Tom Jonard's Size of Things.

The difficult thing about building a scale model of the solar system is getting the scale right.  There is such a disparity in the relative sizes of things in space that it is difficult to imagine much less model.  If you wanted to make the whole thing small enough to fit in your child's room the planets would be infinitismally small.  On the other hand if you want a model that allows you to see the smaller planets (like the earth) you had better start thinking about your neighborhood rather than a room in your house.

Cosmic distances are, well, astronomical when compared with what we are used to day-to-day.  Units of measure like the mile and kilometer that work on the surface of the earth are useless off it because it takes too many of them to express a distance.  Instead astronomers deal in light years* (lys) -- the distance light traveling at 186,000 miles/second (300,000 kilometers/second) travels in a year (31,536,000 seconds).  This is the incredible (and unintelligible) distance of 5,865,696,000,000 miles (9,460,800,000,000 kilometers).  Even with such gigantic units of measure you still hear astronomers using large numbers of them to measure and describe the distance to various objects.

(*--Real astronomers also use a unit of measure named the parsec which is roughly 3.2 lys.  The parsec derives from the basic trigonometric method used to measure the distance to the nearest stars.  Which is all I'll say about that here.)

Building a model (either in reality or mentally) provides a way of grasping some of these distance scales.  One of the best for the solar system in Guy Ottwell's Thousand-Yard Model which describes how to build such a model using commonly available objects.  Since Pluto ends up being 1,000 yards from the Sun in this model (hence the name) it is an exercise in more ways than one.

The best model of the galaxy for my money comes from the video Voyage to the Milky Way where astronomer Richard Terrile imagines an average star such as the Sun modeled by a grain of salt.  The mass of the Milky Way is about 200 billion suns and there are about the same number of grains of salt in 20 tons of salt (40,000 one-pound boxes, which if you poured into a pile would neatly fill the average living room).  To make the model spread the salt into a 50,000 mile-diameter disk about 500 miles thick.  Individual grains should be about 7 1/2 miles apart.

This raises and interesting point.  Can you see a grain of salt at a distance of 7 1/2 miles?  The answer is,  No, it would be too small to see.  Yet we see the stars and they appear no larger than this.  How is that?  The answer is that the stars are simply too bright to ignore.  We perceive them not because they are large but because their power output is so massive.

Another way of grasping relative distances is to use the astronomer's light travel time concept in another way.  The distance to the sun (average) is approximately 8 1/3 minutes.  So imagine (or even find out) how far you can walk (or drive if you must) in 8 1/3 minutes.  By comparison Jupiter at opposition (opposite the Sun and therefore closest to the earth) is about 35 minutes away.  Again imagine (or find out) how far you can go in 35 minutes.  Distances to other planets at closest approach are:

  • Mercury      5 minutes
  • Venus         2 1/3 minutes
  • Mars           4 1/3 minutes
  • Saturn        1 hour, 11 1/8 minutes
  • Uranus       2 hours, 31 1/2 minutes
  • Nepture      4 hours, 2 minutes

  • The moon for comparison is only 1.87 seconds away.  But the nearest star is 4 1/2 years (lys) away.  How far can you go in 4 1/2 years (without stopping -- for the restroom, to eat, or even to sleep)?

    (All the planet distances above are based on the mean orbital radius, which won't work for Pluto whose orbit is highly eccentric and inclined and which is sometimes closer to the Sun than Neptune.)

    For the sports-minded Edward Packard's Imagining the Universe (Perigee, 1994) develops several models of the large and small starting with a baseball metaphor.

    Speaking of the large and small brings me to the best journey through the universe in both directions -- Powers of TenPowers of Ten is available in many formats.  The book (W. H. Freeman, 1982) and the video are in my opinion the most managable.

    Other resources on this topic:

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    Created April 25, 2001, 
    © 2001, Thomas A. Jonard