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Line 176: − == Causes of seasonal changes ==+ − The reason for the change of seasons is the revolution of the Earth around the Sun and the axial tilt of the Earth. The Earth's D rotational axis forms an angle of about 66.5 ° to the plane formed by its orbital path, causing one of the northern or southern hemispheres to be tilted towards the Sun. This inclination varies at different times of the year due to the orbit around the Sun, creating an annual cycle of fluctuations in day and night sunlight periods. This is the root cause of seasonal changes. − −
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Synonyms of Rtu found in various texts include Kaala, Beejam, Saterakam, Samayam, and Maasadvayatmaka kala. [3]
Synonyms of Rtu found in various texts include Kaala, Beejam, Saterakam, Samayam, and Maasadvayatmaka kala. [3]
== Seasons ==
== Factors involved in formation of seasons ==
Many countries reckon four seasons, namely, spring, summer, fall and winter each having three months. Indian system of time reckoning has two additional seasons, rainy and pre-winter. Indian Astronomical treatises take into consideration, the Precession or Ayana movements of the earth in the calculation of seasons and hence the system is called Sayana (Sa -with, Ayana -precession). Ayana is the movement of the earth about its vertically tilted axis due to which the earth wobbles like a spinning top. The movement of the earth around the sun with constellations of stars (zodiac) as reference point and where precession is not considered is called Nirayana (i.e., nir - without, ayana - precession) calculation.
Many countries reckon four seasons, namely, spring, summer, fall and winter each having three months. Indian system of time reckoning has two additional seasons, rainy (Varsha Rtu) and pre-winter (Hemanta Rtu). Generally, seasons are based on solar time in Western systems of time keeping. Bharatiya [[Kalamana (कालमानम्)|Kalamana]] or Indian system of time keeping considers nine ways to measure time. At least five factors influence the formation of seasons.
Seasons primarily occur due to the earth's tilt on its axis relative to the orbital (ecliptic) plane, the invisible line on which most objects in the solar system orbit the sun.
# Axial tilt of the earth or obliquity
# Earth's revolution around the sun
# Angle of sun's radiation
# Daylight period
# Atmosphere
=== Axial Tilt and Precession of Earth ===
[[File:Axial-tilt.png|right|frameless]]
Planetary bodies revolving around objects exhibit an axial tilt toward the object around which it orbits. [[Axial tilt]] refers to an imaginary axial line formed where a planet's vertical axis is tilted to a certain degree towards the ecliptic of the object it orbits (sun in the case of solar system). Also referred to as obliquity, in case of earth, it orbits the sun on a slant about 23.4 degrees. The red imaginary red line in the image is the Earth's axis. NASA's findings suggest that Uranus tilts almost 98 degrees in the solar system, and that most planets other than earth are also tilted, some more than the others.
Seasons happen because Earth orbits the sun at an angle causing variable amounts of solar radiation to reach different parts of the planet.
Due to axial tilt
- the Northern and Southern Hemispheres experience opposite seasons at the same time
- there is variation in the angle of sun's rays thereby affecting the amount of solar radiation received in any particular location
- the number of daylight hours are different at different times of the year in different places on earth, thus longer days in summer and longer nights in winter
Without axial tilt (or obliquity) day and night are not evenly divided on earth. Had earth's axis been perpendicular (no tilt) to its orbital plane around the Sun, all places on Earth would experience equal amounts of day and night (12 hours each) every day of the year. There would be no seasonal variability nor any way to measure time.
Indian astronomers, the ancient seers,<ref>Dutta, Amartya. K. (2006) Aryabhata and Axial Rotation of Earth - Khagola (the celestial sphere). [https://www.ias.ac.in/article/fulltext/reso/011/04/0056-0074#:~:text=The%20sidereal%20day%20differes%20slightly,duration%20between%20two%20successive%20sunrises. Indian Academy of Sciences]</ref> however, went ahead to find the reason why the seasons changed over time. Even though the Earth's tilt is relatively stable over long periods of time, it does undergo a slight rocking or swaying motion called nutation (a nodding motion like a gyroscope). Earth's oblique axis is subject to a slight wobble (like a spinning top) which is causing its orientation to change over time. Known as Precession or [[Ayana (अयनम्)]] by Indian astronomers, this process is causing the date of the seasons to slowly change over a 25,800 year cycle. Indian astronomical treatises take into consideration, precession of the earth in the calculation of seasons and hence the system is called Sayana (Sa- along with, Ayana -precession) or tropical year. Precession is the reason for the difference between a sidereal and tropical year. The movement of the earth around the sun with constellations of stars (zodiac) as reference point where precession is not taken into consideration is called Nirayana (i.e., nir - without, ayana - precession) or Sidereal system.
=== Earth's Revolution ===
The axial tilt being largely constant (it changes over a long time period) the Earth's revolution around the sun brings about the exposure of different hemispheres being pointed towards the Sun. When the north pole is pointed towards the Sun, the northern hemisphere experiences summer and the southern hemisphere experiences winter. When the south pole is pointed towards the Sun, in the next six months the season is reversed.
=== Angle of Sun's Radiation ===
Axial tilt of the earth results in there being a difference in how much sunlight reaches a given point on the earth's surface during the course of a year. The angle at which sun's radiation falls on the earth influences seasonal temperatures.
== Classification of Seasons ==
== Classification of Seasons ==
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== Seasonal changes and Indian Culture ==
== Seasonal changes and Indian culture. ==
Earth's movements lead to seasonal changes, seasonal changes lead to various cultural events. Through cultural programs, our life is enlivened with fun and frolic. Dramas play a major role in the conduct of cultural events, as the plays are enacted on religious festivals. Seasonal activities are mentioned in the plays of Kalidasa, Bhasa, etc.
Earth's movements lead to seasonal changes, seasonal changes lead to various cultural events. Through cultural programs, our life is enlivened with fun and frolic. Dramas play a major role in the conduct of cultural events, as the plays are enacted on religious festivals. Seasonal activities are mentioned in the plays of Kalidasa, Bhasa, etc.