HOW TO OBSERVE THE HEAVENS. 



345 



To ascertain the aspect of the firmament at any hour of the night, it is now 

 only necessary to turn the globe upon its axis until the mark indicating the 

 place of the sun shall be under the horizon in the same position as the sun 

 itself actually is at the hour in question. To effect this, let the globe be turned 

 until the mark indicating the position of the sun is brought under the meridian. 

 Observe the hour marked on the point of the equator which is then under the 

 meridian. Add to this hour the hour at which the observation is about to be 

 taken, and turn the globe until the point of the equator on which is marked the 

 hour resulting from this addition is brought under the meridian. The position 

 of the globe will then correspond with that of the firmament. Every object 

 on the one will correspond in its position with its representative mark or 

 symbol on the other. If we imagine a line drawn from the centre of the globe 

 through the mark upon its surface indicating any star, such a line if continued 

 outside the surface toward the heavens would be directed to the star itself. 



For example, suppose that when the mark of the sun is brought under the 

 meridian, the hour 5h. 40m. is found to be on the equator at the meridian, and 

 it is required to find the aspect of the heavens at half past ten o'clock in the 

 evening. 



H. M. 



To 



Add 



5 



10 



1G 



40 

 60 



00 



Let the globe be turned until 16h. 0m. is brought under the meridian, and the 

 aspect given by it will be that of the heavens. 



We have frequently spoken of stars of the first, second, and inferior magni- 

 tudes. It is necessary that the just application of this term magnitude be 

 clearly understood. The Creator of the universe has not made the visible stars 

 in six moulds, so as to give them as many exact and distinguishable magnitudes. 

 Among these objects there is every gradation of brightness from the splendor 

 of Sirius down to that of those stars which are barely perceptible without a 

 telescope. Between those stars which astronomers have consigned to the 

 first and second classes, respectively, there is no distinct and decisive line of 

 separation. The stars of the first magnitude are not equally bright ; nay, it is 

 probable that no two of them could be selected, which, if submitted to pho- 

 tometric tests, would prove to be of exactly equal splendor. The least splen- 

 did- of them is not distinguishable from the most splendid of the stars of the 

 second magnitude, and in general it may be said that the least bright stars of 

 any magnitude are not distinguishable from the largest and brightest of the 

 class next below them. The classification of stars into magnitudes is there- 

 fore arbitrary, and not founded on any distinction really existing in nature. 

 Still, when properly understood, the classification of stars by magnitudes is 

 not without considerable utility as means of record and of reference, and it is 

 accordingly adopted by astronomers of every country. 



The term magnitude, however, is applied to stars in a very different sense 

 from that in which it is used as applied to planets. The latter present, when 

 seen in a telescope, a perfectly-defined disk, the diameter of which is capable 

 of pretty exact measurement. Before the invention of the telescope, stars 

 were supposed also to have sensible magnitudes, and it was an unanswerable 

 argument against the probability of the Copernican system that, admitting 

 (which was necessarily supposed), that the fixed stars must be so distant that 

 the entire orbit of the earth seen from them would seem but a point, their ap- 

 parent magnitude rendered it necessary to admit that the largest of them at 

 least must be many times larger than a globe which would fill the orbit of 



