Why is distance in Space measured in lightyears?

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Space is so enormous that you need a very large unit to measure it. Light, as far as we know, is the fastest thing in the universe, so the distance it travels in one year’s time makes for a consistent and easy to contextualize measurement.

Like, to put it into context, one lightyear is around nine and a half trillion kilometers. that’s almost incomprehensibly large, and even then we are measuring some distant stars and galaxies in millions or even billions of lightyears.

It simplifies extremely large numbers. The distances between galaxies and solar systems are incredibly vast. One light year is equal to 5,879,000,000,000 (5.879 x 10^12) miles. It is much easier to say “one light year”, rather than “five trillion, eight hundred and seventy-nine billion miles”.

OK so the first thing is that a lightyear is a unit of **length**, like a mile. A lightyear is how far light travels in a year (25,310,000,000,000 miles). That’s a long way! But the important part is a lightyear is a *distance*.

So why do we use such a big distance unit in space? Because space is HUGE and the distances between places in space are very long. It’s the same reason you give your height in feet (or cm), the distance to the store in miles (or km) ; **we’re picking a unit that’s the right size for the numbers to be “nice” when you use that unit.**

* You *could* say that your height is 0.0009469 miles. That’s correct, but it’s messy. Saying “5 feet” just works better.

* You *could* say that your commute to work is 164042 feet. But “50 km” is much easier for people to understand and work with.

It’s the same with lightyears. You *could* say that from here to the next nearest star is 24,900,000,000,000 miles away, but that’s so huge it’s kinda meaningless, and a cumbersome number to start doing math with. Saying “4.24 lightyears” has many advantages. If we say one star is 4.2 lightyears away and another is 12 lightyears away, you can immediately realize “oh, that’s almost 3 times farther!”, but if you see 24900000000000 miles vs 70500000000000, your brain is like “…?” And since astronomy also involves lots of math, it’s nice to use a unit where the lengths are smaller more manageable numbers rather than using something like miles and then all the distances are 10+ digits long.

And as user Oblivious said, space is so big that even in lightyears some of the distances to stuff *still* gets into the trillions. Using any smaller length unit means those would be like 20+ digits long and we’d have to get familiar with the nonillion/decillion part of the numberline.

For the same reason we measure distance on Earth in feet/miles/kilometers instead of nanometers. Appropriate scale makes numbers easier to work with. You could say your drive to school is about 15,000,000,000,000 nanometers (roughly 15 quadrillion), but it’s a lot easier to discuss that distance as “about 10 miles”.

Space is *unfathomably enormous*. A light year is a *gigantic* distance (the speed of light multiplied by the time in a year, or the distance light would travel in a year). So light years allow us to give a unit that makes conversation easier and more intuitive (especially since things at astronomical distances start often having temporal effects to consider, since light and causality take time to travel to us).

Because space is really large and light (technically causality) is the fastest thing. It lets you say something like 4 light years instead of 25,310,000,000,000 miles (or 37,840,000,000,000 kilometers if you live in one of those fictional countries outside the US).