Tag Archives: Size

The Size of the Cosmos

by Shane L. Larson

As many of you know, I ascribe much of my aspirations in life as a scientist to being exposed to Cosmos at a very early age.  Within the first five minutes of the first episode, Carl said a very big thought: “The size and scale of the Cosmos are beyond ordinary human comprehension.”

As I have grown into my career in science, I have lost sight of this simple fact. I’ve learned to write big numbers. I’ve learned to convert between meters and kilometers and lightyears when needed. I’ve even learned to use “crazy relativist units” and measure distance, time, energy and mass all in meters (something that confounds my students, my parents, and many of my astronomer friends!). I’ve done this enough now that when I calculate numbers, I know if they sound right.  Two million lightyears to a galaxy in the Local Group? Sure that sounds fine.  750 Megaparsecs to a quasar? Sure, I’m down with that.  1.3 billion kilometers to Saturn?  Word.

Developing a sense for big (and small!) numbers and whether they “sound right” is an essential skill for scientists, and we spend inordinate amounts of time training ourselves and our students to be facile with them.  But that completely bypasses Carl’s point — these numbers are HUGE.  They encode how utterly small we are on the grand scale of the Cosmos!

One of my hobbies is walking Solar System Walks when I encounter them (here is a long list at Wikipedia; another list at Air & Space).  These scale models lay out the Solar System, marking the location of planets at the appropriate spatial scale to give you a sense of how large the Solar System is (forget the Universe itself).  My favorite is one in Anchorage, Alaska, known as the “Lightspeed Planet Walk”  — if you walk at normal speed, the time it takes you to reach each planet is the same time it would take light to make the journey you made.  That is awesome.  Start at Earth, and shine a laser pointer at Neptune the moment you start walking; you’ll reach Neptune at the same time your feeble green laser beam reaches the real planet Neptune!

The center of the Lightspeed Planet Walk in Anchorage, Alaska, with a scale model of the Sun.

The center of the Lightspeed Planet Walk in Anchorage, Alaska, with a scale model of the Sun.

Despite the large physical scale of these walking models, I still often feel like they don’t capture the immensity in a way that really shocks my brain. I’ve thought about this fact a lot, and suspect it is because when I’m walking the model, it feels quite ordinary.  As I’m meandering from Mars to Jupiter, I’m not really thinking about how far I’m walking. I’m distracted by my daughter prattling about why Pluto should still be a planet, and watching ducks eat algae, and avoiding speeding mountain bikers.

But a couple of weeks ago, one of my astronomy friends showed me something that blew my socks off.  It’s a very simple demonstration you can do right at home that captures how messed up my mental picture (and I’ll bet yours!) of the solar system is!  I think my mental pictures are messed up because we often show the family of the Sun all together, to better show the relative size of the planets, like the image below.

A typical representation of the Solar System, often used in books, online references, and mass media.

A typical representation of the Solar System, often used in books, online references, and mass media.

What this image fails to show, is the spacing between the worlds.  We’ve known the relative spacing of the planets for some time, the distances having been worked out using basic geometry together with clever observations (many of which can easily be done in your own backyard), and through application of the laws of physics (notably Kepler’s Laws of Motion, and Newton’s Universal Law of Gravitation).

Folding pattern to make a reasonably spaced representation of the planetary orbits in the Solar System on a long strip of paper.

Folding pattern to make a reasonably spaced representation of the planetary orbits in the Solar System on a long strip of paper.

Let me teach you the trick my friend showed me.  Get a long strip of paper (adding machine paper, or other strip paper works well), about 1 meter long.  On one end, write the word SUN and on the other end write PLUTO.  Now fold the strip in half, and unfold it again.  What object in the solar system lies halfway between the Sun and Pluto?  It is the planet Uranus; write this on the fold.  Now fold the end marked Pluto down to Uranus.  Label this as the location of the orbit of Neptune.  What does this show us?  There isn’t  much in the way of planets in the outer half of the solar system!

Now fold the end marked Sun down to Uranus.  On  this new fold write Saturn.  Fold the Sun down to  Saturn and label the new fold Jupiter.  Fold the  Sun to Jupiter and label the new fold  Asteroids.  At this point, about 93% of your strip is  between the asteroids and Pluto.  This is the part of the solar  system that is euphemistically called “The Outer Solar  System.”  Fully half of the known planets in the solar  system are still to be squeezed between the Asteroids and  the Sun!  Let’s do that next.

Fold the Sun to Asteroids, and label this fold  Mars.  The last part is two folds before labeling: fold the Sun to Mars, then fold the end over to Mars again.  The result is three folds.  Starting at the  Sun, label them Mercury, Venus and Earth.  The entire procedure creates a map with amazingly accurate spacing between the worlds (yes! I calculated the errors; I was curious!).

The results of all your folding endeavours!

The results of all your folding endeavours!

Now stare at your model for a moment.  The solar system is a lot of empty space!  The places that are easiest to get to are close to Earth, but are still very far away.  The distance to the Moon is about the width of a pencil line, and it took Apollo astronauts 4 days to cross that gulf.  Mars is six to eight months away by rocket.  Look how close it is to Earth!  It took the Cassini spacecraft almost seven years to get to Saturn.   When the New Horizons spacecraft flies by Pluto in 2015, it  will be have been outbound for almost nine-and-a-half years!  The  solar system is a big place. And the Cosmos is far vaster.

I think what amazes me the most about this model is that places I normally think of as very far away are much closer to Earth than my brain normally thinks of them.  Consider Jupiter; it is in the Outer Solar System.  But on the map, it is only 1/8th the distance between the Sun and Pluto!  Wow.

“The size and scale of the Cosmos are beyond ordinary human comprehension.”  Perhaps; certainly outside the realm of our everyday experiences. But our ingenuity gives us ways to push our brains to try to understand, and clever demonstrations like this one give you ways to ponder and think.  So get out your scissors, and start folding.

(L) The full length of the Solar System model. (R) My own version of this model, shown next to a typical Earthling.

(L) The full length of the Solar System model. (R) My own version of this model, shown next to a typical Earthling.

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