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The term Uttarāyaṇa (commonly Uttarayan) is derived from two different Sanskrit words – "uttara" (North) and "ayana" (movement) – thus indicating the northward movement of the Sun. In the Gregorian calendar, this pertains to the "actual movement of the sun with respect to the earth." Also known as the six month period that occurs between the winter solstice and summer solstice (approximately 20 December - 20 January). According to the Indian solar calendar, it refers to the movement of the Sun through the zodiac. This difference is because the solstices continually precess at a rate of 50 arcseconds per year due to the precession of the equinoxes, i.e. this difference is the difference between the sidereal and tropical zodiacs. The Surya Siddhanta bridges this difference by juxtaposing the four solstitial and equinoctial points with four of the twelve boundaries of the rashis.
The complement of Uttarayana is Dakshinayana (the southward movement of the Sun). It is the period between Karka Sankranti and Makara Sankranti as per the sidereal zodiac and between the summer solstice and winter solstice as per the tropical zodiac.
There is a common misconception that Makar Sankranti marks the beginning of Uttarayana. This is because at one point in time Sayana and Nirayana zodiac were the same. Every year sidereal and tropical equinoxes slide by 50 seconds due to axial precession, giving birth to Ayanamsha and causing Makar Sankranti to slide further. When equinox slides it will increase ayanamsha and Makar Sankranti will also slide. This misconception continues as there is not much difference between actual Uttarayana date which occurs a day after winter solstice (of Dec 21) when the Sun makes the northward journey, and 14 January. However, the difference will be significant as equinoxes slide further. In 272 AD, Makar Sankranti was on 21 December. In 1000 AD, Makar Sankranti was on 31 December and now it falls on January 14. After 9000 years, Makar Sankranti will be in June. Then Makar Sankranti would mark the beginning of Dakshinayana. However Makar Sankranti still holds importance in Hindu rituals. All Drika Panchanga makers like mypanchang.com, datepanchang, janmabhumi panchang, rashtriya panchang  and Vishuddha Siddhanta Panjika use the position of the tropical Sun to determine Uttarayana and Dakshinayana.
Mayasura, the composer of Surya Siddhanta, defines Uttarayana, at the time of composition, as the period between the Makara Sankranti (which currently occurs around January 14) and Karka Sankranti (which currently occurs around July 16). Lātadeva describes this as half revolutions of the Sun, using the terms Uttarayana and Dakshinayana to describe the "northern and southern progress" respectively. Bal Gangadhar Tilak, a scholar and mathematician, proposes an alternative, early vedic definition of Uttarayana as starting from Vernal Equinox and ending with Autumnal Equinox. This definition interprets the term "Uttara Ayana" as "northern movement" instead of "northward movement", i.e. as the movement of the Earth in the region North of the Equator. In support of this proposal, he points to another tradition that the Uttarayana is considered the daytime of the Gods residing at the North Pole which tradition makes sense only if we define Uttarayana as the period between the Vernal and Autumnal equinoxes (when there is Midnight Sun at the North Pole). Conversely, Dakshinaya is defined as the period between the Autumnal and Vernal Equinoxes, when there is midnight sun at the South Pole. This period is also referred to as Pitrayana (with the Pitrus (i.e. ancestors) being placed at the South Pole).
This festival is currently celebrated on the 14th or 15 January but due to axial precession of the Earth it will continue to shift away from the actual season. The season occurs based on tropical sun (without ayanamsha). The Earth revolves around Sun with a tilt of 23.44 degrees. When the tilt is facing the Sun it is defined as summer and when the tilt is away from the Sun it is called winter. That is the reason when there is summer north of the equator, it will be winter south of the equator. Because of this tilt, the Sun appears to travel north and south of the equator. This motion of the Sun transitioning from south to north is called Uttarayana (the Sun is moving towards north). Once the Sun reaches north, it begins moving south and is called Dakshinayana – the Sun is moving towards south. This causes seasons which are dependent on equinoxes and solstices.
Uttarayana is referred to as the day of new good healthy wealthy beginning. In the Mahabharata, this day marks the death of Bhishma. Bhishma had the ability to choose the time of his death and although mortally wounded in war, he chose to delay his death until uttarayan. According to the Bhagavad Gita, a Hindu scripture, those who die when the Sun is on its northward course (from south to north) attain nirvana. This explains the choice made by Bhishma to wait until Uttarayana to die.
According to the Hindu tradition the six month period of Uttarayana is equivalent to a single day of the Gods, while the six month period of Dakshinayana is equal to a single night of the Gods. Thus a year of twelve months is single day of the Gods. This refers to the six months of single day at the North pole and concurrent six months of night at the south pole.
During the Uttarayana, devotees often undertake certain rituals to benefit during the auspicious time. Devotees often take part in pilgrimages to bathe in Prayag, where the Yamuna, Ganga and Saraswati rivers meet.
Pongal is celebrated as a harvest festival in the southern states of India like Tamil Nadu. Although rituals and customs may vary, it is generally celebrated as a four-day festival. On the first day, unwanted household items are discarded and burned in bonfires to symbolize starting anew. The second day, people dress in new clothes and prepare pongal, a sweet dish that is made of rice, milk and jaggery, and offer it to Surya, the Hindu sun deity. On the third day, cattle are worshipped because they are seen as a symbol of prosperity. And, on the last day, some regions host bull-fighting and farmers offer prayers for the new, fresh harvest.
Known as Lori in the northern states, children go door-to-door asking for sweets and money, and in the evening, people gather around huge bonfires to sing, dance, and make offerings to Agni, the fire deity, for future prosperity. Traditional dishes made from flatbread and mustard leaves are shared with offerings of sesame brittle, peanuts, popcorn, and jaggery. It is celebrated in other North Indian states like Haryana, Delhi, and Himachal Pradesh.