COMMON THINGS TIME. 



and fifty-four minutes below the horizon, while oil the 18th it 

 was eleven hours and three minutes above, and twelve hours and 

 fifty-seven minutes below the horizon. On no day of the month 

 was it exactly twelve hours above, and twelve hours below the 

 horizon ; and the same result would be found by examining in the 

 same manner the almanacs for other years. 



It appears then that rigorously equal day and night is a phe- 

 nomenon that never exists. It is no answer to this to say that 

 the day and night in the instance produced and others differ only 

 by a minute or two, because the question here involves only the 

 consideration of those very minute intervals. 



Since then the "day of the equinox" cannot mean a day on 

 which day and night are equal, what is its exact meaning ? "We 

 reply that it means very obviously the day on ivhich the equinox 

 takes place. But then what is in that case meant by the word 

 equinox? We reply by turning back upon the explanation 

 already given, that the equinox is that precise moment when the 

 centre of the sun's disc has such a position that, supposing it 1o 

 retain that position unchanged, it would be twelve hours above, and 

 twelve hours below the horizon, during a revolution of the heavens. 



93. But since the sun's disc has a continual easterly motion 

 upon the heavens, moving at the rate of nearly 1 per day, or 2|' 

 per hour, it does not retain the position in question more than an 

 instant. It moves round the heavens as the hand of a clock 

 moves round its dial, passing incessantly from point to point. 

 The exact point at which the centre of the sun is at the moment 

 above described, is therefore called the equinoctial point, as the 

 moment of time at which it passes through that point is called 

 the equinox. 



94. There are two equinoxes and two equinoctial points. The 

 first takes place about the 21st March, and the other about the 

 23rd September.* 



The former is called the vernal equinox, and the latter the 

 autumnal equinox, because it has been agreed to fix the beginning 

 of spring at the one epoch, and the beginning of autumn at the 

 other. 



The two equinoctial points are situate at opposite sides of the 

 heavens, separated one from the other by an entire hemisphere, r.s 

 must be evident when it is considered that the sun takes six 

 months to move from the one point to the other. 



95. Having thus conveyed a distinct notion of the meaning of 

 the equinoxes, and of the equinoctial points, we shall find less 



* In the tables of sunrise and sunset given in the almanac, the effects of 

 refraction are taken into account. These are omitted, however, in fixing 

 the position of the equinoxes. 

 160 



