354 GENERAL SCIENCE 



sun rises farther and farther south on the eastern horizon, is lower and 

 lower down from the zenith at noontimes, and sets farther and farther 

 south on the western horizon. From December to June these changes 

 are in reversed order. The duration of the sunlight period, i.e., the 

 length of the days 1 , varies with these changes. 



In the northern hemisphere the longest days are in June when the 

 sun's path across the sky is highest up toward the zenith, and the 

 days are shortest when those "diurnal circles" are lowest down toward 

 the south. This is due to the progress made by the earth in its revolu- 

 tion about the sun. The observer is brought into new positions in the 

 earth's orbit day by day from which to look out toward the sun as the 

 earth's rotation brings him around on the daylight side. 



Upon the duration of the period of sunlight, and upon the obliquity 

 of the sun's rays for any observer, depends the amount of solar heat 

 received by him, and the changes of seasons for that locality. 



The year as the period of the earth's revolution, and the day 

 as the period of the earth's rotation, are natural time units of unchang- 

 ing values. It is true that the earth's velocity in different parts of its 

 orbit varies, but the length of the year period does not change. The 

 earth is "on time" so far as completion of its journeys of revolution 

 are concerned. 



The lengths of the times from passage of the centre of the sun's disc 

 across the meridian of any observer till it is next on meridian vary a 

 little at different times of year. This necessitates the use of an imagi- 

 nary day known as the "mean solar day." Its length is the average of 

 the lengths of all the solar days of a year. Our clocks and watches 

 keep mean solar time. 



Exercises 



1. Why do we experience so much difficulty in realizing the fact that we live 

 upon a rotating earth? 



2. What is meant by (a) horizon; (b) sunrise? 



3. Since the daily paths of the stars across the heavens, i.e., their diurnal 

 circles, result from the earth's rotation, and since the direction of the 

 earth's axis is at all times parallel to itself and always points toward the 

 same place in the heavens, what must be true of the relative positions of 

 the diurnal circles (a) of different stars; (b) of the same star? 



1 It usually is possible to distinguish from the context whether the word 

 "day" has reference to the period of the earth's rotation (24 hours), or to 

 the duration of daylight (not night-time). 



