A light year is a common unit of measure in the vast universe. Many people often mistakenly think that it measures time. However, it is actually a unit of distance.
When referring to the distances of stars beyond our Sun in the night sky, we do not use miles or kilometers, but rather a measurement called a light year. Light travels at the fastest speed in the universe, moving at 300,000 kilometers per second – incredibly fast. If you could travel at the speed of light, you could circle the Earth’s equator 7.5 times in just one second.
- What is a light year? A light year is the distance that light travels in one year.
- How many kilometers is a light year? A light year is equal to 9,460,528,400,000 kilometers (9.5 trillion kilometers), which is about 5,878,499,810,000 miles.
The Orion Nebula (or Messier 42), located in the Orion constellation, is approximately 1.34 light years away from us.
A light second is the distance that light travels in one second – which is 7.5 times the distance around the Earth’s equator. So, how far is a light year? To find out, multiply the distance light travels in one second by 31,556,926 – the number of seconds in a year. The result is approximately 9.4605284 × 10^12 kilometers, or about 9.5 trillion kilometers.
While this number is indeed large, it is hard to visualize just how vast it really is. In the 20th century, astronomer Robert Burnham, author of Burnham’s Celestial Handbook, devised a way to make the distance of a light year more comprehensible. He compressed the light year down to the Astronomical Unit (AU) – the distance between the Earth and the Sun, which is about 150 million kilometers – equivalent to 8 light minutes.
Interestingly, the number of astronomical units in a light year is equal to the number of inches in a mile, meaning there are 63,000 astronomical units in a light year, and there are also 63,000 inches in a mile. This coincidence helps us easily grasp how far a light year is; if we consider the distance from Earth to the Sun to be 1 inch, then the nearest star to us – Alpha Centauri, which is 4.4 light years away, would be about 4.4 miles (approximately 7 kilometers) away.
For us to see an object, light must hit that object and then travel to our eyes. For instance, the Earth is about 8 light minutes away from the Sun, which means that light from the Sun takes 8 minutes to reach Earth. Therefore, when we look up at the sky, we are seeing an image of the Sun from the past – specifically, from 8 minutes ago.
Similarly, the closest star to Earth after the Sun is Proxima Centauri, which is 4.25 light years away. That means the light from Proxima Centauri takes 4.25 years to reach Earth, and what we see of this star is actually its image from 4.25 years ago.
When we look deep into space through telescopes, we are essentially looking back in time. Light from distant celestial objects can take millions, and even billions of years to reach Earth.
The Hubble Space Telescope can detect planets whose light has taken billions of years to arrive at Earth. Thus, the images captured by Hubble show us planets as they were billions of years ago, sometimes coinciding with the time when Earth was formed.
Light from distant stars takes billions of years to reach Earth. (Image: NASA/JPL-Caltech).
Human history has recorded many interesting events related to light years. In November 2021, NASA’s Voyager 1 spacecraft transmitted signals from a distance of 21.31 light hours, equivalent to 23.12 billion kilometers. This is the farthest distance that an artificial object has ever transmitted a signal in space.
The closest galaxy to the Milky Way is the Canis Major Dwarf Galaxy, which is about 25,000 light years away from the Sun.
The farthest known galaxy from Earth is GN-z11, discovered by the Hubble Telescope in 2016. At that time, it was believed to be 13.4 billion light years away from Earth.
Earendel, the most distant star we know of, is located 28 billion light years away from Earth. When converted to miles, this number is enormous and hard to comprehend: 270 sextillion kilometers, which is a figure difficult to visualize (one sextillion has 21 zeros).
In addition to light years, other units used to measure astronomical distances include: Astronomical Unit (AU), Lunar Distance (LD), and Parsec (pc). The lunar distance is often used to represent the distance between objects close to Earth.
Meanwhile, parsec is used to measure distances beyond the Solar System, such as the distances between galaxies. An Astronomical Unit (AU) is approximately the average distance from the center of the Earth to the center of the Sun.
Exploring space is the quest for things beyond Earth and the stars. As a unit of distance, a light year also serves as a reminder of just how vast the universe is, and how much humanity and science still have to learn.
