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Pythagoras (6th Century B. C.) was apparently the first to say that the Earth was round. Anaxagoras (sth Century B. C.) stated that the moon shone by reflected sunlight. Heraclides of Pontus (4th Century B. C.) was the first to explain movements of celestial bodies by Earth's rotation. Aristarchus (3rd Century B. C.) proposed that Earth revolved round the sun. Eratosthenes (3rd Century B. C.) actually measured Earth's diameter with less than 1% of error. Thus two millenniums, more or less, before Galileo, Columbus and Copernicus, the primary facts about Earth were observed or shrewdly guessed by scattered pioneers.
The Moon is about 240,000 mi. from Earth and some 2,000 mi. in diameter. Physical theory precludes the possibility of water or atmosphere on the moon. The moon is studded with extinct vol canoes, some enormous. Analysis by single spectrum colors shows its surface to be composed of volcanic ash, with a lone patch of sulphur near the crater of Aristarchus. Volcanic ash is a poor conductor of heat. So when Earth eclipses the moon the temperature of the lunar surface (as measured from Earth with sensitive thermocouples) drops from 200° F. to —150° in a few minutes. During the lunar night the temperature is —250°.
The Sun could contain more than a million bodies the size of Earth. Its dis tance from Earth is 93,000,000 mi., its diameter 864,000 mi. Spectroscopic analysis of its light shows the surface temperature to be 5,600° C. It rotates once every 26 days. Its spots (whirlpools of cooling gas) make earthly summers cool and wet. The Sun's huge mass makes it likely that the pressure at its core is 735,000,000,000 Ib. per sq. in., and the temperature is probably 50,000,000° C. A pinhead heated to that point would radiate energy at the rate of three quadrillion horsepower, knock down fortresses by sheer pressure of radiation, burn to a crisp any human within 1,000 miles.
The Planets are nine in number. Six were known before history began: Pluto, a vague ghost wandering outside all the others, was not located until 1930. All rotate in the same direction around the sun, and very nearly in the same plane.
The canals of Mars are an illusion. Schiaparelli thought he saw fine, straight markings with a low-power telescope in 1877, and his Italian word canali (channels) was translated as "canals," which partly accounts for the subsequent uproar.
Innermost and smallest planet, Mercury keeps the same face always toward the sun, as the moon does toward Earth. Theory and observation agree that the temperature of the sunward face is 675° F.
The rings of Saturn seem to be streams of tiny moons which occasionally collide with one another, scattering fragments upon the surface of the planet.
The faces of Venus, planet most like Earth, and of Jupiter, biggest planet, are hidden by densely clouded atmospheres. Five times farther from the sun than Earth, Jupiter is inhospitably cold and its atmosphere is acrid with methane (marsh gas) and ammonia. Venus, warmer than Earth, seems to Sir James the most propitious place for extra-terrestrial life to evolve in the long stretch of the future. Quantities of carbon dioxide have lately been discovered in its atmosphere. This would provide food for plants, which in turn would furnish oxygen for higher forms of life.
* Macmillan ($3).