Man's physical universe

xanabras

106 THK UNIX'KRSK A \'AST SYSTEM OF PARTS

An eclipse is caused by the shadow of one body falling across another.

When the moon lies between the sun and the earth, a shadow is cast on

the earth and there is a solar eclipse; when the earth lies between the

sun and the moon, a lunar eclipse results.

The only solar eclipse visible in the United States up to 1950 will be

the eclipse of July 9, 1945. Under average conditions a solar eclipse

lasts about two minutes, and the width of the shadow is less than a

hundred miles.

Partial eclipses of the sun by planets, caused by their passage

between the sun and the earth, are rare events called transits.

Total eclipses of planets or stars caused by the passage of the moon

between them and the earth are called occultations .

The occultations

of the brighter planets — Venus, Mars, Jupiter, and Saturn — are

very spectacular.

Mars Has Become Well Known as a Possible Abode of Human Beings.

Percival Lowell (1855-1916) observed regular markings on the surface

of Mars which he believed to be canals built by a race of men to conduct

water from the melting edges of the ice-capped poles to the warmer

portions of the planet.

Other observers have failed to see that these

"canals" were so straight that they could not be explained in terms of

ancient river valleys or other natural phenomena. The considerable

seasonal changes in size, shape, and color of these markings, however,

indicate that it is possible that vegetation exists on Mars. This idea

is also supported by the fact that oxygen is found to be present in the

atmosphere of Mars, although the amount present is not sufBcient to

support animal life as we know it.

Suppose that man could make a trip to Mars.

What a sight-seeing

excursion that would be! Phobos, the inner of Mars' two moons,

would certainly be one of the main attractions. This moon, which is

only ten miles in diameter, revolves about Mars at a distance of 3700

miles once every seven hours and thirty-nine minutes. Phobos will

rise in the west and set four hours later in the east, passing through

all of its phases during this time. No other known satellite revolves as

Phobos does in a shorter interval than the rotation of its primary. On

the other hand, Deimos, the second moon, will rise in the east and not

set for several days because its period of revolution is only slightly

greater than that of Mars.

The Asteroids Are a Group of Minor Planets.

Journeying outward from Mars, we come to the Asteroids, which

include 400 or more minor planets. One of these Asteroids, Eros,

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