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 Sunday, 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 1:45 a.m. ET Sunday. That may be the middle of the night in New York - but astronomically speaking, it's the moment when the sun is at its most northerly position in the sky as seen from Earth.

    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.

    季節變化是影響我們生活的強大力量。季節影響著我們的各種行為，我們喜歡的食物，我們的穿衣-并且經常，影響我們的各種情緒。隨著北半球夏天的開始和南半球進入冬季，在周日季節再一次發生了顯著變化。

    是什么造成的季節的變化？

    預測季節變化的能力-通過全年追蹤日出和日落點-是古代人得以生存的關鍵。巴比倫，瑪雅和其它文明發展出了復雜的系統以監測季節的變化。但直到幾個世紀以后人類才能從科學的角度來闡述季節變化幕后成因。

    尼古拉。哥白尼（1473-1543）在他提出太陽而不是地球是太陽系的中心后，日心理論完全改變了人們對于天文學的理解。并引領出關于太陽和地球關系的現代科學認識。

    注：尼古拉·哥白尼（1473年2月19日-1543年5月24日）波蘭天文學家，現代天文學創立者。

    我們現在知道地球以橢圓型軌道繞日運行，同時以傾斜于軌道面的縱軸自轉。這意味著不同的半球在全年中受到的光照強度并不相同。因為太陽是光，能量和熱的源頭，太陽光照強度和密度的變化帶來了春夏秋冬季節的改變。

    二至點和二分點（冬至，夏至和春分，秋分）

    季節變化是以二至點和二分點來標記的-這些天文學詞匯與地球傾斜的自轉軸有關。

    二至點的劃分是依據地軸傾斜角的兩極最大程度地朝向或者背離太陽。這時白天和晚間的時間長度差別最大。二至點時間發生在每年六月二十或者二十一日以及十二月二十一或二十二日，標志著夏天和冬天的正式開始。

    春分和秋分，分別預告著春季和秋季的開始。在一年中的這些時刻太陽直射著赤道，并且白天和夜晚的長度在這個行星的絕大多數地方是相等的。

    在每年的三月二十或者二十一日，北半球到達春分時節并欣喜地迎接春天到來的跡象。同時，南半球秋分來臨風中寒意漸濃。

    一年中另外一個二分點發生在九月二十二或者二十三日，當夏季在北半球落幕凋零時，在南半球冬季的寒意在早春的時節開始消退。

    每年的二至點和二分點總會發生一些變化因為地球正改變著相對于繞日軌道面的自轉軸角度。今年夏至的精確時間是美國東部時間周日凌晨1:45,這多半在紐約的午夜發生-但從天文學上講，夏至是從地球上看太陽在天空中的位置處在一年中最北端的時刻。

    對氣候的影響

    這部分介紹季節變化如何影響天氣：在夏至前后北極點地軸傾斜朝向太陽并且北半球開始享受夏天的陽光。因為直接來自頭頂，陽光輻射能的密度升高-也就是說陽光集中照射在更小的地表區域。白天也變得更長，在同樣的24小時周期中北部地區吸收了更密集的太陽輻射能量。而另一個要考慮的因素是太陽輻射以更短的路徑穿越吸收其能量的大氣到達地表。

    美國國家航空和航天管理局

    這些衛星圖像顯示了地球植被如何隨季節變化。

    同時北半球完全處于夏季，南極地軸的傾斜方向與太陽方向相反，南半球開始感覺到冬天的寒冷。太陽光傾斜的照射分布在（南半球）更廣大的地表并且在光線到達地表前必須大氣中穿行更長的距離。

    這種情形在十月份會發生逆轉，當南半球沐浴在太陽光線的直接照射下時北半球得到的光照輻射密度變小并且時間變短。

    竟管二至日代表著夏季和冬季光照強度的兩個峰值時刻，它們不意味就是最溫暖和寒冷的日子。這是由于氣溫不但取決于大氣所接受陽光的熱量還取決于它損失的被陸地和海洋所吸收的熱量。

    直到陸地和海洋吸收了足夠的能量使之升溫與大氣的溫度達到平衡我們才能夠感覺到冬天最冷的或者夏天最炎熱的那幾天已經來臨。