Geoscience Reference
In-Depth Information
reaches its position of greatest tilt toward the Sun. Meanwhile,
the Southern Hemisphere is tilted away from the Sun. Dur-
ing this solstice the subsolar point is located at the
Tropic of
Cancer
(23.5° N). In Figure 3.13, notice that all latitudes above
66.5° N (the Arctic Circle) receive 24 hours of daylight during
this period of time; this is why these high latitudes are called
the
land of the midnight Sun
. At the same time the northern arc-
tic regions are in perpetual sunlight, all locations above 66.5°
S (the Antarctic Circle) experience 24 hours of darkness. This
continual darkness occurs because the Southern Hemisphere is
tilted away from the Sun during this time and those latitudes
above the Antarctic Circle never rotate into the illuminated part
of Earth.
2.
The subsolar point is the place where the Sun's rays
strike Earth most directly. This occurs because the Sun
angle is 90° at that point and the rays are perpendicular
to the surface.
3.
Due to the axial tilt, the subsolar point migrates between
the Tropic of Cancer and Tropic of Capricorn over the
course of the year.
4.
A solstice occurs whenever the subsolar point is at
either 23.5° N or S latitude. An equinox, in contrast,
occurs when the Sun is directly over the Equator.
5.
The position of Earth relative to the Sun changes as the
orbit progresses.
September 22-23
This date is the
Fall Equinox
(also called
the
autumnal equinox
) and represents the first official day
of fall in the Northern Hemisphere and spring in the South-
ern Hemisphere. At this time the Earth's orbit has progressed
such that the subsolar point is now once again at the Equa-
tor (Figures 3.12 and 3.13). As at the Spring Equinox, all lo-
cations on Earth get equal hours of day and night (12 hours
daylight, 12 hours darkness) because neither the Northern nor
the Southern Hemisphere is tilted toward the Sun and the great
circle of illumination extends from pole to pole.
Human Interactions: How We See
Earth-Sun Geometry on Earth
In the previous section you examined Earth-Sun geometry as it
looks when viewed from space. Now it is time to explore how
this geometry looks from the surface of Earth—in other words,
how
we
see and experience it. Let's begin with the basic prin-
ciple of day and night, which is related to the Earth's rotation
on its axis.
December 21-22
This date is the
Winter Solstice
and rep-
resents the first official day of winter in the Northern Hemi-
sphere and summer in the Southern Hemisphere. Given the
progression of the orbit since the Fall Equinox, Earth is now
positioned such that the Northern Hemisphere is at its great-
est tilt away from the Sun, while the Southern Hemisphere is
tilted toward it. At this time the subsolar point is located at
23.5° S (Figures 3.12 and 3.13), which is also known as the
Tropic of Capricorn
. As a result of this subsolar position, all
latitudes above the Arctic Circle experience 24 hours of dark-
ness, whereas those higher than the Antarctic Circle experience
24 hours of daylight.
Day and Night
Over the course of the day, you can “see” the Earth's rotation by
noting the position of the Sun in the sky. Everyone knows that
the Sun “rises” in the east and then arcs across the sky toward the
west. As the arc progresses over the course of the day, the Sun
reaches its highest position at
solar noon,
and then gradually
lowers as it “sets” in the west. Solar noon at any given place is not
necessarily equivalent to standard clock time because the time of
sunrise and sunset varies across a time zone due to the curvature
of Earth. Although it appears that the Sun is actually rising in
the morning and moving across the sky during the course of the
day, the fact is that the apparent motion of the Sun is caused by
the rotation of Earth on its axis. In short, the Sun rises, wherever
you may live, because your part of Earth has rotated to a position
where the Sun begins to illuminate your home.
KEY CONCEPTS TO REMEMBER
ABOUT THE SEASONS
1.
If the axis of Earth were not tilted, there would be no
seasons.
Tropic of Cancer
The line of latitude at 23.5° N where the
subsolar point is located (and Sun angle is thus 90°) on the
Summer Solstice in the Northern Hemisphere.
Tropic of Capricorn
The line of latitude at 23.5° S where the
subsolar point is located on the Winter Solstice (and Sun angle
is thus 90°) in the Northern Hemisphere.
Fall Equinox
Assuming a Northern Hemisphere seasonal ref-
erence, the Fall (or autumnal) Equinox occurs on September 22
or 23, when the subsolar point is located at the Equator (0°).
Solar noon
The time of day when the Sun angle reaches its
highest point as the Sun arcs across the sky.
Winter Solstice
Assuming a Northern Hemisphere seasonal
reference, the Winter Solstice occurs on December 21 or 22,
when the subsolar point is at the Tropic of Capricorn (23.5° S).
