100 Days

Primaria has been counting the days of school since it started in
September, to give the kids practice counting.  Last week they got up
to 100, which was a big milestone they celebrated.  So I sneaked in an
extra science activity when they did PE.  The Sun is about 100 times
bigger than the Earth, which I thought made a nice connection between
science and what they were celebrating.  I brought a 12-inch (1-foot)
globe to represent the Earth, and marked out a 100-foot diameter
(actually, 109 feet, to be more accurate) circle in the park to
represent the Sun.  I marked it out with about a dozen cones, and to
help visualize a real circle I draped a 50-foot rope around the edges
of the first 2-3 cones; more would have been better.

It's one thing to say the the Sun is 109 times bigger than the Earth,
but another to see it!  If you are familiar with the park, imagine the
Sun covering all the grass in the narrower east-west direction, and
most of the grass in the north-south direction.  Compare that to
little old Earth, the 12-inch globe in my hand!  This is the beginning
of a scale model, but to be true to the scale model we would have to
put Earth a few miles away, like in downtown Davis!  The Sun looks
small only because it is really far away.

Part of the reason that saying "the Sun is 109 times bigger than the
Earth" has less impact than it should is that we are comparing
diameters, not volumes.  The large area of grass I marked is like a
cross-section of the Sun, 109 times Earth's size in one dimension AND
109 times Earth's size in the second dimension as well, for over
10,000 times the area.  But we should really try to visualize a sphere
109 feet tall as well, which would make for a volume over a million
times the volume of the globe in my hand.  So we could just as easily
say that the Sun is over a million times Earth's size, if we are
talking about volume.  See my post(s) on scaling relations
for more on this.  I didn't try to get the Primaria kids to think
abstractly about this, but I did ask them to imagine the circle
representing the Sun as extending 100 feet high.  Then we ran around
the Sun a few times.