National Geographic : 1900 Feb
PRACTICAL EXERCISES IN GEOGRAPHY 77 grams that one usually finds in text-books, although they are much less serviceable than globes.* Whether children of under fourteen years of age can discover this solution of the problem or not remains to be proved. At least they should have a good chance to show their capacity to discover it, a carefully prepared chance, approached by the slow accumulation of pertinent observations, all familiarized by repetition. A simple construction of the earth's orbit is also serviceable at this stage. Draw upon a sheet of paper about a foot square a line through its middle parallel to one side. Locate the middle point of the line. Construct a scale whose units are 20 of the side of the paper, so that two pins, three units apart, can be driven into the middle line sym metrically on either side of the middle point. Lay a loop of thread or fine string 189 units in perimeter over the pins; stretch it tight with a pencil, and draw a curve thus guided. This curve shows the true pattern of the earth's orbit, the units of the scale being millions of miles. The orbit is as sensibly circular as are the earth's meridians. Take out one of the pins, and around the other draw a little circle, a trifle less than a unit in diameter, to represent the sun; a good sized pin-head will not be much too small for it. Assuming that the North star is above the plane of the orbit (or paper), the earth moves around the orbit so as to pass from right to left when viewed from the sun. Find the point on the orbit that is nearest to the sun (it must lie where the orbit is cut by that half of the middle line which passes through the sun). Conveniently for our memories, the sun celebrates New-Year's day by passing through this near-sun point, or perihelion. July 1 sees the earth at the opposite far sun point, or aphelion. Go backward along the orbit from perihelion one-ninth of a quadrant arc; this is the point occupied on Decem ber 21, the date of the sun's least midday altitude, or the winter solstice. Draw a line from this point through the sun; it intersects the orbit at the summer solstice, which the earth passes on June 21. Draw a line through the sun at right angles to the solsticial line; it intersects the orbit in the equinoctial points. Set up a small ball on a vertical axis to represent the earth at the winter solstice; the sun can then be imagined to illuminate the near half of the earth; the day-and-night circle will separate the illuminated half from the dark half of the earth. As the earth now stands, with a vertical axis, the * A simple, small and cheap " elementary globe," divested of nearly all names, and showing only the most general relief, is published by A. Donnelly, Oxford, N. Y .