COMMON THINGS THE ALMANACK. 



sunrise and sunset is intermediate between the actual beginning 

 and end of the sun's appearance or disappearance. 



All the observations which have been here made respecting the 

 rising and setting of the sun are equally applicable to the rising 

 and setting of the moon, the apparent diameter of which is equal 

 to that of the sun. 



The interval between the moment at which the sun or moon 

 begins to rise or set, and that at which it has completely risen 

 or set, varies in different places and at different seasons, but is 

 generally something more than two minutes. 



58. The optical property of the air, by which the effects above 

 described are produced, is called refraction ; and the displace- 

 ment which is produced in the position of an object is called its 

 refraction. 



The refraction is greater or less according as the altitude is 

 greater or less, and disappears altogether when the object is in the 

 zenith, that is, when it is directly above our heads. 



The effect, therefore, of refraction Is to make the sun rise earlier 

 and set later than it would if no atmosphere existed. The days 

 are thus at all seasons rendered longer, and the nights shorter, 

 than they would be if the earth were not surrounded by an 

 atmosphere; and as the effect of refraction retards the setting 

 and accelerates the rising by about two minutes, it increases the 

 length of the day, and decreases that of the night by about four 

 minutes ; this, however, is subject to variation depending on the 

 latitude of the place and the season of the year. 



59. The equinoxes, as commonly understood, are those days 

 in March and September on which the intervals of light and 

 darkness are equal, the sun rising and setting at 6 o'clock. 



. Now any one may convince himself by reference to the 

 columns of sunrise and sunset in an almanack that no such days 

 ever exist. 



Yet the very name of equinox is taken from the supposition of 

 equal day and night. How then is the equinox to be understood, 

 and from whence has it derived its name ? 



It may perhaps be supposed that although there be no case of 

 day and night absolutely equal, the equinoxes may be those 

 days in March and September in which the day and night are 

 least unequal. 



But if the columns of sunrise and sunset be examined in any 

 almanack, it will be found that the day upon which the intervals 

 of light and darkness are least unequal, precedes the day of the 

 equinox in March and follows it in September, by one or two days. 



This is so contrary to the commonly received notions that the 

 point will require some explanation. 

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