The Earth's seasons are driven by the tilt of the planet, but the planet does not, as is generally assumed, tilt back and forth over the course of a year. Rather it remains at a constant angle of 23½° to the plane of its orbit throughout the year (this does alter on a longer cycle, but not one that matters on human timescales), but this tilt is disconnected from its orbit about the Sun, with the effect that the Northern Hemisphere is presented towards the Sun from one side of the orbit (creating the northern summer and the southern winter), and the southern hemisphere from the other (creating the southern summer and the northern winter).
Simplified diagram showing the tilt of the Earth throughout the year. Not to scale. NASA.
At two points in the year the Earth presents the two hemispheres equally to the Sun (though it is still tilted at 23½° to the plane of its orbit), creating the equinoxes, which fall in March and September each year. Sunday 22 September 2013 (today) is the September Equinox, marking the end of summer in the northern hemisphere (where it is the Autumn or Fall Equinox) and the beginning of summer in the southern hemisphere (where it is the Spring Equinox).
See also The Lunar Apogee, The Earth's aphelion, The Lunar Perigee, The Northern Solstice and Total eclipse of the Sun visible from Australia, Papua New Guinea and the Solomon Islands, 9 May 2013.
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