One Big-Ass Universe

March 19, 2012

Part of the problem in understanding the universe is its immense size.  Numbers measured in divisions like parsecs, light years and astronomical units mean little to nothing to most of us.  To help understand, here are some truly nerdy facts about our universe broken down so the average human can understand them:

  • Light travels 186, 282 miles in one second. That means a ray of light could circle Earth 23.5 times in one second.
  • As Earth orbits the Sun, its distance is between 91 million and 94 million miles.  It takes light from the Sun about 8 minutes and 20 seconds to reach us.  That same ray of light would circle Earth about 11,689.5 times in the same period.
  • Earth’s average diameter is 7,913 miles.  The sun’s average diameter is 864,300 miles. That is 109 times larger than Earth.
  • One of the brightest stars in the night sky is Rigel, in the constellation Orion.  Its diameter is 65 million miles.  That is 75 times larger than the Sun.

Now remember, light makes 23.5 trips around Earth in one second.  It takes light 5 minutes and 48 seconds to travel around Rigel one time. In fact, Rigel takes up about 70% of the distance between the Earth and Sun. Just image the tan we’d get if we orbited Rigel!

  • Our solar system is pretty big, to us at any rate.  Counting Pluto as its edge ( I know Pluto is not the real edge but all that gets “nerdier” than I wish to go ) That same ray of light that circled Earth 23.4 times in a second takes about 5.5 hours to reach Pluto.  Look at it this way; it is almost 3 billion miles from the Sun to Pluto.  That means you would need to circle Earth over 379,122 times to cover the same distance.

We have only reached the edge of solar system, using Pluto of course, and the numbers based on a unit we know and understand, like a mile, is useless.  After all, when was the last time you took a 375 million mile trip?  This is why astrophysics comes up with measurements like parsecs.  It helps define the great distances of space in numbers easier to digest.  Here is what some of the common units really mean:

  • Astronomical Unit: au for short. To avoid the lengthy and uber-nerdy definition of au, think of it as the distance between the Earth and Sun.  Or about 92 million miles.  In early astronomy, it sort of made sense as a handy break point.
  • Light-year: ly for short. It is simply the distance light travels in 365.25 days. It is a big number, remembering light circles Earth 23.5 times a second and a year has 31,556,926 seconds; light makes 741,587,761 trips around the Earth in a year.  Another way to see it; a light-year is about 5.9 trillion miles or 1,966 times longer than from Pluto to the Sun.  Having said all that, and just to show how far above mere mortals astrophysicists are, light-years is a term they use for us laymen.  Astrophysicist like to use the next measurement unit – parsec.
  • Parsec: pc for short.  Unlike au and ly, a parsec is based on pure mathematical theory.  Au uses an average distance and is sort of arbitrary; ly uses a distance measurement based on time.  Parsec measures distance based on good ol’ geometry.  A parsec is the distance from the Sun to an object in space which has a parallax angle of one arcsecond.  The name comes from the parallax of one second.   I know, I know, you did not sign up for a math class.  For us it simply means it is about 3.2 ly or 19 trillion miles. Do I really need to figure out how many orbits around the Earth that would be? Ok, it’s just over 2.4 billion trips about Earth.

There are other even larger, units of measure for space.   Trying to convert them down to size is pointless as the reference number itself become a laughably large.  We are talking billions of stars spread over trillions of miles.  The universe is one really big place.

When we think about the universe, we must account for its size.  Just in our solar system alone there are immense size differences to deal with.  If we think of the size of Earth as a peppercorn, Jupiter would be a golf ball and the Sun a volleyball.  At the same scale, Earth would be about 25 paces from the sun and Jupiter about another 110 paces.  I guess Walt Disney was right; it is a small world after all.  Maybe it’s just one big-ass universe.



  1. Thanks for posting Michael. V-A-S-T to grasp! Eh?

    • Yes, to say the least.

  2. salutations from over the sea. precise post I will return for more.

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