THE EARTH'S PLACE IN THE UNIVERSE 25 



that is, as far as the poles. Daylight then extends to both 

 poles, and day and night are equal everywhere. 



All that has been said about the direction of the light 

 is also true about the heat received from the sun. From 

 spring to autumn, the north pole is turned toward the sun. 

 The sun's rays, giving light and heat, are then received ver- 

 tically at places in the torrid zone north of the equator, and 

 obliquely at places within a distance of 90, that is, reaching 

 to places beyond the north pole. It is summer in northern 

 latitudes, and the days are longer and warmer there than 

 in the other half of the earth. Just the reverse is true in 

 the period between autumn and spring. It is winter in 

 northern latitudes, and the days there are shorter and 

 colder than in southern latitudes. 



16. The Circles and Zones of Light. If, by drawing 

 a line upon the earth, we could connect all the places where 

 the sun shines vertically at noon on March 21, we should 

 have a circle around the earth equally distant from both 

 poles. Because it is equidistant, it is called the equator. 



A similar circle drawn on June 21 gives the tropic of 

 Cancer, 23 1 north of the equator. The sun never shines 

 vertically any farther north than the tropic of Cancer. On 

 June 21, it shines obliquely upon the earth beyond the 

 north pole just as far as the tropic of Cancer is north 

 of the equator that is, 23 J. Consequently the arctic 

 circle marks the line 23J from the pole and 66| from the 

 equator. 



Within the arctic circle the sun is visible every day during 

 the six months of summer, and during the winter it nowhere 

 comes above the horizon for more than a short time each 

 day. At the north pole it is visible for the whole day during 

 the summer and is not seen at all during the winter. The 

 nearer a place is to the pole, the longer are its summer days 

 and the shorter its winter days. At Hammerfest, Norway, 

 the most northern town of Europe, the sun never sets from 



