GOOD TIME AND ITS ASCERTAINMENT. 519 



GOOD TIME AND ITS ASCERTAINMENT. 



By Professor ISAAC SHARPLESS. 



THE natural divisions of time are the year and the day. The week 

 is arbitrary, being probably derived from considerations first 

 suggested by the first chapter of Genesis. The month, though origi- 

 nally intended to be the time from one new moon to the next, has, of 

 necessity, departed from this idea, in order to make an even number 

 in the year. The decade and the century are purely artificial, deduced 

 from our system of numbering. But the day and the year, the one de- 

 rived from the reappearance of light and darkness, the other measur- 

 ing the round of the seasons, are universally adopted units of time, 

 suggesting themselves alike to cultured and savage, and which we can 

 not think will ever be superseded. 



The year is the time of the revolution of the earth around the sun. 

 Its measure is most easily obtained by the reappearance of the sun at 

 the same altitude in the sky. Every one knows that it is higher in 

 summer than in winter. If the circle of the earth's equator were 

 extended right out from the center of the earth into the sky, it would 

 cut out a circle there which is called the celestial equator. Now, the 

 sun crosses this line in the spring northward, arriving at its greatest 

 altitude in the middle of summer ; thence it descends, crossing the line 

 southward in the fall, and reaching its lowest point in midwinter. 

 The ancients, by measuring the length of the shadow cast by a verti- 

 cal stick on different days of the year, arrived at surprisingly correct 

 results as to the length of the year. In 450 b. c, Democritus asserted 

 the year to be 365 days long, which is within about eleven minutes 

 of the truth. Another ingenious device for the same purpose was that 

 of the Egyptian astronomers, who set up a wheel parallel to the plane 

 of the equator. When the sun was in this plane, the shadow of the 

 sunward side of the wheel would be exactly intercepted by the other, 

 and the interval between two such occurrences would measure the 

 year. Owing to the fact that the sun does not "cross the celestial 

 equator in the same place each year, this year which measures the 

 seasons is a few minutes shorter than the exact time of the earth's 

 motion around the sun. 



To measure the day troubled the ancients much more. It is, per- 

 haps, a common idea that the shadow of a vertical rod cast by the sun 

 is always exactly northward at twelve o'clock noon. Any one desir- 

 ous of trying this can easily do so, and he will find that such a shadow 

 would be sometimes eastward and sometimes westward of the merid- 

 ian-mark at noon. Moreover, he will find that the time between two 

 passages of the sun over his meridian is not the same, so that, if this 

 time were taken as the day, there would be no uniformity. It was, 



