The points where the pins pierce the paper are called the foci, and, as will be seen, each of these points is nearer one half of the curve than the other. If a body were placed at one of these points and another body moved around it in the line of the curve, the two bodies would be nearest each other when passing one point in the line extending through the foci, and farthest apart when passing the opposite point in the same line.
Now the sun is at one of the foci of the ellipse in which the earth moves, so the distance between the sun and the earth varies during the year. This variation is about three millions of miles, the average distance of the earth from the sun being about 93,000,000 miles. Strange as it may seem, we are nearest the sun in January and farthestFig. 10.
Fig. 9.
away in July (Fig. 9). We are not warmest in the northern hemisphere in January because our hemisphere is then pointed away from the sun and therefore there are fewer heat rays falling upon a given area in this hemisphere than there are when we are farther away in July.
11.Latitude Zones. -As the axis
of the earth is inclined 23.5° from the perpendicular to the plane ofits orbit, the rays of the sun will fail vertically at some times during a year upon all points within 23.5° of the equator both north and south. To this region we have given the name of the torrid zone. There will be a day during the year when no direct sunlight will fall upon points within 23.5° of the poles. The areas inclosed by lines drawn around the earth, 23.5° from the poles, are called frigid zones. The areas betweenA LAPLANDER"S HUT.
Made of thick sod to keep out the cold of the frigid zone.
the frigid zones and the torrid zones are called the temperate zones. We live in the north temperate zone.
Although as far as the direct influence of the sun is concerned, these zones are easily separated and bounded by parallels of latitude, yet, on account of other influences, the temperatures of the zones thus bounded are very irregular.
For instance, at some places like Hammerfest within the frigid zone the average temperature is much higher than at places like Labrador within the temperate zone, so that as regards temperature, the parallels of latitude are uncertain boundaries. This will be more fully discussed later.
12.The Cause of the Seasons. -Since the earth moves aroundthe sun with its axis inclined to the plane of its orbit, the hemispheres will at different times be in-clined toward and away from the sun. When the north- ern hemisphere is inclined toward the sun, the rays of the sun cover the north pole continuously for six months, so that at this point thereS.P.is no night for all that time. The days are longer and the THE PATH OF THE EARTH AROUND THE SUN.
Showing roughly the four positions mentioned in the text.
nights shorter throughout all the northern hemisphere, and more than this, the rays then fall upon this hemisphere more nearly vertically than during the rest of the year.
The nearer vertical the rays, the greater the number that fall upon a given area and the greater the amount of heat received by that area. More heat is received in the northern hemisphere not only because the rays fall more nearly vertically but also because the length of theday is increased. The amount of heat received from the sun continues to increase as long as the sun appears to move north, or until its rays fall vertically upon the tropic of Cancer. This occurs on the 21st of June, which is called the summer solstice. At this time our days are the longest and our nights the shortest. But the days are not the hottest, as the heat gradually accumulates for some time, more being received each day than is given off.
As the earth proceeds in its orbit from this point, the inclination of the north pole toward the sun becomes less and less, until on the 23d of September the sun is directly over the equator. The north pole nowHUT IN THE TROPICS.
Made of thin walls but a heavy thatched roof to keep out the rain.
begins to point away from the sun. On December 21 the direct rays of the sun fall upon the tropic of Capricorn, and the sun has reached its farthest point south, our days being then the shortest and the days in the southern hemisphere the longest. From this point until March 21, when the sun is again vertical over the equator, the inclination of the north pole away from the sun decreases. The days when the sun is over the equator are called the autumnal (Sept. 23) and vernal (March 21) equinoxes, since the days and nights are then of equal length all over the earth.
The greater heating of the hemisphere at one part of the year than at another gives us the changes which we call the seasons. Since the change in the length of the day and in the direction .of the sun"s rays is very small within the tropics, the change in the amount of heat received is very slight, so that in this region there is almost no change of seasons. But at the poles, where for six months there is continuous night and for six months continuous day, the change of seasons is exceedingly great. At middle latitudes the changes, though marked, are not excessive.
There are then two causes which combine to give us our change of seasons: the revolution of the earth around the sun, and the inclination of the earth"s axis to the plane of its orbit.
13.The Measurement of Time.
Experiment 9. -On a fair day place a sundial in an exposed position, and after carefully adjusting it, compare its readings with those of an accurate watch. Probably your watch is set to railroad time and the readings therefore are not alike, unless you are on the time meridian.
Although the exact determination of time is a difficult task and requires great skill and very accurate instruments, yet it is not very hard to determine quite satisfactorily the length of a solar day.