National Geographic : 1900 Feb
PRACTICAL EXERCISES IN GEOGRAPHY stage is reached, better values of the sun's declination may be taken from the Nautical Almanac for the current year, accessible in the larger public libraries. If it is not accessible there, ask the librarian to get it. The teacher of mathematics should be able to explain how to use it in finding the sun's declination on any date. The Year.-The time and direction of sunrise and sunset should be tabulated and diagramed. The correlation of the day's length, the direction of the sun at rising and setting, and the changes in midday altitude are most instructive. Each quantity affords occasion for prediction and verification of its future values. All the changes in these quantities are run in a period of 365 days, and in the same period the star group first seen in the east shortly after sunset is again seen there at the same hour. Now let the scholars try to explain this return to a previous condition, suggesting to them that a line may be imagined starting at the sun, passing through the earth, and extend ing to the distant stars. This line has been found to sweep through the sky, pointing to one star group after another, and to return to the original group in the same period as that in which the noon altitude and its correlated quantities run through their variations. Then the earth must go around the sun once in 365 days. The time unit, called a year, has long been familiar to the scholars ; they have prob ably heard or read that the earth goes around the sun in a year, but those words are now fuller of meaning than they were ever before.* The sensible constancy of the sun's diameter apparent (determined by letting a ray of sunlight pass through a pin-hole in one sheet of paper and fall upon another sheet at a fixed distance from the pin hole) should serve to give a good idea of the form of the curve or orbit that the earth runs around. Inclined Attitude of Axis to Orbit.-The facts regarding noon altitude and the correlated quantities can all be explained if it be suggested that the axis on which the earth has been found to turn does not stand vertical to the plane of the orbit in which it has been found to revolve. Here again a globe is of value as a mental aid and an aid in visualizing the necessary geometrical relations. So are the dia * In order to give a better determination of the length of the year than can be obtained merely by general inspection of the eastern constellations after sunset, the following plan may be adopted: Observations in September and October will show that the stars occupy more and more western positions at a given hour on successive evenings. Let the more skillful scholars make record of the position of some recognizable star with respect to a roof or chimney at a certain hour on a certain evening, then ask them to discover when the star will again be in that position at that hour. It will be well to have records of this sort made on several differ ent evenings, so as to lessen the possible trouble from cloudy evenings in the following year.