The solar system

It took me a few weeks to prepare a unit on the solar system.

We started by looking at the sun using specially designed kids' welder goggles (do not use regular sunglasses - you can find inexpensive kids' solar eclipse observation glasses on Amazon) and clocked how long it takes for light to travel from the sun to the earth (8 minutes and 20 seconds). This in itself was an exciting exercise. For bigger kids, Khan Academy has a good video on the scale of the earth and the sun.

The most amazing thing about space is how astonishingly vast it is. Yet, despite its immense scale, we can design spaceships and travel in it because we can measure its geometry really accurately. Astronomy is all about the math of distances. I wanted the children to get a feel for that.

There is a whole industry of educational solar system kits out there. But I couldn't find one that was proportionally accurate. So I cobbled up my own model using the following scale, adapted from this Finnish document (it's in meters and centimeters; we've used the metric system for science from the start, and our children are used to it).

Planet	Size (cm)	Distance (m)	Note
Sun	100.0	0	If the diameter of the sun was 1 meter...
Mercury	0.4	43	Mercury would be 40 meters from the sun
Venus	0.9	78	Venus would be about 80 meters from it
Earth	0.9	107	The earth would be about 100 meters from it and just under 1 cm in diameter
Mars	0.5	164	It would be 160 meters to Mars
Jupiter	10.3	559	It would be 560 meters to Jupiter, which would be 10 cm in diameter
Saturn	8.6	1,025	1 kilometer to Saturn, which would be 9 cm in diameter
Uranus	3.7	2,061	2 kilometers to Uranus
Neptune	3.5	3,229	3 kilometers to Neptune
(Pluto)	0.2	4,237	The distance to Pluto would be 4 km and it would be the size of a pinhead

I found a sun roughly the right diameter from this inflatable kit. The smaller, rocky planets were about the right size in a kit similar to this Smithsonian one that I picked up on the sale rack at Lawrence Hall of Science in Berkeley. Since these planets are so small I wanted the surface to look detailed. The bigger gaseous planets were roughly the right size in this unpainted styrofoam kit.

I hung the sun in the school tower room. Cool!

We then painted the larger gaseous planets (the smaller ones from the other kit were already painted). While painting, we talked about what they are made of, and why each one is a different color. And peculiarities like Jupiter's Great Red Spot, and of course Saturn's rings.

We played with them to get a better feel for their relative sizes. Compared to Earth the sun is SO HUGE! The children didn't seem to ever get tired of marching in ellipses to show how they orbit round and round.

Also, by now we had memorized their order.

The next day we went to the beach to measure orbits. While R held the sun up high, the children measured the distance to Uranus, then Venus, then the Earth and Mars using a big outdoor tape measure I had ordered for this purpose.

When we got to the orbit of the Earth (107 meters from the sun) one of the girls exclaimed "it's the same size as the real sun!" Which was true - the real sun, its sphere visible through cloud cover, was the same size as our inflatable one, which R was still holding where we had started. We could easily blot either sun out with our thumb.

After Mars (160 meters) the children had had enough walking. It was humbling to think that we still had another 400 meters to get to Jupiter at the end of the beach. And that sand grain-sized Pluto was whizzing by Point Bonita Lighthouse four kilometers away, on the other side of Golden Gate Bridge.

Later this spring I plan to connect this to our ongoing interest in ancient temples and cultures. Stonehenge, the pyramids, and the Mayan temples all stand testimony to the crucial importance of the celestial bodies to people, their religion and societies throughout history. I'm planning to use the book Celestial Geometry: Understanding the Astronomical Meanings of Ancient Sites as one of the resources for this upcoming unit.