The Tilting of the Earth and Various Climatic Seasons
In most parts of the temperate zones, there are four divisions of the year called climatic seasons (divisions of the year based on average temperature and the amount of time that the sun is in the sky each day): winter, spring, summer, and autumn. Winter has the shortest days, which means that the sun is in the sky for the least amount of time each day compared to the other seasons. Winter also has the coldest days. Spring follows winter and has medium length cool days. The days get longer and warmer leading into summer, the season with the longest and hottest days. Autumn follows summer and like spring has medium-length cool days, but each day of this season gets shorter and colder, leading to winter, and the seasonal cycle begins again. The seasons in the temperate zone of the Northern Hemisphere are the opposite of those in the temperate zone of the Southern Hemisphere. This means that when it is spring in the Northern Hemisphere, it is autumn in the Southern Hemisphere.
Seasons based on climate are affected by changes in the amount of sunlight Earth receives and the angle of the sunlight hitting Earth. Direct or perpendicular sunlight warms the atmosphere the most. Sunlight that is more angled warms the atmosphere the least. The angle of sunlight for each region on Earth depends on the position of Earth in relation to the Sun. The Earth's axis is at a tilt of 231⁄2° in relation to its orbit around the Sun. In the Northern Hemisphere, the summer solstice (first day of summer) occurs on or about June 21, when the North Pole has its greatest tilt toward the Sun, as shown in position A in the diagram. On this day, the Sun's rays shine most directly on the Tropic of Cancer, and summer begins for the Northern Hemisphere.
Earth revolves (moves in a curved path around another object) around the Sun once each year. As Earth moves in its orbit (curved path of one body about another) from position A to position B, the daylight hours decrease in the Northern Hemisphere and the Sun's apparent path in the sky becomes lower. On or about September 23 in the Northern Hemisphere, the autumnal equinox (first day of autumn), neither pole is tilted toward the Sun. On this day the Sun's rays shine directly at the equator. All places on Earth have equal hours of daylight and night.
Continuing its orbit, Earth moves from position B to C. During this time the North Pole tilts farther away from the Sun each day, and in the Northern Hemisphere the days become shorter. On or about December 22 marks the winter solstice, the first day of winter and the shortest day of the year in the Northern Hemisphere, when the North Pole is tilted farthest from the Sun. The Sun's path is lowest in the sky and the Sun's rays shine most directly on the Tropic of Capricorn in the Southern Hemisphere.
Each day as Earth moves from position C to D, the days in the Northern Hemisphere become longer and the Sun's path rises a little higher in the sky. On or about March 21, the vernal equinox, as on the autumnal equinox, neither pole is tilted toward the Sun. Again, on this day all places on Earth have 12 hours of daylight. This marks the beginning of spring in the Northern Hemisphere. As Earth continues its journey there is more sunlight each day in the Northern Hemisphere. On or about June 21, Earth reaches position A, thus completing its yearly orbit around the Sun, and heads toward position B again.