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.

季节与我们的生活真的算是形影不离.他们会影响到我们所涉及的活动,所需要的饮食,以及我们的日常穿着.尤其是我们的心情.常规上来讲,季节每变换一次就发生在周五.随着北半球正着夏季而南半球就步入了冬季.

那么是什么导致全球四季的发生呢?

对四季的预测,在远古时代一直是通过追踪太阳全年间升起和降落点来实现的.巴比伦居民是通过幻觉和其他演变过来的文化结丛方法去监测季节的变化.然而,想要能挖掘出在季节背后科学的变化规律还要经历数年.

Nicolai .哥白尼当他确定是太阳而非是地球在太阳系中心位置的时候,他就从根本上改变了我们对天文学的认识.这才导致了我们现在对太阳和地球之间关系的正确理解.

我们现在所了解的是地球是沿着太阳椭圆型的轨迹走的.与此同时,位于主轴上的坚硬纤维就定义了他们运动的轨迹.这就意味着不同的地球半球在全年间吸收不同的太阳光照,能量和热度,转变的强度和光照集中程度导致了春夏秋冬的形成.

至点和昼夜平分点

季节的标志就是至点和昼夜平分点--这种天文学概念是根据地球的倾斜而定的.

至点表示极点偏向或者远离太阳呈最大限度的点.这种现象在太阳光照时间和夜晚时间的表现最为明显.至点在每年11月的20或21号,或者是21或22号发生.是夏冬季到来的表现.

春分和秋分分别预示着春秋季节的开始.在一年中的这个时节,太阳光直接照耀在地球的轨道上,日照长度在大部分行星中是一样的.

没每年的3月20或21日,北半球到了春分时节迎接着春天的到来.与此同时,在南半球冷风袭来,秋分光临.

其它的几个昼夜平分点是在9月的22或23日,当北方迎来了夏季的时候.南方则开始远离开了寒冷的冬天进入了温暖的春天.

年复一年,春分或秋分与至点交替变化.那是因为地球的倾斜是与其饶转太阳的轨道相互匹配的.

气候效应

这就是一年四季如何影响气候的原因--在大概6月左右的至点,北极朝向太阳一边,因此北半球享受着夏季的温暖.太阳光的辐射密度比较强的原因是直接的照射.换句话说就是太阳的辐射只照射一小块区域.白昼时间长也意味着更多的辐射集中的被北方的地域24小时不停循环的所吸收.其它的原因就是辐射出的能量在到达显现在地球之前就直接被大气层所吸收.

与此同时，北半球也进入了夏季，南极开始远离太阳，南半球开始能够感受得到冬天的寒冷了。太阳光的照耀能够传送到地球的大多数面积，在它到达地球之前必须经历多种气候。这样能都接受到白昼24小时的光照时间也就越来越少了。