THE UNIVERSE IN WHICH WE LIVE 9 



much lighter there than here. But this is all mere speculation 

 even if it is fascinating. We know nothing about such inhab- 

 itants, or if there really be such. 



We know even less about the four outer major planets than 

 the minor ones, our near neighbors. Jupiter is some 1,300 

 times as large as the earth, and is probably still in a partly gaseous 

 condition. Not long ago, astronomically speaking, it was glow- 

 ing with its own heat, but now has largely cooled. Saturn has 

 some unique rings about it, composed of myriads of tiny bodies 

 that whirl about the planet in parallel orbits. 



All the planets revolve about the sun in orbits that are practi- 

 cally circles, that of Neptune being most nearly such. The orbits 

 are really ellipses, curves with one axis larger than the other. 

 Such curves may readily be drawn thus: Take a 1 6-inch length 

 of string and tie the ends together, making a loop. Stick two 

 pins through the paper into the drawing board, 5 inches apart, 

 and place the loop over the pins. Set the pencil point within 

 the loop and hold the loop out taut, so the string forms a triangle 

 with the pins a.t two corners, the pencil at the third. Now move 

 the pencil about, as if trying to draw a circle, when an ellipse will 

 be the result. The shape of the ellipse will vary as the distance 

 between the pins is altered, or as the length of the loop of string 

 is changed. The points occupied by the pins are known as the 

 foci, and in the solar system the sun occupies one focus of each 

 planetary orbit. 



If the diameter of the orbit of Mercury be represented by 

 a line i inch long, then that of Venus would be approximately 

 1.9 inches; of the earth, 2.6 inches; of Mars, 3.9 inches; of Jupiter, 

 13.4 inches; of Saturn, 24.6 inches; of Uranus, 49.5 inches; and 

 of Neptune, 77.5 inches. In the case of the earth's orbit the differ- 

 ence between the long and short diameters is about 3,000,000 

 miles, not a great departure from a circle when it is remembered 

 the total diameter is sixty times this. 



The plane passed through the orbit of the earth is known 

 as the plane of the ecliptic. If we should think, as did the 



