秋分那天的地球
Equinox on a Spinning Earth
Image Credit: Meteosat 9, NASA, earthobservatory, Robert Simmon
Explanation: When does the line between night and day become vertical? Today. Today is an equinox on planet Earth, a time of year when day and night are most nearly equal. At an equinox, the Earth’s terminator — the dividing line between day and night — becomes vertical and connects the north and south poles. The featured time-lapse video demonstrates this by displaying an entire year on planet Earth in twelve seconds. From geosynchronous orbit, the Meteosat 9 satellite recorded these infrared images of the Earth every day at the same local time. The video started at the September 2010 equinox with the terminator line being vertical. As the Earth revolved around the Sun, the terminator was seen to tilt in a way that provides less daily sunlight to the northern hemisphere, causing winter in the north. As the year progressed, the March 2011 equinox arrived halfway through the video, followed by the terminator tilting the other way, causing winter in the southern hemisphere — and summer in the north. The captured year ends again with the September equinox, concluding another of billions of trips the Earth has taken — and will take — around the Sun.
Tomorrow’s picture: 22 minute moonrise
秋分那天的地球
影像提供: Meteosat 9, NASA, earthobservatory, Robert Simmon
说明: 白画与黑夜的分隔线何时会垂直上指？明天就会，因为明天是地球的一个分点（秋分），在这一年里白画与黑夜几乎等长的一天。在分点时，地球分隔白画与黑夜的明暗界线，垂直上指连接北极与南极。这部把地球的1年压缩成12秒的主题缩时影片，就证实了这种景观。从在地球同歩轨道上，气象9号卫星每天在相同的地方时.记录这些红外光影像。影片起始于2010年9月的秋分，当时明暗界线呈垂直指向，而随着地球绕行太阳，明暗界线向右倾斜，导致北半球的日照减少，带来北半球的冬天。随着日子一天过去，2011年3月的春分在影片中点时降临。接着，明暗界线往反向侧倾，带来南半球的冬天和北半球的夏天。在影片的终结时，2011年9月的秋分降临，让地球已绕行太阳数十亿圈的旅程划下小句点，而在未来，这样的旅程也会再发生数十亿次。
明日的图片: 22 minute moonrise
