Gayatri Mantra appears in the Rig veda (3.62.10), attributed to the great Sage Viswamitra. This mantra is composed in a very auspicious metre named Gayatri (8 syllables x 3 lines) and hence the name Gayatri Mantra, the original name being Savitri Mantra. There are many Gayatri Mantras in Hindu scriptures, but Savitri Mantra is of special significance and superiority according to the Gita and endorsed by the Upanishads.
This Mantra is recommended to be chanted three times a day - during sunrise, at noon and at sunset. Each of these timings is called a 'sandhi', representing the transition of sun during the day. The recommendation is to chant 108 times in the morning, 32 times at noon and 64 times at sunset. One is recommended to chant according to ones ability and with no fear of inadequacy in practice.
Savitri mantra has a special way of chanting. Traditionally a teacher or one's own dad initiates the student to Gayatri teaching the correct pronunciation, meaning and intent.
Savitri Mantra is a prayer to the all knowing God (Ishvara) for clarity of thinking, sharpness of mind and guidance, through acquisition of knowledge. The word used to indicate God is Savitur (hence the name Savitri) meaning Sun in the worldly sense and Ishvara in the sense of the entity that illuminates all.
Savitri Mantra is not chanted after sunset. The Mantra must be chanted in the correct intonation (svara) for the best results. There is no restriction imposed by Vedas in chanting Gayatri mantra.
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