EQUATION OP TIME. 123 



Civil months are those which are framed to serve the uses 

 of life, and approach nearly to the quantity of astronomical 

 months either lunar or solar ; being made, with the excep- 

 tion of February, to consist of thirty and thirty-one days. 

 To the days of a week, the Pagans gave the names of the 

 sun, moon, and planets ; and for the first two days and last 

 day of our weeks, those names are still retained. 



A natural or solar day is the time which the sun takes in 

 passing from the meridian of any place till it comes round 

 to the same meridian again ; or it is the time from noon to 

 noon. A sidereal day is the'time in which the earth revolves 

 once about its axis. The rotation of the earth is the most 

 equable and uniform motion in nature, and is completed 

 in twenty-three hours, fifty-six minutes, and four seconds, 

 for any meridian on the earth will revolve from a fixed star, 

 to that star again in this time. Sidereal days, therefore, 

 are all of the same length ; but solar or natural days are not. 

 The mean length of a solar day is twenty-four hours, but it 

 is sometimes a little more, and sometimes less. The reason 

 of the difference between the solar and sidereal day is, that 

 as the earth advances almost a degree eastward in its orbit, 

 in the same time that it turns eastward round its axis, it 

 must make more than a complete rotation before it can come 

 into the same position with the sun that it had the day be- 

 fore ; in the same way, as when both the hands of a watch 

 or clock set off together, as at twelve o'clock, for instance, 

 the minute hand must travel more than a whole circle before 

 it will overtake the hour hand, that is, before they will be 

 in the same relative position again. It is on this account 

 that the sidereal days are found to be, on an average, shorter 

 than the solar ones by three minutes and fifty-six seconds. 



As a clock is intended to measure exactly twenty-four 

 hours, it is evident that, when a solar day consists of more 

 than twenty-four hours, it will not be noon by the sun till it 

 is past noon by the clock ; in which case the sun is said to 

 l)e slow of the clock. But when a solar day consists of less 

 than twenty-four hours, it will be noon by the sun before it 

 is noon by the clock ; and the sun is then said to be fast of 

 the clock. Time measured by a clock is called equal or 

 mean time, and that measured by the apparent motion of the 

 sun in the heavens, or by a sun-dial, is called apparent time. 

 The adjustment of the difference of time, as shown by a 



