                        Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                              2025 December 21

                        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: strange lightning
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Amber Straughn Specific rights apply.
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                           NASA Science Activation
                             & Michigan Tech. U.

