61 



The Elliptical Path op the Moon. 



The moon, like many other of the heavenly bodies, in her orbit 

 round the earth obeys Kepler's first and second laws of planetary 

 motion — that is, the moon's path is an ellipse, of which the 

 earth occupies a foci, and that the radius vector of the moon 

 describes equal areas in equal times. When the moon is 

 furthest from the earth she is said to be in apogee, when nearest 

 she is said to be in perigee. 



The Phases of the Moon. 



The sun always enlightens one half of the moon, and though 

 sometimes its whole enlightened hemisphere is seen by us, yet 

 sometimes only a part is discernable, and at other times none 

 at all, according to her different positions in the orbit with 

 respect to the earth. When the moon comes between the sun 

 and the earth no part of its enhghtened hemisphere is visible to 

 the earth. It is then new moon or change, and is said to be in 

 conjunction. When the earth comes between the sun and the 

 moon the whole of the enlightened hemisphere is then turned 

 towards the earth, and this is called full moon, and then it is 

 said to be in opposition. It passes through all its phases between 

 new moon and full moon, and between fuU moon and new 

 moon. It might be supposed that one half of the moon was 

 sell-luminous and the other half was dark, and that the phases 

 of the moon were produced by a rotation of the moon on its 

 axis. Now, if this were the case, those parts that are called 

 seas and other markings upon the disc of the moon would not 

 always occupy the same portion of the visible disc of the moon, 

 but the illuminated hemisphere would always contain the same 

 objects, and the boundary between light and darkness would 

 always occupy the same position with reference to the lunar 

 markings. Observation shows that this is not the case. The 

 light may be seen creeping along the surface of the moon and 

 lighting up one lunar mountain and valley after another. One 

 most interesting feature of the moon is that it always turns the 

 same face to the earth. 



The Eotation of the Moon. 



The surface of the moon possesses certain well-marked features 

 visible to the unaided eye, but still more clearly discerned in a 

 telescope. The simplest observations sufiSce to show that these 

 objects remain constant in position, in other words that the face 

 of the moon which is turned towards the earth is always the 

 same. It follows, therefore, that the moon turns upon its axis 

 in the same time that it takes to move round the earth. 



