Geoscience Reference
In-Depth Information
know that certain dates mark the first days of autumn, spring,
or winter on the calendar. Why do these dates exist, and what is
their significance?
These dates are approximately the same every year be-
cause they indicate key periods within the Earth-Sun geo-
metrical relationship. Remember that the Earth's axial tilt,
in combination with its orbit, causes the subsolar point to
move between 23.5° N and 23.5° S over the course of a year
(Figure 3.11). In other words, the Sun angle is 90° at some
point on the Earth's surface between these latitudes every day.
Although this migration is a seamless process that we barely
notice from day to day, early astronomers found it useful to
mark the passage of the seasons by noting the date when the
Sun's rays are perpendicular to the Equator and to the Tropic
of Cancer (23.5° N) and Tropic of Capricorn (23.5° S). These
two latitudes represent the highest latitude that the subsolar
point reaches in each hemisphere during its seasonal cycle.
The term
equinox
refers to the time when the subsolar point
is at the Equator and all localities on Earth experience equal
hours of daylight and darkness (12 of each). In contrast, the
term
solstice
is used to denote when the Sun angle is 90° at
either of the tropical boundaries.
In an effort to better understand when these dates occur
and how they are related to Earth-Sun geometry, let's examine
the passage of the seasons by beginning in March and moving
forward in time through the calendar. It will help if you refer
to Figures 3.12 and 3.13 to view the process and assume that
your viewpoint of time is from the perspective of living in the
Northern Hemisphere.
Figure 3.11 Range of seasonal subsolar point movement.
The subsolar point migrates between the Tropic of Cancer and
Tropic of Capricorn over the course of the year.
spring season in the Northern Hemisphere and the fall season
in the Southern Hemisphere. This noteworthy date occurs be-
cause the subsolar point is located at the Equator and neither
the Northern nor the Southern Hemisphere is tilted toward the
Sun on this date. Notice that the circle of illumination extends
through each pole in Figures 3.12 and 3.13.
June 20-21
This date is the
Summer Solstice
and represents
the first official day of summer in the Northern Hemisphere and
winter in the Southern Hemisphere. The Summer Solstice oc-
curs because, following the Spring Equinox, Earth continues to
orbit the Sun in a counterclockwise direction when viewed from
above. As the orbit progresses, the Northern Hemisphere slowly
March 20-21
This date is the
Spring Equinox
(also called
the
vernal equinox
) and represents the official beginning of the
Spring Equinox
Assuming a Northern Hemisphere seasonal
reference, the Spring (or vernal) Equinox occurs on March 20
or 21, when the subsolar point is located at the Equator (0°).
Summer Solstice
Assuming a Northern Hemisphere seasonal
reference, the Summer Solstice occurs on June 20 or 21, when
the subsolar point is located at the Tropic of Cancer (23.5° N).
March 21:
Spring in Northern
Hemisphere, Autumn
in Southern Hemisphere
December 21:
Winter in Northern
Hemisphere, Summer
in Southern Hemisphere
June 21:
Summer in Northern
Hemisphere, Winter
in Southern Hemisphere
September 21:
Autumn in Northern
Hemisphere, Spring
in Southern Hemisphere
Plane of the ecliptic
Figure 3.12 The orbital positions of Earth during the solstices and equinoxes.
Note that the tilt of Earth does
not change; instead, the position of Earth relative to the Sun changes. Due to this relationship, the intensity of the solar
radiation that strikes any particular place on Earth varies over the course of the year.
