Along the ecliptic lies the belt known as the zodiac

Along the ecliptic lies the belt known as the zodiac. It is sixteen degrees wide, eight degrees on either side of the Sun's path, and within this belt the moon, the planets and all the other constituents of the solar system have their home. Except for the erratic comets, Pluto and some of the asteroids, no member of the Sun's family wanders more than eight degrees away from the Sun's annual path.

If we projected the equator of the Earth onto the heavens, we should find that it does not coincide with the path of the Sun. The ecliptic and the celestial equator form two great circles which intersect each other at two points, forming oblique angles. The two intersections form the "spring" and "autumnal" equinoxes. We know well enough, in ordinary terminology, what this means. When the Sun crosses the equator coming north, we say that spring has begun. The angle between the path of the Sun and the equator is known as the obliquity of the ecliptic.

In 5000 B. C. this angle was 24° 22', whereas today it is only 23° 27'. This reduction is so slight, that it was never noticed until recent times; but it is great enough to have brought about historical changes. Because of the decrease in the angle, we can no longer see the Sun down the central line of the great Temple at Karnak. In fact there is a very slight bend in the temple itself, as if during the fifteen hundred years of construction the builders had noticed something wrong and tried to make an alteration. Perhaps the later architects even rose up and called their ancestors careless, whereas it was their divinity, the Sun, who was really at fault. If the angle of the obliquity of the ecliptic had increased throughout the centuries we could now see the Sun down the length of the temple a few days before and after the solstice; but it has decreased and the Sun never shines the full length. The little statue behind the niche has disappeared, and the god has proved himself to have--if not feet of clay--at least more variability than his worshipers ever imagined.

Today the Sun comes northward, crossing the equator at an angle of 23° 27'. At the beginning of the sign of the Crab, it pauses for a few hours above the terrestrial latitude of 23° 27' North and then begins to swing back. That time is June 21. It stands still again for a short while above latitude 23° 27' South, as it enters the sign of the Sea Goat. These latitudes are the Tropics of Cancer and Capricorn and it is only between them that the Earth can receive the rays of the Sun vertically.

In all history and literature the only creature who claimed to have run through the Tropics of Capricorn and Cancer was Kipling's old man Kangaroo--but "he had to." The names along the Tropic of Capricorn are the names of deserts and oceans. Here and there a few well-known cities stand on the line: Rio de Janeiro; Walfish Bay in Africa; Rockhampton, Queensland; Antifagasta; but for the most part the southern tropic is desolate: northern Transvaal, southern Madagascar, the Andes and the desert in the middle of Australia (where old man Kangaroo quite naturally began to hop-"he had to"). They sound as hot and cheerless as the names along the equator: Congo, British East Africa, Borneo, Sumatra; but the equator cools off a bit at Quito, Ecuador, nine thousand feet in the air; and gets a taste of fresh water where the Amazon runs down to the sea.

The Tropic of Cancer is a different matter--it was nearer to the civilized portions of the Earth in ancient times, and its climate gave it a better chance. Where the southern tropic is barren and desolate, the northern is romantic--the line of trade routes and famous cities, cities that are the focal points for the great civilizations that lie around them. Mecca is on the Tropic of Cancer; so are Calcutta, Canton, Mexico City and Assuan, where now the dam stretches across the Nile. From Assuan were gauged the first observations ever made on the ecliptic.

About 240 B. C. Eratosthenes, the energetic librarian of Alexandria, heard that the Sun at the summer solstice was vertically above the city which he called Cyene, and which we know as Assuan. With the aid of an armillary sphere he calculated the width of the tropics as 47° 42' 39¨. He divided that figure in half and said that the angle of the obliquity of the ecliptic was 23° 51'. Next he decided that since the Sun was a fiftieth part of the circumference below the zenith on midsummer at Alexandria, the distance between Assuan and Alexandria must be a fiftieth part of the circumference of the Earth. On this basis he proceeded to make a geography.

Considering the crudity of his instruments, the difficulties of scientific travel, and the lack of precedents under which Eratosthenes labored, his conclusions were perfectly marvelous. He never rested content with any half-solved problem, and for forty years he continued to measure the Earth, write geographies, and compose poetry in his spare time, until, at the age of eighty, he starved himself to death because he was afraid of going blind.

The two other divisions of the Earth, the Arctic and Antarctic circles are likewise set by the obliquity of the ecliptic. As we go farther north, the summer Sun is visible for a longer time; the length of twilight increases, and at a distance of 23° 27' away from the poles, the Sun at midsummer's day never sets at all. It drops to the horizon, but not below. Still farther north there are a number of nights when the "Midnight Sun" shines clear. And exactly at the North Pole, the Sun is visible throughout the summer.