280 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



MARS 12-6 



THE FOUR INNER PLANETS 



SCALE 20 TIMES THAT 

 USED IN TOP DIAGRAM 



Figure 1. — ^The solar system. 



THE FIVE OUTER PLANETS 



Numbers give the distance of the planet from the sun in 

 light minutes. 



number of asteroids is not known but there are probably between 

 50,000 and 100,000. 



The dimensions of the solar system are determined by the extreme 

 orbit of Pluto which is slightly elliptical. Although at its closest 

 approach to the sun the orbit of Pluto lies inside that of Neptune, the 

 most distant point of its orbit takes it 4,566 million miles away. At 

 this point the light from the sun to Pluto takes about 61/^ hours on 

 its journey. And so the size of the earth's abode in space is epitomize<i 

 by the 93 million miles or 8 light minutes which separate us from the 

 sun and the 6i/^ light hours of the farthest point of Pluto's orbit from 

 the sun. Although these distances are enormous by terrestrial stand- 

 ards it has to be remembered that once we move out from this system 

 we have to travel for 4i/^ years at the speed of light before we get to the 

 nearest star and then for 100,000 years to the extreme reaches of the 



The parent of the solar system — the sun — has a diameter of 865,000 

 miles (that is excluding the solar atmosphere) and is 332,000 times 

 the mass of the earth. The weight of the sun is about lO^'^ tons ; that 

 is, a thousand billion billion ^ tons. The energy output of the sun 



' The use of "billon" in this article refers to the English billion ; that Is, one million mil- 

 lion (as distinct from the American billion, which is one thousand million). 



