Science /Science News
Radwan Abu-Elmagd / AFP - Getty Images
Happy Winter Solstice! Get the Scientific Reasons for Earth's Seasons
The
seasons are a powerful force in our lives. They affect the activities
we do, the foods we crave, the clothes we wear — and quite often, the
moods we are in. The seasons officially change once again on Sunday,
with winter beginning in the Northern Hemisphere and autumn starting in
the south.
What is it that causes the change in seasons?
The ability to predict
the seasons — by tracking the rising and setting points of the sun
throughout the year — was key to survival in ancient times. The
Babylonians, the Maya and other cultures developed complex systems for
monitoring seasonal shifts. But it took centuries more to unravel the
science behind the seasons.
Nicolai Copernicus
(1473-1543) radically changed our understanding of astronomy when he
proposed that the sun, not Earth, was the center of the solar system.
This led to our modern understanding of the relationship between the sun
and Earth.
We now know that Earth
orbits the sun elliptically and, at the same time, spins on an axis that
is tilted relative to its plane of orbit. This means that different
hemispheres are exposed to different amounts of sunlight throughout the
year. Because the sun is our source of light, energy and heat, the
changing intensity and concentration of its rays give rise to the
seasons of winter, spring, summer and fall.
Solstices and equinoxes
The seasons are marked by solstices and equinoxes — astronomical terms that relate to Earth's tilt.
The solstices mark the
points at which the poles are tilted at their maximum toward or away
from the sun. This is when the difference between the daylight hours and
the nighttime hours is most acute. The solstices occur each year on
June 20 or 21 and Dec. 21 or 22, and represent the official start of the
summer and winter seasons.
The vernal equinox and
autumnal equinox herald the beginning of spring and fall, respectively.
At these times of the year, the sun appears to be directly over Earth's
equator, and the lengths of the day and the night are equal over most of
the planet.
On March 20 or 21 of
each year, the Northern Hemisphere reaches the vernal equinox and enjoys
the signs of spring. At the same time, the winds turn colder in the
Southern Hemisphere as the autumnal equinox sets in.
Darren Staples / Reuters
The year's other equinox
occurs on Sept. 22 or 23, when summer fades to fall in the north, and
winter's chill starts giving way to spring in the south.
From year to year, there
is always some variability in the equinoxes and solstices because of
the way Earth's changing tilt matches up with its orbit around the sun.
This year, the precise moment of the December solstice comes at 6:03
p.m. ET Dec. 21. That's the moment when Earth's south pole is tipped
precisely toward the sun — and the north pole is tipped away, in
darkness.
Sunday thus offers the
longest night of the year for the Northern Hemisphere's residents, and
the longest duration of daytime south of the equator.
How seasons and weather are linked
Here's
how the seasonal change affects the weather: Around the time of the
June solstice, the North Pole is tilted toward the sun and the Northern
Hemisphere is starting to enjoy summer. The density of the solar
radiation is higher because it's coming from directly overhead — in
other words, the sun's rays are concentrated over a smaller surface
area. The days are longer, too, meaning that more radiation is absorbed
in northern climes during the 24-hour cycle. Another factor that may
come into play is that the radiation takes a shorter path through the
energy-absorbing atmosphere before striking Earth's surface.
At the same time that
the Northern Hemisphere is entering summer, the South Pole is tilted
away from the sun, and the Southern Hemisphere is starting to feel the
cold of winter. The sun's glancing rays are spread over a greater
surface area and must travel through more of the atmosphere before
reaching the earth. There are also fewer hours of daylight in a 24-hour
period.
NASA / EUMETSAT file
The situations are reversed
in December, when it's the Southern Hemisphere that basks in the most
direct rays of the sun, while the Northern Hemisphere receives less
dense solar radiation for shorter periods of time.
Although the solstices
represent the pinnacles of summer and winter with respect to the
intensity of the sun's rays, they do not usually represent the year's
warmest or coldest days. This is because temperature depends not only on
the amount of heat the atmosphere receives from the sun, but also on
the amount of heat it loses due to the absorption of this heat by the
ground and ocean.
It is not until the
ground and oceans absorb enough heat to reach equilibrium with the
temperature of the atmosphere that we feel the coldest days of winter or
hottest days of summer.
— Paige Williams
This is an updated version of an article originally published in March 1999.
DigitalGlobe / AAAS
Satellite Images Show Widespread Looting At Syrian Landmarks
Ongoing
conflict in Syria has given looters the opportunity to run free among
the country's most valuable archaeological sites, new satellite imagery
shows. The American Association for the Advancement of Science studied
images taken of several proposed World Heritage sites in Syria and found
that some sites that were practically untouched in 2011 are now riddled with thousands of pits dug by looters.
The ancient city of
Mari, founded in the early third century B.C., is pictured above, with
some of its more than 1,300 pits circled. Earlier imagery, below, shows
that it was well-preserved just a few years ago.
DigitalGlobe / AAAS
The newer city of
Dura-Europos, founded in the third century B.C., had even more damage,
with 3,750 pits identified from satellite data. Some buildings were
totally demolished, presumably by the forces of the extremist Islamic
State, or ISIS.
"These images show the
destruction of ancient artifacts, architecture, and most importantly,
archaeological context that is the record of humanity's past," said the
University of Pennsylvania's Katharyn Hanson. The report should help
inform future preservation efforts and, with luck, prevent extensive
looting from occurring in other war zones.
IN-DEPTH
- Archaeologists Race to Save Syrian Landmarks as War Rages
- How Terrorists Tap a Black Market Fueled by Stolen Antiquities
- Storyline: ISIS Terror
—Devin Coldewey
Hebrew Univ. of Jerusalem via EPA
Archaeologists Unearth Entryway to King Herod's Palace
Israeli
archaeologists are showing off the monumental, many-arched corridor
that led to Herod the Great's hilltop palace near Jerusalem in the first
century. The corridor in the ancient fortress of Herodium, about 7
miles (12 kilometers) south of Jerusalem, has been excavated to reveal a
space 65 feet long, 65 feet high and 20 feet wide (20 by 20 by 6
meters).
In King Herod's day,
the corridor was designed to lead directly into the palace courtyard —
but archaeologists determined during the excavations that it didn't get
much use. Instead, they say it was back-filled when the palace was
converted into Herod's burial monument.
Herod the Great — who's infamous for his role in the Bible's Christmas stories — died in 4 B.C., reportedly after an excruciating illness. (The Herod mentioned in the stories of Jesus' crucifixion was his son, Herod Antipas.)
Hebrew Univ. of Jerusalem via EPA
The dig team also
uncovered a palace vestibule that was decorated with painted frescoes,
as well as hidden tunnels that were used by Jewish guerrillas during a second-century revolt against the Romans.
Even though Herodium is in the West Bank, the Israeli government has designated the site as a park and wants to turn it into a tourist destination.
"The excavation of the
arched corridor will allow visitors direct access to the Herodium
hilltop palace-fortress, in the same way that Herod entered it 2,000
years ago," the Hebrew University of Jerusalem said in a news release.
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