The first motion is rotation. Earth rotates on its axis
once every 24 hours. One-half of the Earth’s surface is
therefore facing the Sun at all times. Rotation about
Earth’s axis takes place in an eastward direction. Thus,
the Sun appears to rise in the east and set in the west.
(See fig. 1-3.)
The second motion of Earth is its revolution around
the Sun. The revolution around the Sun and the tilt of
Earth on its axis are responsible for our seasons. Earth
makes one complete revolution around the Sun in
approximately 365 1/4 days. Earth’s axis is at an angle
of 23 1/2° to its plane of rotation and points in a nearly
fixed direction in space toward the North Star (Polaris).
Solstices and Equinoxes
When Earth is in its summer solstice, as shown for
June in figure 1-4, the Northern Hemisphere is inclined
23 1/2° toward the Sun. This inclination results in more
of the Sun’s rays reaching the Northern Hemisphere
than the Southern Hemisphere. On or about June 21,
direct sunlight covers the area from the North Pole
down to latitude 66 1/2°N (the Arctic Circle). The area
between the Arctic Circle and the North Pole is
receiving the Sun’s rays for 24 hours each day. During
this time the most perpendicular rays of the Sun are
received at 23 l/2°N latitude (the Tropic Of Cancer).
Because the Southern Hemisphere is tilted away from
the Sun at this time, the indirect rays of the Sun reach
only to 66 1/2°S latitude (the Antarctic Circle).
Therefore, the area between the Antarctic Circle and
the South Pole is in complete darkness. Note carefully
the shaded and the not shaded area of Earth in figure 1-4
for all four positions.
At the time of the equinox in March and again in
September, the tilt of Earth’s axis is neither toward nor
away from the Sun. For these reasons Earth receives an
equal amount of the Sun’s energy in both the Northern
Hemisphere and the Southern Hemisphere. During this
time the Sun’s rays shine most perpendicularly at the
equator.
1-6
Agf0103
MIDNIGHT
SUNSET
SUNRISE
NOON
Figure 1-3.—Rotation of the Earth about its axis (during equinoxes).
