Today the summer ends. The season is changing because Earth is tilted. As it rotates around the Sun, the light reaches regions at a different angle for a longer or shorter time. This is how it looks from space:
This video shows how the line between the night and day—called the terminator—changes its angle from December 21 2011 to September 20, 2011. This doesn't mean the Sun changes its position in relation to Earth. It's Earth that moves. And since our planet is tilted on its vertical axis 23.5 degrees relative to the Sun and the ecliptic plane—the plane on which Earth rotates—the Sun's light reaches the Northern and Southern Hemispheres for a different amount of time depending on Earth's position.
When Earth is going through the December's solstice, the axis is tilted away from the Sun. The Northern Hemisphere is exposed to the Sun for shorter time periods than the Southern Hemisphere. The longer nights mean less Sun radiation reaching the Northern Hemisphere, which means colder days during the winter seasons. Meanwhile, the Southern Hemisphere gets its long days of summer. The contrary happens during the June solstice.
Today we are going through the equinox—exactly, it happened at 9:04am Coordinated Universal Time. The terminator is now perpendicular to the orbital plane because the Earth's axis is inclined neither away nor towards the Sun. This means that both the Northern and Southern Hemisphere are getting roughly the same day and night hours. Here in NYC, this means fall. Down there in places like Buenos Aires, that means the beginning of the spring. On March 20 next year, this will happen again but on reverse.