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.

    季节变化是影响我们生活的强大力量。季节影响着我们的各种行为，我们喜欢的食物，我们的穿衣-并且经常，影响我们的各种情绪。随着北半球夏天的开始和南半球进入冬季，在周日季节再一次发生了显着变化。

    是什么造成的季节的变化？

    预测季节变化的能力-通过全年追踪日出和日落点-是古代人得以生存的关键。巴比伦，玛雅和其它文明发展出了复杂的系统以监测季节的变化。但直到几个世纪以后人类才能从科学的角度来阐述季节变化幕后成因。

    尼古拉。哥白尼（1473-1543）在他提出太阳而不是地球是太阳系的中心后，日心理论完全改变了人们对于天文学的理解。并引领出关于太阳和地球关系的现代科学认识。

    注：尼古拉·哥白尼（1473年2月19日-1543年5月24日）波兰天文学家，现代天文学创立者。

    我们现在知道地球以椭圆型轨道绕日运行，同时以倾斜于轨道面的纵轴自转。这意味着不同的半球在全年中受到的光照强度并不相同。因为太阳是光，能量和热的源头，太阳光照强度和密度的变化带来了春夏秋冬季节的改变。

    二至点和二分点（冬至，夏至和春分，秋分）

    季节变化是以二至点和二分点来标记的-这些天文学词汇与地球倾斜的自转轴有关。

    二至点的划分是依据地轴倾斜角的两极最大程度地朝向或者背离太阳。这时白天和晚间的时间长度差别最大。二至点时间发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，标志着夏天和冬天的正式开始。

    春分和秋分，分别预告着春季和秋季的开始。在一年中的这些时刻太阳直射着赤道，并且白天和夜晚的长度在这个行星的绝大多数地方是相等的。

    在每年的三月二十或者二十一日，北半球到达春分时节并欣喜地迎接春天到来的迹象。同时，南半球秋分来临风中寒意渐浓。

    一年中另外一个二分点发生在九月二十二或者二十三日，当夏季在北半球落幕凋零时，在南半球冬季的寒意在早春的时节开始消退。

    每年的二至点和二分点总会发生一些变化因为地球正改变着相对于绕日轨道面的自转轴角度。今年夏至的精确时间是美国东部时间周日凌晨1:45,这多半在纽约的午夜发生-但从天文学上讲，夏至是从地球上看太阳在天空中的位置处在一年中最北端的时刻。

    对气候的影响

    这部分介绍季节变化如何影响天气：在夏至前后北极点地轴倾斜朝向太阳并且北半球开始享受夏天的阳光。因为直接来自头顶，阳光辐射能的密度升高-也就是说阳光集中照射在更小的地表区域。白天也变得更长，在同样的24小时周期中北部地区吸收了更密集的太阳辐射能量。而另一个要考虑的因素是太阳辐射以更短的路径穿越吸收其能量的大气到达地表。

    美国国家航空和航天管理局

    这些卫星图像显示了地球植被如何随季节变化。

    同时北半球完全处于夏季，南极地轴的倾斜方向与太阳方向相反，南半球开始感觉到冬天的寒冷。太阳光倾斜的照射分布在（南半球）更广大的地表并且在光线到达地表前必须大气中穿行更长的距离。

    这种情形在十月份会发生逆转，当南半球沐浴在太阳光线的直接照射下时北半球得到的光照辐射密度变小并且时间变短。

    竟管二至日代表着夏季和冬季光照强度的两个峰值时刻，它们不意味就是最温暖和寒冷的日子。这是由于气温不但取决于大气所接受阳光的热量还取决于它损失的被陆地和海洋所吸收的热量。

    直到陆地和海洋吸收了足够的能量使之升温与大气的温度达到平衡我们才能够感觉到冬天最冷的或者夏天最炎热的那几天已经来临。