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THE MOON. 



MAGNITUDE OF THE MOON. 



When the distance of a visible object is determined, its magnitude may 

 easily be ascertained by comparing it directly with another object of known 

 magnitude and a known distance. To illustrate this by its application to the 

 MOON, let us take, for example, a cent-piece, which measures about an inch in 

 diameter, and let it be placed between the eye and the moon at any distance 

 from the eye. It will be found on the first trial that the coin will appear larger 

 than the moon ; it will, in fact, completely conceal the moon from the eye and 

 produce what may be termed a total eclipse of that luminary. Let the coin be 

 moved however further from the eye, and it will then appear smaller, and will 

 apparently diminish in size as the distance from the eye is increased. Let it 

 be removed until it becomes equal in apparent magnitude to the moon, so that 

 it will exactly cover the disk of the moon, and neither more nor less. If its 

 distance from the eye be then measured, it will be found to be about ten feet, 

 or one hundred and twenty inches, or what is the same, two hundred and forty 

 half inches. But it is known that the distance of the moon is about two hun- 

 dred and forty thousand miles, and consequently it follows in this case, that 

 one thousand miles in the moon's distance is exactly what half an inch is in 

 the coin's distance. Now under the circumstances here supposed, the coin 

 and the moon are similar objects of equal apparent magnitude. In fact the 

 coin is another moon on a smaller scale, and we may use the coin to measure 

 the moon's distance, provided we know the scale, exactly as we use the space 

 upon a map of any known scale to measure a country. But it has been just 

 stated that the scale is in this case half an inch to one thousand miles ; since, 

 then, the coin measures two half inches in diameter, the moon must measure i 

 two thousand miles in diameter. The moon is then a globe whose diameter ' 

 is about one fourth of that of the earth. Its bulk is about one fiftieth of that of ( 

 our globe, its weight a little less than one fiftieth, and its density something < 

 less than three fourths of the density of the earth. 



ROTATION OF THE MOON. 



While the moon moves around the earth in its monthly course, we find by 

 observations of its appearance, made even without the aid of telescopes, that 

 the same hemisphere is always turned toward us. We recognise this fact by 

 observing that the same marks always remain in the same place upon it. Now, 

 in order that a globe which revolves in a circle around a centre should turn 

 continually the same hemisphere toward that centre, it is necessary that it 

 should make one revolution upon its axis in the time it takes so to revolve. 

 For let us suppose that the globe, in any one position, has the centre round 

 which it revolves north of it, the hemisphere turned toward the centre is turned 

 toward the north. After it makes a quarter of a revolution, the centre is to die 

 east of it, and the hemisphere which was previously turned to the north must 

 now be turned to the east. After it has made another quarter of a revolution 

 the centre will be south of it, and it must be now turned to the south. In 

 the same manner, after another quarter of a revolution, it must be turned to the 

 west As the same hemisphere is successively turned to all the points of the 

 compass in one revolution, it is evident that the globe itself must make a single 

 revolution on its axis in that time. 



It appears, then, that the rotaiion of the moon upon its axis being equal to 

 that of its revolution in its orbit, is 27 days, 7 hours, and 44 minutes. The in- 

 tervals of light darkness to the inhabitants of the moon, if there were any, 

 would then be altogether different from those provided in the planets ; there 



