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 changed once again Sunday, with winter beginning in the Northern Hemisphere and summer 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. 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 is reaching the vernal equinox and enjoying the signs of spring. At the same time, the winds are turning cold in the Southern Hemisphere as the autumnal equinox sets in.
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 came at 7:04 a.m. ET Sunday.
Effect on climate
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 somewhat shorter path through the energy-absorbing atmosphere before striking the earth.
NASA
This montage of satellite imagery shows how vegetation changes on Earth with the seasons.
--------------------------------------------------------------------------------
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.
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 represent the 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.
季節(jié)在生活中威力無比，左右著我們的衣食住行，更常常影響我們的情緒。本周日季節(jié)再次正式更替， 北半球進(jìn)入冬季而南半球進(jìn)入夏季。
是什么原因讓季節(jié)交替變更？
在遠(yuǎn)古時(shí)代，預(yù)測(cè)季節(jié)的能力攸關(guān)生存大計(jì)，古人要作出整年日出日落的記錄。古巴比倫，瑪雅文化和其他文明古國(guó)編訂了深?yuàn)W的季節(jié)變化監(jiān)測(cè)系統(tǒng)，而這花了后人好幾百年去弄明白其中的科學(xué)道理。
當(dāng)尼古拉哥白尼（ 1473年至1543年）提出了太陽而不是地球是太陽系中心這一論點(diǎn)后，人們的天文觀被徹底的改變了。太陽中心說奠定了現(xiàn)代太陽地球關(guān)系理論。
現(xiàn)在我們知道，地球繞太陽轉(zhuǎn)的軌道是橢圓的而且其自轉(zhuǎn)軸于軌道面是傾斜的。這就意味著，一年中，地球不同部位接受到不同的太陽直射。太陽是地球光能熱三者的來源，其強(qiáng)度的不停變化造就了地球的春夏秋冬四季更替。
“極至“ 和“均分”
四季由“極至“和“均分“ 劃分。“極至“和“均分“是天文學(xué)術(shù)語。
“極至”是指兩極離太陽最近或最遠(yuǎn)的時(shí)刻，這時(shí)
天亮和天黑時(shí)間相差?！皹O至”出現(xiàn)在每年6月20日或21日和12月21日或22日，表示夏季和冬季正式開始的。
春分和秋分標(biāo)志著春季和秋季的開始，此時(shí)，太陽照在赤道上，地球上大多地方白天和晚上一樣長(zhǎng)。
每年3月20日或21 ，北半球進(jìn)入春分時(shí)刻，享受春天的賜予。與此同時(shí)，南半球正值秋分，秋風(fēng)漸冷。
地球的另一“均分”出現(xiàn)在9月22日或23日，此刻北半球秋到夏離，南半球春到冬離。
參照圍繞太陽公轉(zhuǎn)的軌道平面，地球傾斜度每年都不同。 因而每年“極至”和“均分”時(shí)刻也不盡相同。今年，冬至的準(zhǔn)確時(shí)刻是12月21日星期日上午7點(diǎn)04。
對(duì)氣候的影響
以下是季節(jié)變化如何影響天氣：夏至?xí)r，北極指向太陽，北半球開始了夏天。因?yàn)樘栔鄙溥^來，密度較高，同時(shí)，太陽直射的面積較小。在地球24小時(shí)自轉(zhuǎn)中，北半球被照射得多，北半球白天就長(zhǎng)。陽光射到地球前，要穿過吸收光能的大氣層。夏至?xí)r，陽光因直射而走的距離短，被吸收的熱能少。這也許是北半球較熱的另一原因。
當(dāng)北半球進(jìn)入夏季時(shí)，南極是遠(yuǎn)離太陽的。太陽因?yàn)檎丈淞烁嗟拿娣e，又穿越更多的大氣層才抵達(dá)地球，南半球因此開始了寒冷的冬天，白天的時(shí)間（以24小時(shí)算）比晚上少。
12月情況則剛好相反。那時(shí)太陽直射南半球，北半球得不到太陽直射，日照時(shí)間較短，北半球進(jìn)入冬季。
雖然“極至”代表太陽光在夏季和冬季照射的強(qiáng)度，但并不代表最熱或冷的天氣。這是因?yàn)闅鉁夭粌H取決于所獲得的太陽熱量，也取決于陸地和海洋對(duì)熱量吸收能力。如果陸地和海洋獲得較大熱量后失去較少，那則是夏天里最熱的天氣，反之，則是冬天最冷的天氣。
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. 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 is reaching the vernal equinox and enjoying the signs of spring. At the same time, the winds are turning cold in the Southern Hemisphere as the autumnal equinox sets in.
