Is A Synopsis of the principal 



tion in riglu ascension is 59' 8". This is the case about 

 April J 6th, June Idth, September 1st, and December 25tbi 

 on these davs the difference vanishes, or nearly so. It is at 

 its greatest about November 1st. when it is \6' 16"<J 



The astronomical year is divided into four parts, deter- 

 mined bv the two equinoxes and the two solstices. The 

 interval between the vernal and autumnal equinoxes is (on 

 account of the eccentricity of the Ealth's orbit, and its un- 

 equal velocity therein) near eiglit days longer than the in* 

 terval between the autumnal and vernal equinoxes. These 

 intervals are, at present, nearly as follow ; 



From the spring equinox to the! ^ J^ ^'l 45} .,. ,,. ,„. 



summer solst.ce ■■''■'-'•S K^,^^ 35 2O 



Irom the summer solstice to ] r,- ,- cr 



the autumnal equmox . .... J J 



From the auturhnal equinox to 1 ^,. ,_ ,.^ 



the wmter solstice / S = 178 18 20 



From the winter solstice to the! cr. . ^^ 1 



J- . .89 1 42 1 

 .spnng equinox y J 



7 16 51 



The nutation of the Earth's axis is 19'',3. 



Light takes &' \3",3 to come from the Sun to the Earth, 

 But in this interval the Earth has moved 20",2 in its orbit. 

 This motion of the Earth produces an optical illusion iii 

 the light which comes from ihe stars : and which Bradley 

 calls the aberration of light. 



Tht figure of the Earth is that of an oblate spheroid : the 

 axis of the poles being to the diameter of the equator as 

 331 to 332. The mean diameter of the Earth is about 7916 

 miles : its equatorial diameter is 79-4 miles. 



As a necessary consequence from this circumstance, the 

 degrees of' latitude increase in length as we recede from the 

 equator to the poles. But different meridians, under the 

 same latitude, present difiereut results. The general fact, 

 however, is well ascertained. 



The density of the Earth is 3'9326 times greater than 

 that of the Sun, and is to that of water as 1 1 to 2. 



Its masSf compared with that of the Sun considered as 

 unity, is t^tthts-- 



Tht centrifugal force is greater at the equator than at 

 the poles: in consequence of which, bodies lose part of 

 their weight by being taken towards the equator. If the 

 gravity of a body at the equator be represented by unity, 

 its gravity at the poles will be increased by -00569. A 

 pendulum, therefore, which vibrates seconds in the higher 



latitudes J 



