Equinox on a Spinning Earth. Image Credit: NASA, Meteosat, Robert Simmon. Please click to watch a 12 second time-lapse video (see explanation below).
When does the line between day and night become vertical? In 2016, the spring equinox in the Northern hemisphere ( it is the start of autumn in the Southern hemisphere) comes on March 20 at 4:30 UTC (March 19 at 11:30 p.m. CDT). Also, according to http://www.earthsky.org, the 2016 spring is the earliest spring since 1896. The website explains:
“The March equinox can come on March 19, 20 or 21. And 2016 has the earliest March equinox since the year 1896. Is it a coincidence that 2012 also had the earliest spring since 1896? No. Recall that both 2012 and 2016 are leap years. But 2016’s spring comes even earlier than the spring of 2012.” 
At that time the 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 above time-lapse video from 2010-2011 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. 
SPRING TIME PLEASURE FOR LITTLE GIRLS AT A VANCOUVER BEACH
On a lovely spring like day at the Kitsilano beach in Vancouver, Canada, a little girl attempts to “pop” one of the several soap-bubbles released moments earlier by her mother. Photo: Nurin Merchant, Guelph, Canada.
This long stretch of soap-bubble eludes the girl, as her sister watches her but…
…then a similar dolphin like bubble reaches her at the right height which she is about to pop to her delight. Photo: Nurin Merchant, Guelph, Canada.
After a very brief explanation by the mother, the keen girl wastes no time in learning the art of creating a soap-bubble, and manages to release a little bubble of her own as her sister watches her. Photo: Nurin Merchant, Guelph, Canada.
Date posted: March 18, 2016.
 Read more at http://earthsky.org/astronomy-essentials/everything-you-need-to-know-vernal-or-spring-equinox
 Please visit http://apod.nasa.gov/apod/ap140319.html