what is oldest and most important hindu book ?
The oldest hindu book is RIGVEDA. Scholars date it around 1,500 CE, but that is debatable. Even a later book, Taittiriya Brahmana, mentions the change of the year from the asterism of Orion to the asterism of Pleiades. This happened around 2,500 CE. RIGVEDA is a book belonging to Indo-Aryans who later merged with hindus. The beginning of the year in the asterism of Orion is from 4,000 CE, RIGVEDA gives an indication of the beginning of the year in the asterism of Castor and pollux, which was even earlier.
There are other reasons for considering an older date for RIGVEDA. It seems that Indo-Aryans hailed from a region near North Pole. RIGVEDA mentions a year of ten months with seven months of continuous sun-light. It mentions a long dark and cold Arctic night of two months, a dawn which lasted 30 days, and priests who completed their annual ritual cycle in nine or ten months. That corresponds to the ancient Italian calendar which was of 304 days and consisted of only ten months. This theory is discussed in the book 'The Orion or the Antiquity of Vedas', Chapter VIII - Conclusion, by a famous Indian scholar and leader of Indian independence struggle, Bal Gangadhar Tilak, which can be accessed at http://archive.org/stream/orionortheantiqu021979mbp#page/n1/mode/2up
. Therefore, definitely older than 4,000 CE,
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