Robert Simmon / NASA Image

Understand the Equinox: See It From Space

NASA Video: A year in 12 seconds

September 21, 2018
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From VisibleEarth.NASA.gov...

One of the most frequently misunderstood concepts in science is the reason for Earth’s seasons. 

As we experience the September equinox Saturday —anyone try to balance an egg yet?—we thought we’d offer a space-based view of what’s going on.

When does the line between day and night become vertical? Tomorrow, September 22nd!

The day 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 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.