Light Years for Dummies like Me

on Tuesday, 03 January 2017.

An easy to understand method for deep sky distances

Throughout the year at the Oregon Observatory, the staff is constantly telling our guests that such and such deep sky objects are “x” number of light years away from us.  We go on to explain that a light year is approximately 6 trillion miles, the distance that light will travel in a year at the speed of light (light is the fastest moving thing in the universe – about 186,000 miles per second).  The reason celestial objects beyond our solar system are measured in light years is because they are just so far away.  The closest star system to us, Alpha Centauri, is about 4.4 light years away from us or about 25,863,000,000 miles from our Sun. That’s almost 26 TRILLION miles! These numbers get unwieldy in a hurry!

Robert Burnham Jr., author of a standard astronomy reference book series – Burnham’s Celestial Handbook - figured out a very simple method for sharing the distance of a light year and ultimately distances in our universe in a very familiar scale – namely inches and miles.

Burnham discovered that the number of Astronomical Units (distance from Earth to the Sun) in a light year and the number of inches in a mile are almost exactly the same. Generally, there are 63,000 Astronomical Units in a light year and 63,000 inches in a mile. This wonderful coincidence allows us to interpret the vast distances in the universe into a scale we use almost every day – inches and miles.

For example, the Astronomical Unit between the Earth and the Sun is 93 million miles, in Burnham’s scale, that is equal to one inch. It takes light about 8 minutes to travel that one inch.  The Alpha Centauri system, at 4.4 light years away scaled in this way, is about 4.4 miles away from us. As objects get further and further away, the numbers still get big but not anywhere near as large as they would be in regular miles that are translated from light years.


Based on the above system, some of the objects we frequently view at the Observatory would translate into these numbers - all distances are approximate:

The Sun – 1 inch

Mars - .37 inches (varies)

Jupiter – 4.3 inches (varies)

Saturn – 8 inches (varies)

Arcturus – 37 miles

Albireo – 39 miles

North Star/Polaris – 43.4 miles

Beehive Cluster – 54.7 miles

The Pleiades – 440 miles

M11/Wild Duck Cluster - 554 miles

M42/Great Orion Nebula 1,344 miles

M13/Great Hercules Cluster 24,000 miles

Galaxies are much farther away but Burnham’s scale brings them down to size as long as you remember the one inch Earth to Sun distance

Andromeda – 2 million 500 thousand miles

Whirlpool – 37 million miles

Sombrero – 65 million miles

Deep Sky distances are hard to fathom.  I know in my mind, it’s difficult to even conceive the vast stretches of space between us and other places.  “Star Trek” made it somewhat simple using a warp speed drive but even then, most places they visited were within our Galaxy.  As noted above, Light travels faster than anything else in the universe that we know of. Time to travel through the deep sky miles is another whole article. Hopefully the above makes it a LITTLE easier to understand.

--Larry Cerullo



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