June 21st marks the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere. In the North it's the longest day of the year. At mid-latitudes there is sunlight for over 16 hours. Above the Arctic Circle the sun doesn't set at all!
Above: The Earth's axis (the black line) is tilted by 23.5 degrees relative to the plane in which our planet orbits around the Sun. That's why we have seasons. For three months of the year, centered on June 21, the north pole is tilted toward the Sun and the south pole is tilted away. Six months later the situation is reversed. The tilt of the Earth's spin axis is exaggerated in this figure
Left: Click on the image to see an animated gif simulating the Earth's rotation on a single day - June 21 - as seen from the Sun. The North Polar Cap is clearly visible throughout this 24 hour period - the sun doesn't set. And Antarctica cannot be seen - there, the sun doesn't rise. The images were generated by JPL's Solar System Simulator; the animated gif is 349KB.
While the sun beats down on the North Pole from its highest point today, it is pitch dark at the other end of the Earth. It is midnight at the South Pole where the long night is just reaching its midpoint. The temperature there is a frigid -57 deg. C and the sun won't rise again for 3 months.
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This page is reproduced courtesy of Science at NASA.