FORM, MOTIONS, LATITUDE, AND LONGITUDE 213 



the horizon at noon in summer than in winter. The same is true 

 in all latitudes similar to those of the United States. 



This variation of the angle at which the sun's rays strike the 

 earth at a given time and place, as well as the unequal lengths of 

 days and nights in most places, is the result of the inclination of 

 the axis on which the earth rotates as it revolves around the sun 

 (Fig. 202). The position of the axis is constant throughout the 

 year. The effect of the inclination of the axis is illustrated by 

 Fig. 202, which represents the earth in four positions in its orbit. 

 In the position marked March 21st, the half of each parallel (the half 

 toward the reader) is illuminated. At this time, therefore, days 

 and nights are equal everywhere. In the position marked June 

 21st, more than half (the part not shaded) of every parallel of the 

 northern hemisphere is illuminated, and there the days are more 

 than 12 hours long and the nights correspondingly less. In the 

 southern hemisphere the nights are longer than the days. In the 

 third position, September 22d, the days and nights are again equal 

 everywhere, for the circle of illumination divides every parallel 

 into two equal parts. In the figure, the lighted part is away from 

 the reader. In the fourth position, December 22d, more than 

 half of each parallel in the southern hemisphere is in the light, and 

 there the days are longer than the nights, while in the northern 

 hemisphere the nights are longer than the days. Twice during 

 the year, therefore, on March 21st and September 22d, the days 

 and nights are equal everywhere. These times are known as the 

 equinoxes. The equinox in March is the vernal equinox; that in 

 September is the autumnal equinox. 



When the earth is in the relation to the sun shown in the posi- 

 tion marked June 21st, Fig. 202, the days are longest in the northern 

 hemisphere, and the sun is highest in the heavens at noon, and its 

 rays fall perpendicularly on the surface of the earth farther north 

 (23 27'+) than at any other time. This is the summer solstice 

 (Fig. 207). The winter solstice occurs six months later, when the 

 sun's rays strike the earth vertically 23^ (nearly) south of the 

 equator (Fig. 208), and when the days of the southern hemisphere 

 are longest and those of the northern shortest. Figs. 207 and 208 

 also show that the days and nights are always equal at the equator, 



