overhead and where solar energy entering the top of theatmosphereis most
intense. This latitude is known as the solar declination. Declination progressively
changes because of the consistent tilt of Earth’s axis. In June, the north end of
Earth’s axis is pointed toward the sun and the south end is pointed away from the
sun. This causes the sun to appear at a higher angle and have a longer path length
in the sky in the Northern Hemisphere compared to the Southern Hemisphere. Solar
declination varies between 23.5°north latitude and 23.5°south latitude. The “tropi-
cal” year is the time it takes for the sun to make one round trip between these two
latitudinal extremes. The northern extreme is also called the Tropic of Cancer and
the southern extreme the Tropic of Capricorn. The equator at 0°latitude functions
as the middle of the range of solar declination. These lines are frequently drawn
onmapsand globes. One complete orbit of the sun is a revolution and causes the
migration of the vertical rays of the sun. Revolution is not the same as rotation,
which is the movement of Earth about its axis. Whereas the former is related to sea-
sons, the latter is related to day versus night.
Temperatures at individual locations are typically highest just after the time of
highest solar angles because a high-angle sun imparts much more energy than a
low-angle sun. So, timekeeping has long been based on the solar path through the
sky. The starts and ends of seasons are reckoned from knowledge of the solar path.
The third week of December is a solstice at which time the sun’s vertical rays are
overhead at the Tropic of Capricorn (23.5°south latitude). This marks the beginning
of winter with the shortest days of the year in the Northern Hemisphere. By the third
week of March, the declination has progressed northward to the equator and the day
is known as an equinox—the start of spring in the Northern Hemisphere. The third
week of June holds another solstice and the beginning of summer in the northern
hemisphere as the vertical rays of the sun reach the Tropic of Cancer (23.5°north
latitude). Then the declination progresses southward and crosses the equator in the
third week of September to start the northern fall with another equinox. A round trip
of the declination is completed as the vertical sun reaches the Tropic of Capricorn.
Inconveniently, this round trip takes approximately 365 ¼ rotations of the Earth
on its axis so that the modern calendar has added a leap day every four years.
It is important to understand that the delineation of major lines of latitude are a
direct function of the tilt of Earth’s axis. The tropics include the latitudes the solar
declination crosses. In that the sun is always close to beingoverheadat midday,
the tropics are always warm. The polar latitudes are circumscribed by the Arctic
and Antarctic circles that are the equatorward limits where the sun can remain
above or below the horizon for more than a day at a time depending on the season.
In that the sun is either low in the sky or not above the horizon the polar regions are
cold to frigidly cold depending on the season.
Seasons 299