Geoscience Reference
In-Depth Information
S U M M A R Y O F K E Y C O N C E P T S
1.
Earth revolves around the Sun within an elliptical orbit in
the plane of the ecliptic. As the Earth revolves, it rotates
on its axis, which is tilted at 23.5° from a line perpen-
dicular to the plane of the ecliptic. It takes approximately
365 days for one complete revolution to occur.
migrates between the Tropic of Cancer and Tropic of Cap-
ricorn between June 21-22 (Northern Hemisphere Sum-
mer Solstice) and December 21-22 (Northern Hemisphere
Winter Solstice), respectively. The Sun is directly over the
Equator at solar noon on both the Spring and Fall Equi-
noxes (March 21-22 and September 21-22, respectively).
2.
The subsolar point is the line of latitude on Earth where
the Sun angle is 90° and thus where solar radiation strikes
Earth most directly. Due to the axial tilt, the subsolar point
3.
Day and night on Earth occur because Earth rotates on its
axis. It takes 24 hours for a complete rotation to occur.
C H E C K Y O U R U N D E R S T A N D I N G
1.
Describe Earth's place in the solar system.
2.
What is the shape of Earth's orbit around the Sun, and
how do the terms
aphelion
and
perihelion
apply?
3.
What is the tilt of Earth's axis relative to the plane of the
ecliptic?
4.
Define the concept of the subsolar point and how
it relates to Sun angle. Describe why it moves on a
seasonal basis. What are the four important dates in this
movement, and how do they relate to latitude?
5.
Chicago, Illinois, and Omaha, Nebraska, are both lo-
cated in the Central Time Zone, with Chicago being on
the east side of the zone and Omaha on the west. With
respect to time, which of these two cities is the first to
“see” the sunrise? Why does this variation occur?
6.
If the axis of Earth were perpendicular to the plane
of the ecliptic, where would the subsolar point be on
December 21?
7.
Imagine that the axis of Earth were tilted at 45°. In this
case, where would the Tropic of Cancer be located?
8.
If the axis of Earth were tilted at 40°, what would the
Sun angle be at 40° S latitude on December 21-22?
9.
Which direction, north or south, do shadows project dur-
ing solar noon on December 21-22 at 40° S latitude?
10.
Where does the greatest range in daylight hours occur,
at high or low latitudes?
11.
Assume that you are standing on the Equator and it is
June 21-22. Given your perspective, is the noontime
Sun in the northern or southern part of the sky?
ANSWERS TO VISUAL CONCEPT CHECKS
V I S U A L C O N C E P T C H E C K 3 . 1
Both October and February are logical times of the year when the subsolar point would be at 10° S
latitude. In October, the Sun is in the process of moving from the Equator (0°) to the Tropic of Capri-
corn (23.5° S). After the subsolar point reaches 23.5° S in late December, it begins to move back to the
Equator, which it will reach on or about March 21. To do so, however, it must pass by 10° S, which it
does in February.
V I S U A L C O N C E P T C H E C K 3 . 2
The Sun rises and sets earlier in Boston, Massachusetts, than in Holland, Michigan, because the
surface of Earth is curved. When the Sun rises in Boston, Holland lies on the part of Earth that is
curved away from the Sun's light. As Earth rotates, the Sun ultimately “rises” in Holland, but about an
hour later than in Boston. The Sun sets later in Holland than in Boston because Boston rotates out of
sunlight about an hour before Holland.
