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(jié)與我們的生活真的算是形影不離.他們會(huì)影響到我們所涉及的活動(dòng),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(guī)上來(lái)講,季節(jié)每變換一次就發(fā)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(dǎo)致全球四季的發(fā)生呢?

對(duì)四季的預(yù)測(cè),在遠(yuǎn)古時(shí)代一直是通過(guò)追蹤太陽(yáng)全年間升起和降落點(diǎn)來(lái)實(shí)現(xiàn)的.巴比倫居民是通過(guò)幻覺(jué)和其他演變過(guò)來(lái)的文化結(jié)叢方法去監(jiān)測(cè)季節(jié)的變化.然而,想要能挖掘出在季節(jié)背后科學(xué)的變化規(guī)律還要經(jīng)歷數(shù)年.

Nicolai .哥白尼當(dāng)他確定是太陽(yáng)而非是地球在太陽(yáng)系中心位置的時(shí)候,他就從根本上改變了我們對(duì)天文學(xué)的認(rèn)識(shí).這才導(dǎo)致了我們現(xiàn)在對(duì)太陽(yáng)和地球之間關(guān)系的正確理解.

我們現(xiàn)在所了解的是地球是沿著太陽(yáng)橢圓型的軌跡走的.與此同時(shí),位于主軸上的堅(jiān)硬纖維就定義了他們運(yùn)動(dòng)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(yáng)光照,能量和熱度,轉(zhuǎn)變的強(qiáng)度和光照集中程度導(dǎo)致了春夏秋冬的形成.

至點(diǎn)和晝夜平分點(diǎn)

季節(jié)的標(biāo)志就是至點(diǎn)和晝夜平分點(diǎn)--這種天文學(xué)概念是根據(jù)地球的傾斜而定的.

至點(diǎn)表示極點(diǎn)偏向或者遠(yuǎn)離太陽(yáng)呈最大限度的點(diǎn).這種現(xiàn)象在太陽(yáng)光照時(shí)間和夜晚時(shí)間的表現(xiàn)最為明顯.至點(diǎn)在每年11月的20或21號(hào),或者是21或22號(hào)發(fā)生.是夏冬季到來(lái)的表現(xiàn).

春分和秋分分別預(yù)示著春秋季節(jié)的開(kāi)始.在一年中的這個(gè)時(shí)節(jié),太陽(yáng)光直接照耀在地球的軌道上,日照長(zhǎng)度在大部分行星中是一樣的.

沒(méi)每年的3月20或21日,北半球到了春分時(shí)節(jié)迎接著春天的到來(lái).與此同時(shí),在南半球冷風(fēng)襲來(lái),秋分光臨.

其它的幾個(gè)晝夜平分點(diǎn)是在9月的22或23日,當(dāng)北方迎來(lái)了夏季的時(shí)候.南方則開(kāi)始遠(yuǎn)離開(kāi)了寒冷的冬天進(jìn)入了溫暖的春天.

年復(fù)一年,春分或秋分與至點(diǎn)交替變化.那是因?yàn)榈厍虻膬A斜是與其饒轉(zhuǎn)太陽(yáng)的軌道相互匹配的.

氣候效應(yīng)

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(diǎn),北極朝向太陽(yáng)一邊,因此北半球享受著夏季的溫暖.太陽(yáng)光的輻射密度比較強(qiáng)的原因是直接的照射.換句話說(shuō)就是太陽(yáng)的輻射只照射一小塊區(qū)域.白晝時(shí)間長(zhǎng)也意味著更多的輻射集中的被北方的地域24小時(shí)不停循環(huán)的所吸收.其它的原因就是輻射出的能量在到達(dá)顯現(xiàn)在地球之前就直接被大氣層所吸收.

與此同時(shí)，北半球也進(jìn)入了夏季，南極開(kāi)始遠(yuǎn)離太陽(yáng)，南半球開(kāi)始能夠感受得到冬天的寒冷了。太陽(yáng)光的照耀能夠傳送到地球的大多數(shù)面積，在它到達(dá)地球之前必須經(jīng)歷多種氣候。這樣能都接受到白晝24小時(shí)的光照時(shí)間也就越來(lái)越少了。