Star Points for April, 2007; by Curtis Roelle
How Big is the solar system?

There's no doubt about it.  Regardless of their age, people young
and old are curious about the universe around them.  Although the
specific subject varies, the following questions are among the most
popular: How big? How bright? How far away?   In the limited space
of this column I want to begin a discussion on the first of these.

Let's start with the earth and sun.  If the earth was the size of
a gum ball the sun would be a gum ball machine big enough to hold
over a million balls.  If you could make necklace with pearls the
size of the earth you would need 343 "pearls" to make a necklace
large enough to go around the sun.

When talking in astronomical terms the numbers quickly become too
staggering to comprehend.  Constructing or visualizing a scale
model helps the mind build a bridge between familiar things and
concepts that are difficult to grasp.  

A scale for the solar system I have frequently used is based on
scale models developed by Ray Sterner of the Westminster
Astronomical Society, Inc. (WASI).  In one version one foot
represents the distance between earth and sun, or 93 million miles.
At this scale the sun is just 1/8" across, or about the width of
two letters of newspaper print.  The earth is a nearly invisible
spec smaller than flea dirt.

When demonstrating this model I place a stake at the position of
each planet.  Mercury, Venus, and Earth are all within one foot of
the sun's location.  On it goes until the final stake is pounded
into the ground 400 feet from the sun to mark the location of
Pluto, which by the way is officially now known as a "dwarf planet"
(see StarPoints for Sep. 2006).

I've done this demonstration many times for scouts, students, and
adult groups.  The problem is that it's one dimensional.  It only
shows the radius of the solar system, let alone its diameter or
even circumference.  

When a youth group was visiting the house I modified the model. 
The sun and all the planets were placed in the front yard.  Pluto's
stake was pounded in 400 feet from the sun.  Then we took off for
a walk along a large imaginary circle representing Pluto's 248-
year-long orbital path.  The halfway point coincided with the
location of my back yard observatory on top of a hill overlooking
the solar system.  As they huffed and puffed from the climb I
explained that it would have taken Pluto 124 years to cover the
same distance. Gazing down the hill we could see the stakes marking
the orbits of each planet including Pluto on the far opposite side
of the solar system 800 feet off in the distance.  

Pluto's orbit is so large that even light requires 11 hours to
cross it!  Obviously, at our scale model we had been walking many
times faster than the speed of light.  Then we completed the tour
of Pluto's orbit, continuing the circular path downhill.  We
strolled at a leisure pace at ultra warp speed and arrived back at
our starting point.  On the return trip we covered in just a few
minutes a distance that takes light several hours and Pluto 124
years to complete!

But where does the solar system fit into the greater scheme of
things?  Next month our discussion will continue as we expand the
model farther outward using models developed for NASA's Night Sky
Network (NSN) program of public outreach.  NSN develops programs
for amateur astronomy clubs to use when talking with the public
about space.  WASI is one of the leading NSN clubs in the United
States in terms of the number of outreach programs it conducts on
a yearly basis.

The next opportunity for you to visit the planetarium at Bear
Branch Nature Center (BBNC) is at 07:30 p.m. Friday, April 27. 
Weather permitting, WASI will host an outside "star party" with
telescopes.  The star party is free and the planetarium program is
rain or shine.  For planetarium directions, reservations and
pricing call BBNC at 410-848-2517.