ASTRONOMY. 85 



g. The length of the day for any latitude, and any 

 declination of the sun, may be calculated, like the 

 continuance of a star above the horizon, 27. 



h. The determination of the length of the year, re* 

 quires the time of the sun's crossing the equator to 

 be accurately defined. 



85. The time of the sun's crossing the equa- 

 tor, is found by observing his declination at 

 mid-day, for several days before and after the 

 equinox \ so that an equation can be found, ex- 

 pressing the relation between the time and the 

 declination sufficiently exact for a few days. 

 If, in this equation, the declination be suppos- 

 ed equal to nothing, the value'of the time cor- 

 responding will give the moment of the equinox. 



The manner of finding an equation between the time 

 and any quantity determined by observations, made 

 at given intervals of time, is called the Method of 

 Interpolation. It is much used in astronomy, and 

 will be afterwards explained. See NEWTON'S Prin- 

 cipia, Jib. in. lemma 5. LA LANDE, Astronomic, 

 torn. in. 4< 1 1 3. LA CAILLE, Lemons Elementaires 

 d'Astronomie, 135, &c. 



Nearly in the same way, the moment is found, when 

 the sun's declination is greatest either on the north 

 or the south side of the equator. The only diffe- 

 rence is, that when the equation is obtained be- 

 tween 



