Papier ou 3-D? / Paper or 3-D?
Here we have the English language content component to Lesson Plan 1, of the same name. It works in conjunction with the previous section, as it helps us understand the changing sky-views as we stress the move from two- to three-dimensional space.
What is a constellation? A constellation is a group of stars that can be seen in the sky at night. To our eyes, these stars seem to form drawings in the sky, as if the sky was flat, like a sheet of paper, and someone drew little pictures on the paper of the sky. Of course, we know that the sky is not flat, because we know that the Earth is not flat. The Earth is a three-dimensional sphere, like a big ball. A sheet of paper is only a two-dimensional object. Like the Earth is a three-dimensional sphere, the gas which is all around the Earth, called the atmosphere, is also three-dimensional.
Here is the Earth. [I show a football.] Here is the atmosphere. [I place a big piece of cotton around the ball.] And these are the stars in the atmosphere. [I put some sequins in the cotton.] Although it seems that each constellation is made on a flat surface, in reality that is not the case. In reality, maybe there was one star here, another star here, and another over there. [I place each "star" at a different depth.] To our eyes, all these stars seem to form a two-dimensional, flat surface, but we know that it is truly a three-dimensional space.
[I place a basketball on a stick, and the stick in a bucket filled with sand, for stability.] Here is the Sun. Look how it stays in the same place. Look how the Earth revolves around the Sun. It doesn't make a circle, like this; it makes an oval, like this. A single trip around the Sun takes how many days? [I wait for an answer; if there is no answer, I give it.] It takes 365 days. We have different seasons during the year, because the light from the Sun strikes more directly the hemisphere, and so is more concentrated in the summer, and in the winter, less concentrated, so less strong. [I show the Earth's trip, using a photo like what can be found on page 121 of the book The Stars by H.A. Rey as a guide.] Just like the seasons change during the year, the views of the stars that can be seen also change during the year. Watch: Here is the Sun, the Earth, and over there are the planets of our solar system. The stars that can be seen are over here, on this side of the Earth. There are also stars on this side, but we can not see them, because of all the light from the Sun. Stars are fixed; the Earth is not fixed. The Earth moves, the Earth travels; the stars don't move, don't travel. In fact, stars move a very very little bit, over the course of many many years; it is a negligible movement.
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