THE SOT. 



Since the sun moves over 360 of the heavens in 365| days, its 

 daily apparent motion must be 59' '14, or 3548", which being 

 about twice the sun's apparent diameter, it is easy to remember 

 that the disk of the sun appears to move in the firmament daily 

 over a space nearly equal to twice its own apparent diameter. Its 

 hourly apparent motion is 



^=147-8. 



3. Having explained the distance of the sun, let us now see how 

 its magnitude can be ascertained. There is one general principle 

 by which the magnitudes of all the heavenly bodies can be ascer- 

 tained when their distance is known. This is, in fact, accom- 

 plished by the device of comparing them with some object of 

 known magnitude and which at any known distance will have the 

 same apparent size. As this is important, considered as a general 

 principle applied to all objects in the heavens, it may not be 

 uninteresting to develop it somewhat fully in its application to the 

 present object, the sun. 



The common observation of every one who directs his view to 

 the heavens, will inform him of the fact that the sun and full 

 moon appear to be of the same size. The mere effect of ordinary 

 visual observation is, perhaps, enough to establish this ; but if 

 more be desired, instruments expressly adapted to measure the 

 apparent magnitudes of objects may be applied. We are also 

 confirmed in the fact by the consideration of the well-known 

 phenomena of solar eclipses. A solar eclipse is produced by the 

 interposition of the globe of the moon between the eye and the 

 globe of the sun. The eclipse is said to be central when the 

 centre of the moon is directly in line between the eye and the 

 centre of the sun. When this takes place we find that the globe 

 of the moon generally covers pretty exactly that of the sun. 

 Owing, however, to a slight variation in the apparent size of these 

 bodies, from a cause that we shall explain on another occasion, 

 the moon at one time a little more than covers the sun, and at 

 another time a little less. In short, the average apparent magni- 

 tudes of these bodies are the same, the one exactly covering or 

 concealing the other. 



But we have already stated that the distance of the moon is 

 only a quarter of a million of miles. And since that of the sun is 

 an hundred millions of miles, it appears that the distance of the 

 sun is four hundred times greater than that of the moon ; yet 

 these two globes appear to the eye to be of the same magnitude. 

 The sun, notwithstanding its being four hundred times farther 

 off, appears just as large as the moon. What, then, are we to 

 infer respecting its real magnitude ? If the sun were really 

 100 



