The solstices occur when the Sun reaches its highest position in the sky as seen from the South and North Poles. The word solstice is derived from the Latin "sol," which means sun and "sistere," meaning to stand still. A solstice is the time when the Sun stops moving in one direction in the sky and reverses its motion; it also marks the beginning of winter and summer in the Northern and Southern Hemispheres. The following outlines the differences between summer and winter solstices.
The Earth revolves around the Sun in an elliptical orbit, while at the same time rotating around its own axis. The Earth's axis is tilted 23.5 degrees with respect to the orbital plane of the Earth around the Sun (the ecliptic). The solstices designate the beginning of summer, on June 21-22 and the beginning of winter on December 21-22 in the Northern Hemisphere, as well as the beginning of winter and summer around the same dates, although at distinct months, in the Southern Hemisphere.
The apparent motion of the Sun in the sky, from its highest point in the summer to its lowest point in the winter, is due the inclination of the Earth's axis which makes it appear that the Sun is moving from season to season. If the axis of the Earth were not inclined with respect to the plane of the ecliptic, the Sun would always be above the Earth's equator and there wouldn't be seasons on Earth. The tilt of the Earth's axis allows one half of the planet to receive more direct sunlight than the other half throughout the course of a year, and this is what creates the seasons of the year.
When the Earth's axis is tilted towards the Sun, the Earth receives more direct sunlight from the Sun. In the Northern Hemisphere, this usually occurs in the summer. The summer solstice marks the beginning of the summer season in the Northern Hemisphere. This occurs on June 21, when the Sun reaches its maximum elevation in the sky which occurs directly above the Tropic of Cancer at latitude 23.5 degrees. This is the time of the year when the Northern Hemisphere is tilted towards the Sun at its maximum extent and the Southern Hemisphere (winter solstice) is tilted away from the Sun at its maximum extent above the Tropic of Capricorn 23.5 degrees south of the equator.
The differences between summer and winter solstices are due to the tilt of the Earth's axis and the path that the Earth follows around the Sun in one year. During the summer solstice, the North Pole axis reaches its maximum inclination towards the Sun, receiving the greatest amount of sunlight, and the greatest amount of daytime hours. In the Southern Hemisphere, the South Pole axis reaches its maximum inclination (winter solstice) away from the Sun, receiving the least amount of sunlight and the least amount of daytime hours.
The summer solstice in the Northern Hemisphere is the winter solstice in the Southern Hemisphere. During the summer solstice in the Northern Hemisphere, an observer standing at the Tropic of Cancer observes the Sun directly overhead, while an observer in the Tropic of Capricorn sees the Sun on the northern horizon. If the same observer sees the Sun standing in the Tropic of Capricorn, he will see the Sun directly overhead, while an observer in the Northern Hemisphere will see the Sun on the southern horizon.
During the summer solstice, the Sun remains visible continually for twenty four hours in regions of the Arctic Circle. The Sun doesn't set at the northern latitudes, especially at the poles, where the Sun can continually be seen during half of a year. At the same time, during the winter solstice, in the Antarctic Circle the Sun remains below the horizon, generating long lasting nights of more than 24 hours.
The differences between summer and winter solstices are the amounts of sunlight in the hemispheres and the fact that while in the Northern Hemisphere is summer solstice, in the Southern Hemisphere is winter solstice and vice versa. In the summer solstice, the sun's path is longest in the sky and so are the days, while in the winter solstice, the trajectory of the Sun in the sky is shorter, and so are the days.