EQUAL DAY AND NIGHT. 



The sun's disc makes a circuit of the heavens in a year. Its 

 position from March to September is such as to render the days 

 longer, and from September to March such as to render them 

 shorter than the nights. 



At a certain moment on some day in each of these months, the 

 sun's disc has such a position that if it were to remain stationary 

 in that position, and if there were no atmosphere, sunrise and 

 sunset would take place exactly at 6 o'clock, A.M. and P.M., and 

 consequently the days and nights would be precisely equal, each 

 being twelve hours. 



The moment at which the sun's disc has this position, is that of 

 the equinox. 



60. Before the equinox in March, the position of the sun has a 

 tendency to render the nights longer, and after it to render them 

 shorter than the days. 



Before the equinox in September, its position has a tendency 

 to render the days longer, and after it shorter than the nights. 



If on the day of the March equinox, the equinox take place 

 exactly at noon, the sun will have a tendency for the preceding 

 twelve hours to render the night longer, and, for the succeeding 

 twelve hours, shorter than the day. In that case, these effects 

 will compensate each other, and if there were no atmosphere the 

 day and night would be equal. But in this case sunrise and sun- 

 set would take place not at six o'clock, but a little later. The 

 tendency of the sun for the twelve hours before noon being to 

 render the nights longer than the days, the sun would not rise till 

 .after six, and its tendency during the twelve hours of the noon 

 being to render the day longer than the night, the sun would 

 not set until after six. 



If the equinox of March take place in the forenoon, the tendency 

 of the sun in the interval since the preceding midnight being to 

 render the nights longer than the days, and its' tendency in the 

 longer interval until the next midnight being to render the days 

 longer than the nights, the latter tendency will prevail, and the 

 day would be longer than the night. 



If the equinox of March take place in the afternoon, the con- 

 trary effects will ensue for like reasons, and the night would be 

 longer than the day. 



Similar observations will be applicable to the equinox of 

 September, but with opposite results. If the equinox take place 

 in the forenoon, the night will be longer than the day, and if in 

 the afternoon, the day will be longer than the night. 



It must not be forgotten, however, that these conclusions are 

 such as would follow only on the supposition that the effect of the 

 atmosphere is excluded. 



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