COMMON THINGS TIME. 



will presently appear, it is attended with other circumstances 

 which make it unsuitable for that purpose. 



24. The vertical plane directed north and south, of which we 

 have spoken, if supposed to be extended upwards to the firmament, 

 will meet the visible hemisphere in a semicircle which, passing 

 through the zenith, as the point directly over the observer is 

 called, descends to the horizon at the north and south points. 

 This semicircle is called the MERIDIAN. It divides the visible 

 hemisphere into two equal parts, the eastern on the left of the 

 observer, and the western on his right. By the diurnal rotation 

 of the celestial sphere, all objects upon it rising in the east 

 ascend to the meridian, where they attain their greatest altitude', 

 and then descend to the west and disappear. The interval 

 during which each of them is visible is divided into two exactly 

 equal parts by the meridian, the time which elapses between tho 

 moment at which it rises and that at which it passes the meridian 

 and attains its greatest altitude, being equal to that which 

 elapses between the latter moment, and that at which it 

 disappears. 



This movement of the heavens is more observable by night than 

 by day, because it is then shared by a vast number of objects, 

 having positions infinitely various upon the celestial vault. 

 Countless numbers are every moment rising or ascending towards 

 the meridian, passing it, or descending from it, or setting. 

 Although the objects upon the firmament by day are not leas 

 numerous, they are rendered invisible by the superior splendour 

 of the sun. They may nevertheless be seen even then with suffi- 

 ciently powerful telescopes, and they present exactly the same 

 apparent motion, being still carried round with the common 

 motion imparted by the celestial sphere. 



25. The sun like the rest is carried round with the diurnal 

 motion, and its continuance above the horizon is divided into 

 equal parts by the meridian. Hence it appears that when its 

 centre is on the meridian, it is midday or noon, and at that 

 moment it has its greatest altitude. 



This moment then being the epoch upon which the fundamental 

 unit of time is based, it becomes of great importance to com- 

 prehend the means which have been contrived for accurately 

 observing it. If the meridian were traced by a visible line upon 

 the heavens, the observation of the moment at which any celestial 

 object crosses it would be easy. But that not being the case, it 

 may be asked how the moment at which the sun's centre passes a 

 merely imaginary line, can be ascertained with the ext 

 precision necessary in this case. 



26. Astronomers have accomplished this by a very simple 

 124 



