ON PRACTICAL ASTBONOMY. 537 



the plane of the meridian of that place; and a plane touching the earth in any 

 point is its horizon. With respect to the appearances of the fixed stars, 

 this plane may be considered as passing through the earth's centre in the 

 same direction: and the difference is scarcely sensible with respect to the sun 

 and the primary planets; but in observations of the moon's place, these 

 planes must be carefully distinguished. (Plate XXXV. Fig. 502. )• 



The instruments requisite for astronomical observations are principally 

 referable to geometrical or to optical apparatus, or to the measurement of 

 time. Particular constructions and combinations are, however, necessary 

 for the accommodation of quadrants, graduated circles, telescopes, and 

 transit instruments, to the uses of observatories; and astronomical observa- 

 tions are as necessary to the correct determination of time, as artificial time- 

 keepers afe useful for other astronomical purposes. 



The most accurate standard of time is the diurnal rotation of the earth on 

 its axis, as ascertained by its situation with respect to the fixed stars. The 

 time elapsing between two successive passages of any star over the same 

 meridian, which constitutes a sidereal day, may be measured with great 

 precision; and the star may for this purpose be observed, witli almost equal 

 accuracy, in any other situation, and sometimes with greater convenience. 

 The length of the sidereal day may be considered as perfectly constant, the 

 inequalities arising from the aberration of light, and from the nutation of 

 the earth's axis, being too small to be easily discovered; but the correction 

 for the aberration may be applied when it is necessary. For observations of 

 this kind, it is usual to have a clock adjusted to sidereal time, which not only 

 admits of a more ready comparison with the transits or passages of any one 

 star over the meridian, but, by the difference of the times of any two transits, 

 shows at once the difference of right ascension of the stars or planets, ex- 

 pressed in time instead of degrees. 



The solar days are not only about four minutes longer than the sidereal 

 days, but they are also unequal among tliemselves ; and this inequality 

 arises from two causes; the one, that even if the sun's motion in the ecliptic 

 were uniform, his diurnal changes of right ascension would be different at 

 different times, and the difl^'erence between his path in every sidereal day, and 



VOL. I. 3 z 



