INTRODUCTION TO ASTRONOMY. xlix 



common year, called the solar or tropical year, containing 365 days, 

 5 hours, 48 minutes, and 5:2 seconds, is measured from the time the sun 

 sets out from one of the equinoxes, or solstices, till it returns to the same 

 again ; but the year is completed before the earth has finished one entire 

 revolution in its orbit. This is owing to the spheroidal figure of the 

 earth, the elevation about the equator producing much the same effect as 

 if a similar mass of matter, collected in the form of a moon, revolved 

 round the equator. When this moon acted on the earth in conjunction 

 with, or in opposition to, the sun, variations in the earth's motion would 

 be occasioned, and these variations produce what is called the precession 

 of the equinoxes. The equinoctial points are therefore not quite fixed, 

 but have a retrograde motion : that is to say, instead of being every revo- 

 lution in the same place, they move backwards. Thus, if the vernal equinox 

 be at A (Jig. 15, next page), the autumnal one will be at B. instead of C, 

 and the following vernal equinox at D, instead of at A, as would be the 

 case if the equinoxes were stationary at opposite points of the earth's 

 orbit : so that though the earth takes half a year to move from one 

 equinox to the other, it has not then travelled through half its orbit ; and 

 consequently, when it returns again to the first equinox, it has not com- 

 pleted the whole of its orbit. In order to ascertain when the earth has 

 performed an entire revolution in its orbit, we must observe when the 

 sun retires in conjunction with any fixed star; and this is called a sidereal 

 year. Supposing a fixed star situated atE, the sun would not appear in 

 conjunction with it till the earth had returned to A, when it would have 

 completed its orbit. The sidereal is only about twenty minutes longer 

 than the solar year, so that the variation of the equinoctial points is very 

 inconsiderable. 



In regard to time, we must further add, that the earth's diurnal motion, 

 on an inclined axis, together with its annual revolution in an elliptic orbit, 

 occasions so much complication in its motion as to produce many irre- 

 gularities : therefore true equal time cannot be measured by the sun. A 

 clock, which was always perfectly correct, would in some parts of the 

 year be before the sun, and in other parts after it. There are but four 

 periods in which the sun and a perfect clock would agree, which are the 

 15th of April, the 16th of June, the 31st of August, and the 24th of 

 December. The greatest difference between solar time and true time 

 amounts to between fifteen and sixteen minutes. Tables of the equation 

 of time are constructed for the purpose of pointing out and correcting 

 these differences between solar time and equal or mean time, which is 

 the denomination given by astronomers to true time. 



SECTION IV. On the Moon. 



LET us now turn our attention to the Moon. This satellite revolves 

 round the earth in the space of twenty-seven days eight hours, in an orbit 

 nearly coinciding with the plane of the earth's orbit, and accompanies us 

 in our revolution round the sun. Her motion, therefore, is of a compli- 

 cated nature ; for as the earth advances in her orbit whilst the moon goes 

 round her, the moon proceeds in a sort of progressive circle. There are 

 also other circumstances which interfere with the simplicity and regularity 

 of the moon's motion, but which are too intricate for us to notice at 

 present. 



The moon always presents the same face to us, by which it is evident 







