426 LECTURE XLV. 



usually laid down ; but with respect to the sun and the planets, performing 

 their revolutions in or near the ecliptic, it is most convenient to calculate 

 their latitude and longitude. 



The plane passing through the earth's axis and the place of a spectator is 

 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 he 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 

 referrible 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 

 timekeepers are 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, with almost 

 equal accuracy, in any other situation, and sometimes with greater con- 

 venience. 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 neces- 

 sary. 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, expressed 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 themselves ; 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 difference between his path in every 

 sidereal day, and a whole circle, depending on this change, would also 

 vary ; the other that the sun's motion in the ecliptic is actually variable, 

 consequently the diurnal change of right ascension is liable to a double 

 inequality. Hence it happens that the solar time agrees at four instants 

 in the year only with the mean time, derived from supposing the whole 

 365 days to be divided into as many equal parts ; the difference is called 

 the equation of time, and amounts sometimes to as much as 16 minutes. 

 The term equation is commonly applied in astronomy to any small quan- 

 tity, which is to be added to, or subtracted from, another quantity ; thus 



