Earth, Moon, Sun

Last Friday I had my second astronomy session with the 3-4 graders.  In the first one, we spent a lot of unplanned time on why we don't feel the Earth moving, and also it had been 3 weeks since my last visit, so I spend the first block of time recapping how we know the Earth rotates and how we know the cause of the seasons,  and building on that to analyze the Earth-Sun motion.

Do we go around the Sun or does it go around us?  We know that one of these two things is happening, because a given star rises at intervals of 23 hours and 56 minutes, whereas the Sun does at 24-hour intervals.  (Maven alert: 24 hours is an average which varies with the seasons, but that's too much detail here.) So each day the Sun gets 4 minutes "behind" the stars and over the course of a year it appears to make a complete circuit around the sky relative to the stars.  Ancient people knew this without having accurate clocks; they simply observed that the stars they could see at night (ie when the Sun was below the horizon) shifted slowly throughout the year.  We also know (as a boy mentioned last time) that the Sun's apparent size varies slightly throughout the year, thus indicating that our distance from the Sun varies slightly throughout the year.  [We happen to be closest to the Sun in January; if this shocks you, read about the cause of the seasons.]

I drew two models on the board: one with the Sun going around us in an ellipse (thus varying the distance) and the other with Earth going around the Sun in an ellipse.  What would be the observable differences between these two scenarios? This is a tough question!

Think about sitting in a moving car.  The roadside trees appear to rush by, but the distant mountains appear to move very slowly.  If the Earth moves, we ought to be able to see an effect like this by comparing nearby and distant stars.  I had taped some stars around the room and we had a small circular carpet to orbit around, so we practiced that.  You could also do this activity in the schoolyard. This effect is called parallax; if Earth is still, we will not see it.  The ancient Greeks thought of this, they looked for parallax and didn't see it, so they leaned toward Earth being still.  It turns out that even the nearest stars are so far away that the parallax effect is tiny, and was not measured until modern times.  So the ancient Greeks were not at all ignorant; they just didn't have precise enough tools to measure this really small effect. 

It turns out that the nearest star is about 250,000 times more distant than the Earth-Sun distance, ie the distance which Earth moves. It's as if your car moved one mile but you were asked to discern the difference in your view of mountains 250,000 miles away (eg, on the Moon).  I illustrated this dramatically by asking the kids to drawing a one-inch Earth-Sun model on the board, and then drawing a long line representing the distance to the next star and asking the kids to stop me when they thought I had arrived.  Kids (even most adults) have no idea how much 250,000 times is; they ask me to stop after 5 feet or so, but I keep going.  When I run out of board, I get a roll of toilet paper and start unrolling it, as a way to illustrate a very long line.  I keep going even when they tell me to stop. Then, when I run out of toilet paper, I go to the back room and get a cart full of hundreds of rolls of toilet paper!  It is really dramatic and fun. I also wrote out the number of miles to the next nearest star on the board: about 24,000,000,000,000 miles. (Kids and even most adults have little idea what a "trillion" really means.)

Knowing these distances, we can answer a few questions about the nature of the Sun and stars.  Lights which are further away appear to be fainter, so if we compensate for the enormous distance of the stars, we find that their intrinsic brightness (aka luminosity) is about the same as the Sun.  The Sun is just another star!  And we can compute that luminosity in terms of watt, just like a light bulb.  The Sun's luminosity turns out to be 400,000,000,000,000,000,000,000,000 watts.  It is the ultimate source of nearly all the energy we use on Earth.  If it burned fuel like coal or oil to produce its energy, it would rapidly run out of energy.  Astronomers were stumped for years on what the source of energy could be, until they discovered nuclear fusion (which we may address in more detail next time).

After the break, we looked at kids' observations of the Moon over the past three weeks, and we figured out what model could explain these observations, using as many different ways as possible: kinesthetic activity, going into a dark room with a blacklight and ping-pong balls so each student could move the "Moon" around his/her own head; and a mechanical model I had borrowed. I really think all these ways (and more) are necessary for most people to really get it.  I don't have time to write up this activity now, but if you are interested there are plenty of internet resources to help you understand it.  I just want to say that the kids' own observations over the previous three weeks were key to demolishing the misconceptions that the Moon is only visible at night, and/or that the Moon is always visible at night.  Finally, I want to leave you with some insanely cool pictures of eclipses.

This is a picture of the Moon's shadow falling on Earth during a solar eclipse.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This one shows the Moon at three different times throughout a lunar eclipse (when Earth's shadow falls on the Moon).  The ancient Greeks were able to determine from this that the Moon is about 1/4 the diameter of Earth, or about 2000 miles across. Furthermore, they know that for a 2,000-mile-diameter object to look as small as the Moon looks to us, it would have to be about 240,000 miles away.