When the sun shines down directly over a planet's equator -- that's an equinox. When one of the poles is tilted toward the sun to its maximum extent -- that's a solstice.
The word equinox comes from a Latin word meaning "equal night." Solstice comes from the Latin for "sun stands still."
Contrary to the all-too-popular notion that Earth is closer to the sun during summer and farther away during winter, seasons are not caused by the eccentricity of our planet's orbit. Indeed, during the hottest days of northern summer the Earth is at its greatest distance from the sun.
The primary cause of seasonal extremes on Earth is the 23 degree tilt of our planet's spin axis. When the North Pole is tilted toward the sun, northern days are long and the weather is warm. Six months later, as the South Pole tilts toward the sun, the southern hemisphere takes its turn at summer.
Seasons in the two hemispheres are always reversed. When it is summer in New York, it is winter in Sydney. On a spring day in Paris, autumn leaves are falling in Argentina.
Last Update: 6 Jul 2011 (AMB)