1. Seasons The seasons are a powerful force in our lives. They affect the activities we do, the foods we crave, the clothes we wear — and quite often, the moods we are in. The seasons officially change once again Tuesday, with autumn beginning in the Northern Hemisphere and spring starting in the south. What is it that causes the change in seasons? Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to go into hibernation or to migrate, and plants to be dormant.tilt of the Earth's axis relative to the plane of revolution During June, July and August, the northern hemisphere is exposed to more direct sunlight because the hemisphere faces the sun. The same is true of the southern hemisphere in December, January, and February. It is the tilt of the Earth that causes the Sun to be higher in the sky during the summer months which increases the solar flux. Causes and effects

2. Satellite images of the illumination of the Earth during various seasons Fig. 1 This is a diagram of the seasons, regardless of the time of day (i.e. the Earth's rotation on its axis), the North Pole will be dark, and the South Pole will be illuminated; see also arctic winter. In addition to the density of incident light, the dissipation of light in the atmosphere is greater when it falls at a shallow angle.EarthNorth Pole South Polearctic winterincidentdissipationatmosphere

3. The seasons result from the Earth's axis being tilted to its orbital plane; it deviates by an angle of approximately 23.5 degrees. Thus, at any given time during summer or winter, one part of the planet is more directly exposed to the rays of the Sun (see Fig. 1). This exposure alternates as the Earth revolves in its orbit. Therefore, at any given time, regardless of season, the northern and southern hemispheres experience opposite seasons.Earthaxistiltedorbital planedeviatesdegreesSunnorthernsouthern hemispheres The effect of axis tilt is observable from the change in day length, and altitude of the Sun at noon (the culmination of the Sun), during a year.day lengthaltitudenoonculminationyear Seasonal weather differences between hemispheres are further caused by the elliptical orbit of Earth. Earth reaches perihelion (the point in its orbit closest to the Sun) in January, and it reaches aphelion (farthest point from the Sun) in July. Even though the effect this has on Earth's seasons is minor, it does noticeably soften the northern hemisphere's winters and summers. In the southern hemisphere, the opposite effect is observed. elliptical orbitperihelion Sunaphelion Compared to axial tilt, other factors contribute little to seasonal temperature changes. The seasons are not the result of the variation in Earth’s distance to the sun because of its elliptical orbit. [2] Orbital eccentricity can influence temperatures, but on Earth, this effect is small and is more than counteracted by other factors; research shows that the Earth as a whole is actually slightly warmer when farther from the sun. This is because the northern hemisphere has more land than the southern, and land warms more readily than sea. [3] Mars however experiences wide temperature variations and violent dust storms every year at perihelion. [4]Earthelliptical orbit [2]Orbital eccentricity [3]Marsviolent dust storms [4]

4. From the March equinox it takes 92.75 days until the June solstice, then 93.65 days until the September equinox, 89.85 days until the December solstice and finally 88.99 days until the March equinox. Traditional seasons are easily recognized by insolation (amount of sunlight received in a given day), with summer being the quarter of the year with the greatest insolation and winter the quarter with the least.insolation

5. We now know that Earth orbits the sun elliptically and, at the same time, spins on an axis that is tilted relative to its plane of orbit. This means that different hemispheres are exposed to different amounts of sunlight throughout the year. Because the sun is our source of light, energy and heat, the changing intensity and concentration of its rays give rise to the seasons of winter, spring, summer and fall. Solstices and equinoxes The seasons are marked by solstices and equinoxes — astronomical terms that relate to Earth’s tilt. The solstices mark the points at which the poles are tilted at their maximum toward or away from the sun. This is when the difference between the daylight hours and the nighttime hours is most acute. The solstices occur each year on June 20 or 21 and Dec. 21 or 22, and represent the official start of the summer and winter seasons. The vernal equinox and autumnal equinox herald the beginning of spring and fall, respectively. At these times of the year, the sun appears to be directly over Earth’s equator, and the lengths of the day and the night are equal over most of the planet. On March 20 or 21 of each year, the Northern Hemisphere is reaching the vernal equinox and enjoying the signs of spring. At the same time, the winds are turning cold in the Southern Hemisphere as the autumnal equinox sets in. The year's other equinox occurs on Sept. 22 or 23, when summer fades to fall in the north, and winter’s chill starts giving way to spring in the south.

6. From year to year, there is always some variability in the equinoxes and solstices because of the way Earth's changing tilt matches up with its orbit around the sun. This year, the precise moment of the September equinox comes at 5:18 p.m. ET Tuesday. That's the moment when the sun is exactly overhead, as seen from the point on Earth's equator directly facing the sun at that time. Effect on climate Here’s how the seasonal change affects the weather: Around the time of the June solstice, the North Pole is tilted toward the sun and the Northern Hemisphere is starting to enjoy summer. The density of the solar radiation is higher because it's coming from directly overhead — in other words, the sun's rays are concentrated over a smaller surface area. The days are longer, too, meaning that more radiation is absorbed in northern climes during the 24-hour cycle. Another factor that may come into play is that the radiation takes a somewhat shorter path through the energy-absorbing atmosphere before striking the earth. At the same time that the Northern Hemisphere is entering summer, the South Pole is tilted away from the sun, and the Southern Hemisphere is starting to feel the cold of winter. The sun’s glancing rays are spread over a greater surface area and must travel through more of the atmosphere before reaching the earth. There are also fewer hours of daylight in a 24-hour period.

7. The situations are reversed in December, when it’s the Southern Hemisphere that basks in the most direct rays of the sun, while the Northern Hemisphere receives less dense solar radiation for shorter periods of time. Although the solstices represent the pinnacles of summer and winter with respect to the intensity of the sun’s rays, they do not represent the warmest or coldest days. This is because temperature depends not only on the amount of heat the atmosphere receives from the sun, but also on the amount of heat it loses due to the absorption of this heat by the ground and ocean. It is not until the ground and oceans absorb enough heat to reach equilibrium with the temperature of the atmosphere that we feel the coldest days of winter or hottest days of summer.