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 Friday, with summer beginning in the Northern Hemisphere and winter 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 June solstice comes at 7:59 p.m. ET Friday. That's the exact time when the North Pole is at its maximum tilt toward the sun.

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.

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.

季節與我們的生活真的算是形影不離.他們會影響到我們所涉及的活動,所需要的飲食,以及我們的日常穿著.尤其是我們的心情.常規上來講,季節每變換一次就發生在周五.隨著北半球正著夏季而南半球就步入了冬季.

那么是什么導致全球四季的發生呢?

對四季的預測,在遠古時代一直是通過追蹤太陽全年間升起和降落點來實現的.巴比倫居民是通過幻覺和其他演變過來的文化結叢方法去監測季節的變化.然而,想要能挖掘出在季節背后科學的變化規律還要經歷數年.

Nicolai .哥白尼當他確定是太陽而非是地球在太陽系中心位置的時候,他就從根本上改變了我們對天文學的認識.這才導致了我們現在對太陽和地球之間關系的正確理解.

我們現在所了解的是地球是沿著太陽橢圓型的軌跡走的.與此同時,位于主軸上的堅硬纖維就定義了他們運動的軌跡.這就意味著不同的地球半球在全年間吸收不同的太陽光照,能量和熱度,轉變的強度和光照集中程度導致了春夏秋冬的形成.

至點和晝夜平分點

季節的標志就是至點和晝夜平分點--這種天文學概念是根據地球的傾斜而定的.

至點表示極點偏向或者遠離太陽呈最大限度的點.這種現象在太陽光照時間和夜晚時間的表現最為明顯.至點在每年11月的20或21號,或者是21或22號發生.是夏冬季到來的表現.

春分和秋分分別預示著春秋季節的開始.在一年中的這個時節,太陽光直接照耀在地球的軌道上,日照長度在大部分行星中是一樣的.

沒每年的3月20或21日,北半球到了春分時節迎接著春天的到來.與此同時,在南半球冷風襲來,秋分光臨.

其它的幾個晝夜平分點是在9月的22或23日,當北方迎來了夏季的時候.南方則開始遠離開了寒冷的冬天進入了溫暖的春天.

年復一年,春分或秋分與至點交替變化.那是因為地球的傾斜是與其饒轉太陽的軌道相互匹配的.

氣候效應

這就是一年四季如何影響氣候的原因--在大概6月左右的至點,北極朝向太陽一邊,因此北半球享受著夏季的溫暖.太陽光的輻射密度比較強的原因是直接的照射.換句話說就是太陽的輻射只照射一小塊區域.白晝時間長也意味著更多的輻射集中的被北方的地域24小時不停循環的所吸收.其它的原因就是輻射出的能量在到達顯現在地球之前就直接被大氣層所吸收.

與此同時，北半球也進入了夏季，南極開始遠離太陽，南半球開始能夠感受得到冬天的寒冷了。太陽光的照耀能夠傳送到地球的大多數面積，在它到達地球之前必須經歷多種氣候。這樣能都接受到白晝24小時的光照時間也就越來越少了。