The Scale of the Universe

The Scale of the Universe is literally astronomical. Distances in the universe are large. So large that we have an entirely different set of units to grasp their distances. It is unreasonable to consider cosmic distances in our largest units (kilometres and miles) because just the distance from the Earth to the Sun is 150,000,000 kilometres.

The Light-Year

To measure distances in the universe we use a unit called light-years (ly). A light-year is the distance that light travels in one year. More generally it is the distance that any object travelling at lightspeed will travers over the span of a year of time.

Since light travels at about 300,000 km/s (186,000 Mi/s) this distance is, from our scale, enormous. The distance of a light year is about 9.46×10¹² km (or 5.88×10¹² miles). In normal English this is 9.46 trillion kilometers (5.88 trillion miles).

Given that the circumference of the Earth if you were to walk the entire equator of our planet is only about 40,000 kilometers (and that distance is enormous from our perspective) light-years are significantly larger.

Light-Years In A Cosmic Frame of Reference

Yet while light-years are large from our perspective, in the universe they are perfectly reasonable. To put them in perspective, the nearest star to our solar system is Alpha Centauri at a distance of about 4.2 light years.

In our usual distance units that is about 37.8 trillion kilometers (37,800 billion km or 3.78×10¹³km). You can see how 4.2 light-years compared to 3.78×10¹³ kilometers is much more reasonable.

The Significance of Light In The Universe

In addition to the role of light in measuring distances at the scale of the universe, light is important in a number of other ways. Analyzing starlight — all of the various types of the light in the universe in fact — is one of the main ways that we gather information about the universe.

Light can tell us the chemical elements in the atmosphere of a star, which was crucial in the journey of discovering what stars are, the mechanisms behind them, and their composition.

A stars light can also tell us about a stars motion. If a star is moving away, the wavelengths of its light are elongated, meaning that they are shifted towards the red end of the spectrum. This is called redshift. If a star is moving towards us, its light waves are compressed compared to if it was stationary, shifted towards the blue end of the spectrum. This is called blueshift.

(The Doppler Effect at the scale of the universe when dealing with light.)