492 POPULAR SCIENCE MONTHLY. 



FOREIs^OON AND AFTERNOOiT. 



Bt CHARLES F. DOWD, Ph. D. 



IT is a fact of common observation, at different times of the year, 

 that the forenoon and afternoon, as to daylight, are of unequal 

 length. Along in later autumn the shortness of the afternoons is 

 very noticeable, and the shortness of forenoons along in later win- 

 ter. Whatever makes common facts more intelligible adds to the 

 general intelligence and to the general good. It is to this end that 

 the folloAving brief statements are made. 



Nothing is more evident than that the sun requires just as much 

 time to go from the eastern horizon to the midday meridian as to 

 go from that meridian to the western horizon. But, strange to say, 

 there are but four days during the whole year in which the sun 

 reaches the midday meridian at just twelve o'clock. The true 

 noon point varies from about fifteen minutes before to about six- 

 teen minutes after twelve o'clock. These extreme points in one 

 set of variations fall in the first week of November and in the sec- 

 ond week of February, not to designate exact days for years in 

 general. 



The calendars show that in the latitude of Saratoga (essen- 

 tially Boston latitude) on November 3, 1898, the sun rose at 6.30 

 and set at five o'clock, thereby making the forenoon a half hour 

 longer than the afternoon. On that day the sun reached the mid- 

 day meridian at 11.45. On February 13, 1899, the sun rose at just 

 seven o'clock and set at 5.30, thereby making the afternoon a half 

 hour longer than the forenoon, and on this day the sun reached 

 the midday meridian at 12.15. These are facts plainly open to 

 general view, and therefore need no verifying. 



The causes of the foregoing are not so apparent to common 

 observation. It must be borne in mind that the mean or average 

 solar day is the basis for all time measurements, therefore its exact 

 length is of the greatest importance. Yet the general solar day, 

 from which the average one is derived, is a very indefinite term 

 as to its length. Its length in general may be defined, under view 

 of the sun's apparent motion, as the time extending from the in- 

 stant that the sun's center crosses any given meridian of the earth 

 on one day to the instant that center crosses the same meridian 

 on the following day — i. e., the time intervening betwe'en these 

 two instants is the length of a solar day. 



The motion of the sun, however, is only apparent; the actual 

 motion is in the earth's revolution upon its axis. We should have 

 one day a year long if the earth did not revolve on its axis at all, 



