Geoscience Reference
In-Depth Information
The Earth's Rotation
and Axial Tilt
As Earth revolves around the Sun, it also ro-
tates (spins) on its axis , which is an imagi-
nary line that extends through the center
of Earth from pole to pole. When viewed
from above the North Pole, Earth rotates
in a counterclockwise direction on its axis
(Figure 3.5). It takes 24 hours for Earth to
make one complete rotation, which is the
origin of our day length. During this inter-
val, one-half of Earth is always illuminated,
whereas the other is in shadow. The bound-
ary between day and night is known as the
circle of illumination and is constantly
moving across the surface as Earth rotates.
You can see a good example of the circle
of illumination by looking at a half Moon.
Notice that one-half of the Moon is brightly
illuminated, whereas the other half is completely dark. The divid-
ing line between those two portions of the Moon is the circle of
illumination.
In association with Earth's rotational cycle, humans estab-
lished 24 time zones that span specific portions of the planet
(Figure 3.6). The contiguous United States contains four such
areas, including (from east to west) the Eastern, Central, Moun-
tain, and Pacific time zones (Figure 3.7). Each of these time
zones covers about 15° of longitude because it takes 1 hour for
Earth to rotate that distance (360°/24 h = 15°/h). Given the ir-
regular nature of political boundaries, the edges of time zones
are usually asymmetrical because people in individual states or
countries want to be entirely within a particular time zone. Notice
in Figure 3.7, for example, that the western boundary of the
Eastern Time Zone jogs west to encompass most of Michigan's
Upper Peninsula even though the far western part of the state
nearly reaches the same longitude as St. Louis, which is in the
Central Time Zone. This irregular boundary was drawn because
most people in Upper Michigan identify with Lower Michigan
as far as time is concerned. A sliver of western Upper Michigan
lies in the Central Time Zone because it borders Wisconsin. In
this border area, it makes good economic sense that people on
both sides of the state line be on the same time schedule.
The Prime Meridian in Greenwich, England, was chosen as
the standard for the entire time zone system at the 1884 Interna-
tional Prime Meridian Conference in Washington, D.C. This sys-
tem, referred to as Universal Time Coordinated (UTC), is used
by the vast majority of countries on Earth, although some (such
as Saudi Arabia) adhere to their own time systems. In the UTC
Moon
N
Earth
Direction
of rotation
Direction
of rotation
Revolution
around the Earth
Sun
Revolution
around the Sun
Figure 3.5 The Earth's revolution and rotation. As Earth re-
volves around the Sun, it rotates in a counterclockwise direction
on its axis, when viewed from above the North Pole. Also note the
geometrical relationship of Earth and Moon.
As Earth orbits the Sun, it does not follow a perfectly
circular path, but rather an elliptical one (see Figure 3.4). Note
also that the Sun is not located in the exact center of our orbit
and that the distance between Earth and Sun varies over the
course of the year, with an average distance of approximately
150 million km (93 million mi). The point where Earth is clos-
est to the Sun, approximately 147 million km (91.5 million
mi) away, is called perihelion . In contrast, the term aphelion
refers to the point where Earth is farthest from the Sun, about
152 million km (94.5 million mi) away. At the present time,
perihelion and aphelion occur in early January and July,
respectively.
Given the basic geometric relationship shown in Figure
3.4, what seems odd about the timing of our seasons in the
Northern Hemisphere? The answer is that winter in the North-
ern Hemisphere occurs when we are closest to the Sun, dur-
ing orbital perihelion. Intuitively, you might think that we
would experience our summer when Earth is closest to the
Sun. Since that is not the case, we can conclude that the dis-
tance of Earth from the Sun does not cause our seasons and
that another variable is responsible. This variable is the Earth's
axial tilt.
Perihelion The point of the Earth's orbit where the distance
between the Earth and Sun is least (~147 million km or 91.5
million mi).
Axis The line around which the Earth rotates, extending
through the poles.
Circle of illumination The great circle on Earth that is the
border between night and day.
Aphelion The point of the Earth's orbit where the distance between
the Earth and Sun is greatest (~152 million km or 94.5 million mi).
 
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