OUR EARTH, A PLANET 



191 



Exercise. Explain why the days and nights at the 

 equator are always of equal length.* 



Exercise. Cite evidence to support or disprove this 

 statement: There are places on the earth other than 

 exactly at the poles where during some months each year 

 the sun never rises and during other months it never sets. 



Exercise. A newspaper report recently stated that a 

 baseball game was played in a certain part of North 

 America at ten o'clock in the evening without the aid 

 of artificial illumination. Suggest the general location of 

 the place in which the game might have been played. 



What causes the seasons of the year? The earth 

 revolves in its orbit about the sun. The earth com- 

 pletes 365^4 rotations upon its axis while it is re- 

 volving once around the sun. Thus our year is 365}4 

 days long. 



The seasons are due to the fact that the earth's 

 axis is inclined at an angle of 23^2 degrees towards the 

 plane of its orbit (Fig. 314) and also because the axis 

 always points towards the North Star. If the earth's 

 axis were not inclined or tilted, there would be no 

 seasons, because the direct rays of the sun would al- 

 ways strike the equator, and days and nights would 

 be equal everywhere, every day of the year. 



Figure 314 gives the positions of the earth in rela- 

 tion to the sun at the different seasons of the year. 

 There are only two days in the year, about March 21 

 and about September 22, when the earth's axis is not 

 inclined toward or away from the sun. On these two 

 dates the rays of the sun fall perpendicularly upon the 

 equator, and every place on the earth has twelve hours 

 of daylight and twelve hours of darkness. March 21 

 is called the spring or vernal equinox and September 



Courtesy Science Service 



FIG. 313. PHOTOGRAPH OF THE SUN IN ECLIPSE 



22 the fall or autumnal equinox. As the earth continues 

 its course in its orbit after March 21, the upper end of 

 its axis shifts, from day to day, towards the sun. On 



1 See workbook, p. 71. 



June 21 the rays of the sun fall vertically on the tropic 

 of Cancer, 2Z l / 2 degrees north of the equator. At this 

 time all of the arctic circle receives the sun's rays, 

 while the antarctic circle is in darkness. June 21 is 

 called the summer solstice. At this time the day for 

 all places north of the tropic of Cancer is more than 



March 21 



Equator 



Arctic Circ'le 



Tropn t% Cdnwr 

 Fqu&tor 



December 



|\ Tropic 

 of Capricorn 



wrcht/ 



Circle,'' 



Equator 

 September 22. 



FIG. 314. SEASONAL INCLINATION 



twelve hours long, and we have our summer. The day 

 for all places south of the tropic of Capricorn is less 

 than twelve hours long, and people living in the south 

 temperature zone have their winter. 



From June 21 to September 22 the days again be- 

 come shorter. The upper end of the earth's axis on 

 September 22 is again pointing neither away from 

 nor toward the sun. The sun's rays fall directly on 

 the equator, and days and night are equal everywhere 

 on the earth. September 22 is the fall equinox and 

 marks the beginning of the fall season. 



In the northern hemisphere from September 22 to 

 December 22 the tilt of the earth away from the sun 

 is slightly more each day. On December 22 the sun's 

 rays strike directly on the tropic of Capricorn, 23 1 / 2 de- 

 grees south of the equator. This is the ^vintcr solstice. 

 We have our shortest day and our longest night. The 

 entire antarctic circle is in daylight and the entire 

 region of the arctic circle is in darkness. December 22 

 is the first day of winter for us and the first day of 

 summer in the south temperate zone. Thus the sea- 

 sons in the countries south of the tropic of Capricorn 

 are always the reverse of ours. 



As the earth continues in its orbit from December 

 22, the days increase in length and the seasons are 

 repeated. 



Why is summer warmer than winter? The earth's 

 orbit is not a circle, but an ellipse. The sun is not ex- 

 actly at the center of its orbit. The differences in tem- 

 perature between summer and winter are not due, 



