高明市区有什么好玩的:从太空看春夏秋冬

Around 6 a.m. local time each day, the Sun, Earth, and any geosynchronous satellite form a right angle, affording a nadir (straight down) view of the terminator, the edge between the shadows of nightfall and the sunlight of dusk and dawn. The shape of this line between night and day varies with the seasons, which means different lengths of days and differing amounts of warming sunshine. (The line is actually a curve because the Earth is round, but satellite images only show it in two-dimensions.)

每天早晨当地时间6点钟左右，太阳、地球和同步人造卫星构成的直角勾勒出地球上的明暗界限，也就是黑夜阴影与黄昏或黎明阳光之间的分界线。这条黑夜与白天分界线的形状随着季节的变化面变化，它代表着不同的白天时间长度和不同的阳光照射量。（由于地球是球形的，这条线其实是曲线，但人造卫星拍摄的图片只能显示它为平面上的一条直线。）

The Spinning Enhanced Visible and Infrared Imager (SEVIRI) on EUMETSAT's Meteosat-9 captured these four views of Earth from geosynchronous orbit. The images show how sunlight fell on the Earth on December 21, 2010 (upper left), and March 20 (upper right), June 21 (lower left), and September 20, 2011 (lower right). Each image was taken at 6:12 a.m. local time.

On March 20 and September 20, the terminator is a straight north-south line, and the Sun is said to sit directly above the equator. On December 21, the Sun resides directly over the Tropic of Capricorn when viewed from the ground, and sunlight spreads over more of the Southern Hemisphere. On June 21, the Sun sits above the Tropic of Cancer, spreading more sunlight in the north and turning the tables on the south. The bulge of our spherical Earth blocks sunlight from the far hemisphere at the solstices; that same curvature allows the Sun’s rays to spread over more area near the top and bottom of the globe. 

在春分和秋分日，太阳位于赤道的正上方，地球上明暗界限是一条南北方向的直线。在冬至，太阳位于南回归线的正上方，阳光更多地洒在南半球上。夏至，太阳位于北回归线的正上方，北半球扭亏为盈获得更多的阳光。由于地球的球面形状，在至日突出的表面阻挡了阳光照射照远端的半球; 同样是球面的原因，在至日阳光可以照射两极的更多地方。

Of course, it is not the Sun that is moving north or south through the seasons, but a change in the orientation and angles between the Earth and its nearest star. The axis of the Earth is tilted 23.5 degrees relative to the Sun and the ecliptic plane. The axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly.

当然，这不是太阳随着季节变化而南北移动，而是因为地球与太阳之间的方向与角度在变化。地球自转的轴线与黄道面（黄道面是指地球绕太阳公转的轨道平面）的夹角是23.5度。冬至时该轴线偏离太阳，夏至时该轴线偏向太阳，这就导致阳光在南北半球上分布的不同；同样地，春分和秋分时间，该轴线与太阳-地球成直角，阳光就平均地洒在南北两个半球上。

The equinox and changing of the seasons occurs on September 23, 2011 at 9:05 a.m. Universal Time. (Our September image above is a few days early.) Equinox means "equal night" in Latin, capturing the idea that daytime and nighttime are equal lengths everywhere on the planet. That is true of the Sun's presence above the horizon, though it does not account for twilight, when the Sun's rays extend from beyond the horizon to illuminate our gas-filled atmosphere.

今年的秋分是在格林威治时间9月23日上午9点5分（我们上面的照片稍微提前了几天）。在拉丁语里春分、秋分是“黑夜均等”的意思，表示地球上的所有地方白天和黑夜都同样长。