, in a stationary circular orbit, would have an orbital period of 365.2568983 days (also known as Gaussian Year).

(Visualization of 1 Astronomical Unit)

Significance of Astronomical Unit

The astronomical unit makes it easy to think about the distances between objects in the solar system. They make it easy to see that Jupiter is five times farther from the sun than earth and that Saturn is two times farther from the sun than Jupiter. This is possible because now we are technically expressing each distance as the ratio of the distance from the earth to the sun, which is way more convenient. Astronomical units are a great way to truly compress astronomical numbers into a more manageable size.

Light Year (LY)

A light-year is technically defined as the product of a Julian Year (365.25 days) and the speed of light (299,792,458 m\s). The numeric value of this distance is 9.46 trillion Km or 63,241.077 AU.

As the name suggests, it is the distance which a photon of light travels in the course of one year (more accurately Julian Year). A light-year is the parent unit, from which we can also derive its smaller form of units like light hours, light minutes, and light seconds. 

(Overview of Light Year)

Significance of Light Year

The Light year provides a useful perspective on the distances of the solar system: The sun is about 8 light minutes from Earth. And because light travels from the objects at the speed of light, you see the Sun, Jupiter, or a distant star as it was when light escapes from its surface, be it 8 minutes, ten minutes, or 4.3 years. And this is fundamental to the idea that the more deeply we look into the space web, the more we look into the past.

(Think about it: you see all the stars in the night sky at different times in history, some few years ago, some hundreds of years ago, all at the same time). 

Parsec (PC)

Parsec is the unit that comes into the role when the number of light-years between objects increases to thousands or millions. The value of one Parsec is 3.26 light-years. 

You must be wondering that if there is such a less difference in a light year and a parsec, then what is the need for a new different unit? Well, the origin of this unit of measurement is a bit more complicated, but it has to do with how astronomers measure latitudes in the sky. 

To know more about parsec and its numerical value derivation, visit our blog page Parsec in Astronomy.

Significance of Parsec

The parsec method is the most basic calibration step for determining distance in astrophysics. Since the parallax measurement limit of the ground-based telescope is 0.01 arcsecond, it is not possible to accurately measure stars exceeding 100 parsecs. This is because the Earth's atmosphere limits the sharpness of star images. However, the space telescope is not limited by this effect. Parsec, kiloparsec, and megaparsec are used to indicate the distance to a deep-sky object.