What is that?–LIGHT YEAR-a measure of distance–not a measure of time

Image: Prism refracting white light into a rainbow. Text: LIGHT YEAR--A measure of distance--not a measure of time--the light year helps astronomers describe the vast distances between the stars.

Today is Wednesday, so it’s time for our regular feature What is that? Here’s today’s term:

Light Year – a unit of length used to measure great distances in space

Light Year is a confusing term.  We all know what a year is.  A year is the length of time from your birthday to your next birthday.  It is also the length of time it takes the Earth to travel around the sun one time.  That idea may help us.

A year measures time.  It is how long the Earth takes to travel around the sun.  We could make up a term called an Earth Year.  It would mean how far the Earth travels in one year — or how far that trip around the sun is.  By the way, that is about 584,000,000 miles.  (If you live anywhere besides the United States, then that is about 940,000,000 kilometers.)

If an “Earth Year” (my made up term) is how far the Earth travels in one year, then a Light Year is how far a beam of light travels in one year.  Light travels very, very fast.  It can travel from the sun to the Earth in about 8 minutes.  That is about 92,960,000 miles (149,600,000 kilometers).  It takes me more than 8 minutes just to run one mile.

Sunlight on the Earth. Image: NASA.

According to scientists, light is the fastest traveler in the universe.  In fact, there is a rule of physics that says nothing in the universe can travel faster than light.

Light travels slower through glass and water than through air or a vacuum. Source: NASA.

Light also travels at a constant speed in the vacuum of space.  (It does slow down when it passes through air, a window, or water.  But even then, it travels super fast.  Michael Phelps is fast in water, but light is still faster.)  In the vacuum of space, light does not get tired and stop flying along.

A digital version of the Sun’s spectrum created from observations captured by the Fourier Transform Spectrometer at the McMath-Pierce Solar Facility at the National Solar Observatory on Kitt Peak, Arizona. Source: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF.
Proxima Centauri. Image: NASA.

Because light travels so fast, it travels very far in one year.  That makes a Light Year–a measure of distance–very useful for measuring the vast distances between stars.  For example, the nearest star to our sun is Proxima Centauri.  It is about 25,000,000,000,000 miles away (40,000,000,000,000 kilometers).  It’s much easier to say (and to write) 4.25 light years.

So, when someone says “That is light years away,” they mean it is very far.  They are not talking about time or how quickly a human could travel that distance.  They are just talking about how large the distance is.

Invent your own measurement!

Let’s invent our own measurement of distance.

First, decide what you will use to set your measurement length. Will it be the length of an object? (To measure small distances, we could use the length of a twig we found.) Will it be the distance that something travels? (Like how far you can ride your bike in five minutes?)

What will you use your new measurement length to measure? The length of your hand? Then length of a car? The length of your neighborhood?

Finally, what will you name your measurement length? An “twig unit”? A “bike in 5”? Will you name it after yourself? (A “Matthew Mile”?) Or create a shiny, brand-new name you made up yourself? (A “plenth”?)

Please post your comments below.

Be stellar!

Matthew Cross

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