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 came at 7:04 a.m. ET Sunday.
Effect on climate
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 somewhat shorter path through the energy-absorbing atmosphere before striking the earth.
NASA
This montage of satellite imagery shows how vegetation changes on Earth with the seasons.
--------------------------------------------------------------------------------
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.
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 represent the 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.
季節(jié)在生活中威力無比，左右著我們的衣食住行，更常常影響我們的情緒。本周日季節(jié)再次正式更替， 北半球進(jìn)入冬季而南半球進(jìn)入夏季。
是什么原因讓季節(jié)交替變更？
在遠(yuǎn)古時(shí)代，預(yù)測(cè)季節(jié)的能力攸關(guān)生存大計(jì)，古人要作出整年日出日落的記錄。古巴比倫，瑪雅文化和其他文明古國(guó)編訂了深?yuàn)W的季節(jié)變化監(jiān)測(cè)系統(tǒng)，而這花了后人好幾百年去弄明白其中的科學(xué)道理。
當(dāng)尼古拉哥白尼（ 1473年至1543年）提出了太陽而不是地球是太陽系中心這一論點(diǎn)后，人們的天文觀被徹底的改變了。太陽中心說奠定了現(xiàn)代太陽地球關(guān)系理論。
現(xiàn)在我們知道，地球繞太陽轉(zhuǎn)的軌道是橢圓的而且其自轉(zhuǎn)軸于軌道面是傾斜的。這就意味著，一年中，地球不同部位接受到不同的太陽直射。太陽是地球光能熱三者的來源，其強(qiáng)度的不停變化造就了地球的春夏秋冬四季更替。
“極至“ 和“均分”
四季由“極至“和“均分“ 劃分。“極至“和“均分“是天文學(xué)術(shù)語。
“極至”是指兩極離太陽最近或最遠(yuǎn)的時(shí)刻，這時(shí)
天亮和天黑時(shí)間相差?！皹O至”出現(xiàn)在每年6月20日或21日和12月21日或22日，表示夏季和冬季正式開始的。
春分和秋分標(biāo)志著春季和秋季的開始，此時(shí)，太陽照在赤道上，地球上大多地方白天和晚上一樣長(zhǎng)。
每年3月20日或21 ，北半球進(jìn)入春分時(shí)刻，享受春天的賜予。與此同時(shí)，南半球正值秋分，秋風(fēng)漸冷。
地球的另一“均分”出現(xiàn)在9月22日或23日，此刻北半球秋到夏離，南半球春到冬離。
參照圍繞太陽公轉(zhuǎn)的軌道平面，地球傾斜度每年都不同。 因而每年“極至”和“均分”時(shí)刻也不盡相同。今年，冬至的準(zhǔn)確時(shí)刻是12月21日星期日上午7點(diǎn)04。
對(duì)氣候的影響
以下是季節(jié)變化如何影響天氣：夏至?xí)r，北極指向太陽，北半球開始了夏天。因?yàn)樘栔鄙溥^來，密度較高，同時(shí)，太陽直射的面積較小。在地球24小時(shí)自轉(zhuǎn)中，北半球被照射得多，北半球白天就長(zhǎng)。陽光射到地球前，要穿過吸收光能的大氣層。夏至?xí)r，陽光因直射而走的距離短，被吸收的熱能少。這也許是北半球較熱的另一原因。
當(dāng)北半球進(jìn)入夏季時(shí)，南極是遠(yuǎn)離太陽的。太陽因?yàn)檎丈淞烁嗟拿娣e，又穿越更多的大氣層才抵達(dá)地球，南半球因此開始了寒冷的冬天，白天的時(shí)間（以24小時(shí)算）比晚上少。
12月情況則剛好相反。那時(shí)太陽直射南半球，北半球得不到太陽直射，日照時(shí)間較短，北半球進(jìn)入冬季。
雖然“極至”代表太陽光在夏季和冬季照射的強(qiáng)度，但并不代表最熱或冷的天氣。這是因?yàn)闅鉁夭粌H取決于所獲得的太陽熱量，也取決于陸地和海洋對(duì)熱量吸收能力。如果陸地和海洋獲得較大熱量后失去較少，那則是夏天里最熱的天氣，反之，則是冬天最冷的天氣。