I30 



SCIENCE-GOSSIP. 



CHAPTERS FOR YOUNG NATURALISTS. 



{Continued from page 65.) 



Speed of Solar Planets. 



By Cyril Carr. 



"DECAUSE the Earth does not appear to us to 

 move, we might suppose it to be fixed in 

 space, and that the stars, sun and moon move 

 round it, as thought the astronomers of ancient 

 times. As we cannot see the v/hole of the world 

 at once, it was in those days a very difficult matter 

 to prove. The apparent movement of the constella- 

 tion of the Great Bear, always visible in the north- 

 ern sky on clear nights, served as the key to the 

 mystefy. By watching it revolving every twenty- 

 four hours, a suggestion of the spherical form 

 of the Earth was obtained. This observation, 

 followed up by thoughtful philosophers, told us of 

 the Earth's rotundity. 



Our own planetary rotation appears to be slow, 

 but in reality it deceives us. In order to make 

 the reason more clear, I will illustrate it by simple 

 methods that will indicate the true velocity in the 

 rotation of the Earth and the planets. 



Suppose a high post be placed in the ground on 

 the equator, pointing upwards a little below the 

 brilliant star Regulus, in the zodiacal constellation 

 Leo, in order to mark a spot from which we are to 

 start in a railway train at a rate of sixty miles an 

 hour, and dash along the equator right round the 

 earth. Suppose again, we leave the appointed 

 place on January ist, and travel along without 

 stopping, we shall not arrive back from our trip 

 round the world until the iSth, making eighteen 

 days on the journey. The starting-post turns 

 away as the earth turns round, and passes all 

 through the zodiac till it arrives back in a Uttle 

 less than twenty-four hours. That is to say, this 

 post, in twenty-four hours, has moved with one 

 spin of the Earth entirely round the equator, a 

 distance of about 25,000 miles, at a speed of more 

 than 1,000 miles per hour, while the train takes 

 eighteen days to go over the same distance. So 

 the Earth spins round at a rate more than fifteen 

 times greater than that of the fastest train. 



We will imagine the train now moves along the 

 same railway-line from east to west, in the direc- 

 tion that the stars apparently move across the sky ; 

 and, as we go, we look through a window in the 

 roof of our carriage at the star Regulus, for the 

 purpose of watching the difference between the 

 velocities of the train and of Regulus. We will 

 mark the position of the star by a spot on the 

 window, and then carefully look at the star after 

 some hours. We shall see that it seemingly travels 

 faster than the train, and sinks below the western 



horizon, disappearing from view directly ahead of 

 us ; then, after about eighteen hours, Regulus 

 re-appears in sight behind us, above the eastern 

 horizon, and arrives overhead again in about seven 

 hours. Yet the train has not gone round the world, 

 but has only covered about 1,440 miles in this time. 



Let us imagine a gigantic spear could be sus- 

 pended from the sky -with its point downwards 

 towards London. Now what would become of the 

 spear supposing it were not instantly burnt up by 

 friction of the passing air ? The inhabitants of 

 London would be throwm into great excitement 

 by hearing the sound of some awful roaring 

 noise over the capital as the spear rushed through 

 the Earth's atmosphere. Immediately they would 

 run out and look up at the sky, but would 

 see nothing, as the spear-point would have 

 passed miles away in a second. If the spear 

 remained in position for several days, it would 

 flash across London, from east to west once 

 every twenty-four hours, showing onl}- the faintest 

 streak in the sky. This should ser\-e to illustrate 

 the velocity of the Earth's rotation, which is some- 

 thing like 1,040 miles an hour at the equator. 

 Xow, if we could imagine the same experiment 

 occurring in Saturn or Jupiter, as the spear-point 

 flashed across the sky, the faint streak observable 

 there would be very much finer than on Earth, and 

 nearly invisible. The velocity of Saturn's rotation 

 at the equator is known to be about 22,500 miles 

 an hour, or about twenty-two times quicker than 

 that of the Earth, for Saturn is a much larger body 

 than the Earth, and j-et revolves on its axis much 

 more rapidly. As to the same occurrence in 

 Jupiter, the flash of the spear-point across the sky, 

 would be invisible, as the velocity of this planet's 

 rotation at the equator is i8,ooo miles an hour. 



There is also another kind of motion possessed 

 b\' our planet, which is its onward journey round 

 the sun. We should obtain a verj- good idea of 

 this if we were to imagine ourselves lifted out of the 

 world into space by a gigantic man, in whose hand 

 we could rest in a fixed position. Then, to our 

 great surprise, the Earth would at once rush ofl' 

 with fearful rapidity and would be over eighteen 

 miles away from us in a second, or about 67,000 

 miles an hour. This illustrates the yearly orbit of 

 our planet round the sun. When we remember 

 that the sun also is moving and bearing the Earth 

 and other planets with it, we see that they really 

 travel in a spiral course through space. 



