Geoscience Reference
In-Depth Information
Orbital View and Earth as Viewed from the Sun
Let's reinforce the important concept of axial tilt by interacting
with some simulations. First, go to the Geo Media Library and
select
Orbital View
. This simulation shows Earth orbiting the
Sun, with the Earth's axis indicated. This simulation has close-
up views of Earth at the solstices and equinoxes, showing Earth
rotating (so lengths of daylight periods are indicated), the Earth's
axis (indicating axial tilt), and the portion of Earth that is illuminat-
ed at each of the four times. Click on the “Orbit,” “Fall Equinox,”
“Winter Solstice,” “Spring Equinox,” and “Summer Solstice” but-
tons to see different perspectives. Pay particularly close atten-
tion to the tilt and orientation of Earth, and the fact that these
variables do not change during the orbit. In addition, be sure to
notice where the subsolar point is at any given point in time and
compare this migration to Figure 3.13 in the text. Last, change
the axial tilt and explore how the migration of the subsolar point
changes. Remember that the subsolar point represents the place
where solar radiation is striking Earth most intensely. Notice how,
with increased tilt, the subsolar point migrates a great deal. With
less tilt, the subsolar point migrates less.
After you interact with the Orbital View simulation, select
Earth as Viewed from the Sun
. This part of the module provides
a different view of the Earth-Sun relationship by simulating Earth
as it would appear if viewed from the Sun. As the months progress
in time, notice how the
apparent
tilt of Earth changes, relative to
your view (on the Sun). Remember that the
actual
tilt does not
change; instead, the position of Earth in its orbit around the Sun
progresses. In this simulation, you can also adjust the amount of
axial tilt on Earth and see how the migration of the subsolar point
changes as viewed from the Sun. Again, notice that the subsolar
point migrates more with increased tilt and less with decreased tilt.
Once you complete this simulation, examine Figure 3.13 to review
where the subsolar point strikes Earth, with the tilt at 23.5° on each
of the solstices and equinoxes. Be sure to answer the questions at
the end of the exercise to make sure you understand this concept.
At the same time that your day is beginning on one side of
the circle of illumination, night has begun on the opposite side of
Earth because that place has rotated so that the Sun is no longer
illuminating it. Remember, the Sun is always illuminating half
of Earth at any given time, whereas the other half is in shadow.
of the sky over the course of the day. At solar noon, it is directly
to the south rather than overhead because the subsolar point never
reaches a point higher than 23.5° N at any time in the year. Given
that the Sun is in the southern sky at solar noon, shadows from
buildings and trees project toward the north. In the context of the
seasons, the noon Sun is highest in the sky during summer. In
other words, the Sun angle is greatest during summer. This high
angle occurs because the Sun is in the Northern Hemisphere dur-
ing this time of year.
As the season moves from summer to fall, the position of the
noontime Sun (as you see it) slides farther south in the sky on a
daily basis. Why? This southerly migration occurs because the
subsolar point moves toward the Equator (which is to the south)
as fall approaches and Sun angle decreases. With the gradual
approach of winter, the noontime Sun moves still farther to the
south because the subsolar point is migrating toward the Tropic
of Capricorn. At this time, Earth is in a position within its orbit
where the Northern Hemisphere is tilted away from the Sun. If
you happen to live in the Northern Hemisphere, the noontime
Sun is lowest in the southern sky (in other words, the Sun angle
is the least) on December 21, or the Winter Solstice. From this
point on, the noontime Sun slowly migrates back to the north, as
Sun angle increases, until the Summer Solstice.
You can see the combined daily and seasonal migrations of
the Sun, with respect to the Earth's surface, in a graphical way by
examining Figure 3.14. This diagram is called a
celestial dome
and shows the Sun's daily arc and seasonal migration relative to
the Earth's surface. In this particular diagram, the dome is orient-
Seasonal Changes in Sun Position
(Angle) and Length of Day
In addition to the daily or
diurnal cycle
of day and night, which
is related to the Earth's rotation, we can also see a seasonal
cycle in the Sun's position in the sky, which depends on orbital
progression. Whereas the day-night cycle is obvious and can be
easily seen because of the observable east-west arc of the Sun,
the seasonal migration of the Sun's position is a slow progres-
sion in north-south directions relative to the Equator. This mi-
gration results in a seasonal variation in the angle of the Sun's
rays with respect to any location on Earth. To follow this migra-
tion, the best point of reference is the daily position of the Sun
at solar noon at any given place because that is when the Sun is
at its highest point in the arc at that locality. As we examine the
seasonal migration of the noontime Sun, imagine that you are
living at 45° N (but remember that what you see is opposite for
those who reside in the Southern Hemisphere). Also, keep in
mind that the overall seasonal north-south migration of the Sun
is occurring in combination with the Sun's daily arc.
If you live in the middle latitudes of the Northern Hemi-
sphere, you will notice that the Sun arcs across the southern part
Diurnal cycle
A 24-hour cycle
Celestial dome
A sphere that shows the Sun's arc, relative to
the Earth, in the sky.
