Question
Is the Gayatri mantra used to worship the sun or Goddess Gayatri?
Answer
Gayathri is not a mantra. There are classes of mantra called Gayathri which follow a particular meter called Gayathri.
The most popular Gayathri mantra ॐ भूर्भुव: स्व:तत्सवितुर्वरेण्यं । भर्गो देवस्य धीमहि, धीयो यो न: प्रचोदयात् ।। happens to be one among the many.
Gayathri Chandus is a format of 24 meters, any mantra set to this chandus is called a Gayathri (and there are many gayathries). The one today understood as Gayathri mantra , is Savithri Mantra (because of the deity Savitr) set in Gayathri Chundas . The Rishi for this mantra is Vishvamithra.
This mantra is specifically meant for Thapas (तपस; the highest of Tantras) as it increases the heat in the body burning up all muck at the physical level.
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