Thousands of years ago the ancient peoples found out that days were longer in summer than in winter, and nights were shorter. They knew that this had a great deal to do with the changes of the seasons and the growth of plants and animals. They determined through generations of painstaking observation that the day was shortest in the Northern Hemisphere on the 22nd of December, and after which it gradually grew longer until the 21st of June, when the day was the longest in the year and the night was the shortest. After that, the day would begin to shorten again gradually. In the beginning, the actual dates of those two days had to be calculated for each individual year, and depended on what kind of calendar year was being used.
The first calendar to fix these days on definite dates of the year was the solar calendar, which had 365 days in a year and – every four years – a “leap-year” with one extra day.
To an observer on earth, the sun seems to move farther and farther away from the equator to the north until on June 21st it seems to reach its furthest point north. Then it seems to “pause” for one day before it turns around and goes back. Then it goes further and further south until on December 22nd it appears to “pause” again for one day before swerving back north again.
These two days are called the Summer Solstice and the Winter Solstice respectively.
Now we know that all this is caused by the movement of the earth around the sun. as the earth journeys around the sun, it spins on its axis. This can be illustrated by a simple experiment. If you push a sharp stick through a rubber ball and twirl it with two fingers, the ball spins around in much the same way the earth is spinning at this very moment. The points where the stick comes through the ball correspond to the North and South poles. If you twirl this ball at night directly in front of a bright light, you will notice that half the ball is lighted up while the other half is in the shade. This is just like our night and day. If you keep the stick strictly vertical to the light and twirl it at an even speed, any spot on the ball’s surface will be in the light and in the shade the same length of time.
If the earth were spinning just like this rubber ball, there would only be day and night on earth, but no seasons, and days would be always be the same length as nights – 12 hours each. But that is not how the earth spins. It spins with its axis tilted. Its axis is always at the angle to the plane of its orbit – an angle of about 231/2 degrees.
It is this tilting that accounts for our four seasons and the
lengthening and shortening of days and nights. For this reason also, the Equator (an imaginary line drawn around the earth at equal distance from the two poles) is not always directly under the sun’s rays. For six months the earth is tilted towards the sun, and the Northern Hemisphere gets more than its share of sun light every day. Days are longer than nights, and what is more, the sun’s rays come down more perpendicularly instead of slanting down.
(From English Language Learning)
数千年前古代民族发现，白天夏季比冬季长，而黑夜则夏季比冬季短。他们知道这种现象与季节变化和动植物生长有很大关系。他们通过多少代人辛辛苦苦的观察，确认在北半球十二月二十二日的白天最短，过了那天后，白天就逐渐变长，一直到六月二十一日。那天是一年中自天最长，黑夜最短的一天。此后，白天又会开始逐渐变短。起初，这两天的具体日子必须每年计算，而且要看所使用的历法的种类而定。
最早把这两天固定在每年一定的日子的年历是阳历，按照这种年历，一年有三百六十五天，每四年有一个“闰年”，多一天。
在地球上的观察者看来，太阳似乎离开赤道一步步向北
移动，到六月二十一日，移动到了北面最远的地方。接着太阳似乎“暂停”移动一天，然后就回头移动，一步步向南移动，直到十二月二十二日，太阳似乎又“暂停”一天，再回
过头来往北移动。这两天就分别叫做夏至和冬至。
现在我们知道，所有这一切现象都是由于地球圈绕太阳转动而产生的。地球绕着太阳运行时是围绕自身的轴旋转的。这可以用简单的实验来说明。如果把一根尖头棒插进橡皮球，用两个手指使它转动起来，橡皮球的旋转与地球此时此刻的自转极为相似，尖头棒穿过的两处相当于南极和北极。如果在夜间正对着明亮的灯光转动，就会发现半个皮球被照亮了，而另外半个在阴暗处。这种情况正好同我们的白天黑夜的现象相同。如果使尖头棒完全与灯光垂直，以均匀的速度加以转动，球面的任何一点在亮处和暗处的时间长短是相同的。
如果地球完全象这个橡皮球一样转动，那么地球上就只有白天黑夜，没有季节，而白天永远与黑夜一样长 —— 各是十二小时。但是地球并不是那样转动的。它旋转时地轴是倾斜的。地轴同地球轨道平面总是成一个角度 —— 大约是二十三度半的角度。
就是这样的倾斜角度才使我们有四个季节，有昼夜长短的变化。由于同样的缘故，赤道（离两极距离相等的地方围绕地球的一条假想线）并不总是在阳光垂直照射之下。有六
个月期间地球向太阳倾斜着，北半球就每天得到较多的阳光，白天就比黑夜长，而且，阳光不是斜射，而是以比较直的角度向下照射。
〈摘自《英语学习》
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