106 OUTLINES OF NATURAL PHILOSOPHY. 



tween the mean and the apparent time, and is 

 called the Equation of Time. 



a. The conversion of degrees and minutes, into 

 mean solar time, is performed by a different 

 rule from their conversion into siderial time. For 

 an hour of mean solar time, there must be reckon- 

 ed 15 2' 27".84-7 of the equator ; and so on, in 

 the same proportion. 



I. There are four times in the year, when the mean 

 longitude of the sun, and his true right ascension, 

 are equal to one another j and, at these times, the 

 apparent and the mean time coincide. These times 

 happen, at present, about the 15th of April, the 

 15th of June, the 1st of September, and the 24th 

 of December. From the first of the above periods 

 to the second, the apparent time is before the 

 mean, and the equation of time is subtractive, or 

 must be taken from the apparent time, to give the 

 mean. It is greatest at the 15th of May, when it 

 amounts to 3' 58" to be subtracted. From the se- 

 cond to the third interval, the equation is additive, 

 the mean time being before the apparent ; and it 

 becomes a maximum about the 25th of July, when 

 it amounts to 6' 6". It becomes negative between 

 the third and fourth interval, and reaches its ma- 

 ximum on the 2d or 3d of November, when it 

 amounts to 16' 15" subtractive. Dr HALLEV has 

 given a geometrical construction, for determining 

 the time when the apparent days are longest or 

 shortest, and also when they are equal to the 



mean. 



