Again, the heating power of the Sun in each hemisphere is greatest at the period of the longest days, because of its greater altitude in the heavens; and least at the period of shortest days. Thus long days and a high sun operate together to produce the high temperature of summer; while long nights and a low sun cause the low temperature of winter.
The following table gives the length of the longest day, excluding the time of twilight, and of the shortest night, in the different latitudes, with the difference of duration in hours and minutes, thus exhibiting more clearly the above law.
TABLE OF UNEQUAL DAYS AND NIGHTS
| LATITUDE | Longest Day | Shortest Night | Difference | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Equator | 12 | .0 | hours | 12 | .0 | hours | 00 | .0 | hours |
| 10° | 12 | .7 | „ | 11 | .3 | „ | 1 | .4 | „ |
| 20° | 13 | .3 | „ | 10 | .7 | „ | 2 | .6 | „ |
| Tropics | 13 | .5 | „ | 10 | .5 | „ | 3 | .0 | „ |
| 30° | 14 | .0 | „ | 10 | .0 | „ | 4 | .0 | „ |
| 35° | 14 | .5 | „ | 9 | .5 | „ | 5 | .0 | „ |
| 40° | 15 | .0 | „ | 9 | .0 | „ | 6 | .0 | „ |
| 45° | 15 | .6 | „ | 8 | .4 | „ | 7 | .2 | „ |
| 50° | 16 | .3 | „ | 7 | .7 | „ | 8 | .6 | „ |
| 55° | 17 | .3 | „ | 6 | .7 | „ | 10 | .6 | „ |
| 60° | 18 | .7 | „ | 5 | .3 | „ | 13 | .4 | „ |
| Polar Circles | 24 | .0 | „ | 0 | .0 | „ | 24 | .0 | „ |
| 671⁄2° | 1 | month | 0 | .0 | „ | ... | |||
| 691⁄2° | 2 | months | 0 | .0 | „ | ... | |||
| 73.3° | 3 | „ | 0 | .0 | „ | ... | |||
| 78.3° | 4 | „ | 0 | .0 | „ | ... | |||
| 84° | 5 | „ | 0 | .0 | „ | ... | |||
| North Pole | 6 | „ | 0 | .0 | „ | ... | |||
LAW OF VARIATION OF
DAY AND NIGHT
The inequality of day and night increases slowly in the tropical regions, but more and more rapidly towards the polar circles. Beyond these circles the Sun, in the hemisphere in which it is vertical, makes the entire circuit of the heavens, without sinking below the horizon, for a period varying from twenty-four hours to six months; while in the opposite hemisphere there is a corresponding period of continuous night.
RESULT OF THIS LAW IN
DIFFERENT ZONES
In the tropical regions, where the days and nights vary little in length, the temperature is nearly uniform throughout the year; while the increasing inequality of day and night towards the Poles, causes an increasing difference between the summer and the winter temperature.
Again, the length of the day, in the summer of high latitudes, compensates for the diminished intensity of the Sun’s influence; so that the temperature, in the hottest part of the day, may equal, or even exceed, that within the tropics. A summer day in Labrador or Petrograd may be as warm as one under the Equator; but in the former latitudes there are only a few days of extreme heat in the year, while with increasing nearness to the Equator the number of warm days constantly increases.