240 ASTRONOMY. 



native light she would always appear full; but aa she shines only by 

 reflecting the light of the sun, her luminous parts put on different shapes, 

 according to her situation as respects the earth. The time that elapses 

 between one new moon and another, is greater than the moon's revolution 

 in her orbit; the first, which is called a lunation, being twenty-nine 

 days and a half, and the second, which is denominated a periodic month, 

 only twenty-seven days and a half; the reason for this is obvious, for as 

 the earth during the moon's revolution must have passed through 

 nearly one twelfth part of her orbit, the moon must traverse more than 

 the circumference of her's before she can be in conjunction so as to 

 become a new moon. The moon's orbit is inclined to that of the earth 

 about five degrees, and the two points where they cross each other are 

 called her nodes. If the moon is in one of these at the time of her 

 conjunction, she directly interposes between the earth and sun, and 

 occasions a total eclipse of the latter. If she be full in one of her nodes, 

 she passes through the earth's shadow, and is herself totally eclipsed. 



The moon revolves on her axis in twenty-nine days and a quarter, 

 which being exactly the time from one new moon to another, occasions 

 her to present always the same part of her surface to the earth. In 

 consequence of this, the earth is now seen from one half of the moon, 

 while to the other half it is visible in exactly the reverse order of the 

 moon to the earth. The motion of the moon in her orbit is irregular, in 

 consequence of the attraction of gravitation ; for not only is she acted 

 upon by the earth and sun, but also by the other planets; and as this 

 attraction varies in proportion as they approach to or recede from her, 

 the accurate calculation of her motion is a matter of great difficulty. The 

 moon when viewed through a telescope, presents an appearance which in- 

 dicates that its surface is made up of hills, valleys, &c. Her axis is nearly 

 perpendicular to the plane of the Ecliptic ; that the sun never removes 

 sensibly from her equator, consequently her days and nights must be 

 equal, each more than fourteen times as long as ours, and there can be 

 no variety of seasons. The light emitted by the moon (beautiful as it is) 

 produces no heat; for if her rays, concentrated by a powerful mirror, be 

 thrown on the bulb of a thermometer, no effect is perceptible. Indeed, 

 experiments have shown that the light of the full moon is three hundred 

 thousand times less than that of the sun. Of the moon's influence on the 

 weather, the tides, and the human constitution, is perhaps doubtful, 

 although it has the authority of great names. Madmen are called 

 lunatics, from the moon's Latin name Luna\ Shakspear many times 



