Celestial Bodies 147 
will be 1/1000 of a second longer than it was in 1900. Over 
the course of a century such lengthening of each day adds up 
to quite a bit. Since Julius Caesar's time the earth has lost 
over an hour's time. 
WHAT DO YOU WEIGH? : 
The spinning of the earth on its axis causes a centrifugal 
force acting at the equator which lessens the apparent weight 
of objects. A man who weighs 200 pounds at the North Pole 
would weigh about 198.94 pounds at the equator. 
Besides spinning upon its axis, the earth also moves 
around the sun in an elliptical orbit, completing one revolu- 
tion every year. The average distance of the earth from the 
sun is about 92,900,000 miles; the distance changes slightly 
during the year, being smallest in December, and largest in 
June. The size of the elliptical orbit, its orientation in space, 
and its shape may be described quantitatively by certain geo- 
metric quantities called the “elements” of the earth’s orbit. 
ICE AGES 
These elements do not remain invariable for long periods 
of time because there are other planets in the solar system 
which tend to disturb the motion of the earth by their gravi- 
tational influences. Such slight perturbations of the earth's 
orbit over the course of many hundreds of thousands of years 
cause certain portions of the earth to receive more or less sun- 
light and result in corresponding changes in temperature and 
climatic conditions. There is some evidence that such long 
period perturbations of the earth’s orbit are among the pri- 
mary causes of those great periods of cold in past history 
—the Ice Ages—which have occurred at intervals during the 
