2025年12月21日
Solstice on a Spinning Earth
Image Credit: Meteosat 9, NASA, Earth Observatory, Robert Simmon
Explanation: Can you tell that today is a solstice by the tilt of the Earth? Yes. At a solstice, the Earth’s terminator — the dividing line between night and day — is tilted the most. 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 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: an equinox. 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. At the most tilt, winter solstice occurred in the north, and summer solstice in the south. 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 the billions of trips the Earth has taken — and will take — around the Sun.
APOD Review: RJN’s Night Sky Network Lecture
Tomorrow’s picture: stone sun
自转地球上的至日
影像提供: Meteosat 9, NASA, Earth Observatory, Robert Simmon
说明: 你能从地球的倾斜角度判断今天是冬至吗？是的。在至日时，地球的晨昏线-即昼夜分界线-倾斜角度最大。这段缩时影片展示了这一现象，影片在短短十二秒内呈现了地球一整年的变化。从地球同步轨道上，Meteosat 9卫星每天在相同的当地时间记录下红外线影像。影片从2010年9月的秋分开始，此时晨昏线垂直，即秋分点。随着地球绕太阳公转，晨昏线的倾斜使北半球每天接受到的阳光减少，造成北半球的冬季。在最大倾斜时，北半球迎来冬至，而南半球则是夏至。随着时间推移，2011年3月的春分出现在影片播放到一半时，随后晨昏线向另一侧倾斜，导致南半球进入冬季，而北半球则是夏季。这段记录的年份再次以9月的秋分结束，完成了地球绕太阳的又一次旅程，这只是地球数十亿次（并将持续数十亿次）旅程中的一次。
APOD回顾: RJN的夜空网路讲座
明日的图片: stone sun
