E C L 



Solar eclipses arc more difficult of computation than lunar ones ; nor 

 is it possible to enter here upon the methods that have been employed, 

 We shall .'. conclude this article with an account of the number of 

 eclipses that may take place in a year. 



III. Eclipses, number of. 



In the space of 18 years, there are usually about 70 eclipses, 29 of the 

 moon, and 41 of the sun. 



Seven is the greatest number of eclipses that can happen in a year, and 

 two the least. 



If there are seven, five must be of the sun, and two of the moon. If 

 there are only two, they must be both of the sun j for in every year there 

 are at least two eclipses of the sun. 



There can never be more than three eclipses of the moon in a year ; 

 and in some years there are none at all. 



Though tlie number of solar eclipses is greater than of lunar in the ra- 

 tio of 3 to 2, yet more lunar than solar eclipses are visibla in any parti- 

 cular place, because a lunar eclipse is visible to an entire hemisphere, 

 and a solar is only Visible to a part. 



ECLIPTIC, obliquity of.f Woodhouse, Vines.) 



The mean obliquity of the Ecliptic in January 1, 1827 = 23. 27'. 43".7. 

 For the variations in the obliquity, see Precession. But besides these va- 

 riations in the obliquity, arising- from solar inequality and nutation, the 

 former of which passes through all its changes in the period of half a 

 year, and the latter in 9 years and 3 months, the obliquity of the Eclip- 

 tic has, as far back as observation goes, been diminishing from the action 

 of the planets, particularly Venus and Jupiter. This diminution, called 

 the secular diminution, is at present 52" in a century. There is, how- 

 ever, a mean to the obliquity which it cannot pass, and round which it 

 oscillates backwards and forwards. According to La Grange, the incli- 

 nation will never vary more than 5. 23' from the year 1700. 



Hence if we have given the mean obliquity for any time, and wish to 

 find the true obliquity, we must correct the given mean obliquity by 

 the secular diminution, the solar inequality, and the nutation. The ana- 

 lytical expression for the obliquity, including these corrections, is 



E ~~L 4. 0".4345 X cos. 2 sun's longitude 4. 9".63 X cos. N 



E being the mean obliquity at the beginning of the year, N the supple- 

 ment of the node, and n the number of days from the beginning of the 

 year. 



