Seasons are distinct divisions of a year which are based on annual changes in the weather, amount of daylight and ecology. The seasons are influenced by Earth's orbit around the sun and its axial tilt relative to the ecliptic plane. During the year the angle of the Earth's tilt does not change, the orientation of the tilt, however, changes as the Earth orbits the sun. That means the northern hemisphere, for example, is oriented towards the sun for part of the year and is oriented away for the other part of the year. The same thing happens with the southern hemisphere. Orientation towards the sun, on the northern hemisphere, means that the region becomes warmer because of increased solar radiation from the sun; the sun’s rays during that period strike that region at an angle which is more direct resulting in summer. When the northern hemisphere is oriented away from the sun, then winter is experienced in the region. The southern hemisphere experiences seasons opposite to those of the northern hemisphere during the year.
Spring is experienced between winter and summer. The period is generally defined as extending from the Vernal equinox between March 20th or 21st and the Summer Solstice on June 21st or 23rd in the northern hemisphere. On the southern hemisphere winter is experienced between September 22nd or 23rd to December 22nd or 23rd. During spring, temperatures transition from winter cold to summer only in the high and middle Latitudes. In the polar regions, spring is generally very short while in regions close to the equator experience little variation in temperatures during the year. Across several cultures, the spring season is celebrated with festivals and rites revolving around the importance of food production. In the northern hemisphere, the concept of spring is linked to the sowing of crops. During this time plants that had been dormant during winter begin to grow anew while animals that were in hibernation due to winter begin their reproduction and nesting activities. Birds are also observed to migrate to regions with warmer temperatures.
The warmest season is summer, and it is experienced between spring and autumn. The period extends from the Summer Solstice on June 21st or 22nd to the autumnal equinox on September 22nd or 23rd in the northern hemisphere. The period extends from December 22nd or 23rd to March 20th or 21st in the southern hemisphere. Temperature variation is mainly felt in the high and middle latitudes. The temperature in regions close to the equator experience little variation. In the northern hemisphere, the concept of summer is linked with the maturity and growth of cultivated crops. Areas with sufficient rainfall experience the highest growth of plants. Rites and festivals during this period recognize its significance in food production.
The autumn season occurs between summer and winter seasons. During this period temperatures gradually decrease. In the United States, the period is commonly referred to as "fall" because of the leaves that fall from trees during the season. In the northern hemisphere, the period extends from the autumnal equinox on September 22nd or 23rd to December 21st or 22nd during the winter solstice. In the southern hemisphere, the period occurs between March 20th and 21st and June 21st and 22nd. During this period temperatures transition between summer heat to winter cold. The temperature variations are mainly experienced in the high and middle latitude. Autumn is usually very short in the polar regions while the variations in temperature are minimal in areas close to the equator. In the northern hemisphere, the concept of autumn is associated with the harvesting of crops. During this period animals gather food in preparation of the coming winter while birds migrate towards areas close to the equator to escape the dropping temperatures.
Winter occurs between autumn and spring and is the coldest season in the year. The word winter is derived from a Germanic word that refers to "time of water" due to the snow and rain that is experienced in the high and middle latitudes. The period extends from the winter solstice on December 21st or 22nd to the vernal equinox on March 20th or 21st. The period occurs between June 21st or 22nd and September 22nd or 23rd in the southern hemisphere. The temperature variations are experienced mainly in the high and middle latitudes. In the northern hemisphere, the concept of winter is associated with dormancy, animals especially those which hibernate become dormant during this time.
Distance From the Sun
There have been arguments that the changes of Earth’s distance from the sun during the year causes the four seasons experienced on Earth. The argument contends that temperature changes are experienced when the Earth moves closer or away from the sun at different periods during the year. The argument seems logical but it important to note that the decrease and increase in the distance from the sun are usually too small to cause significant changes. The argument would be more fitting in the case of Mars which has a more elliptical orbit which means that it experiences more dramatic changes in the distance from the sun.
Influence of the Earth's Tilt on the Seasons
The Earth’s tilt is influenced by the distribution of mass over the planet. The northern hemisphere has larger amounts of ice sheets and land masses making it heavier compared to the southern hemisphere. To better understand how that works, imagine spinning a ball with a piece of gum stuck close to the top, the result is that the extra weight will lead to the ball tilting when spun. The angle of the Earth's obliquity usually cycles between 24.4 degrees and 21.4 degrees over long periods of geological time. A cycle lasts an estimated 41,000 years a factor that is thought to have been key to the formation of ice ages. The earth is currently experiencing decreasing obliquity meaning that the planet will experience more moderate seasons including warmer winters and cooler summers. The glaciers on earth tend to grow when the obliquity is decreasing due to cooler summers that fail to melt the snow formed during winter.