At the first Coffee Break Conference held in 2010 in Rome a talk on "Triḥ Sapta and Ekaviṃśati in the Vedic Literature" was given by Luca Picardi, here you find the abstract on p. 85 (the proceedings might be available as well):
http://asiatica.wikispaces.com/file/view/Abstracts+booklet.pdf
Maybe it is of help.

Best wishes,

Gaia Pintucci, M.A.
Universität Hamburg (currently Guest Resarch Associate at Kyoto University)
Asien-Afrika-Institut
Abt. für Kultur und Geschichte Indiens und Tibets
Doktorandenkolleg Geisteswissenschaften
Hamburg (Germany)
gaiapintucci@gmail.com

On Fri, Mar 18, 2016 at 6:28 PM, James Hartzell <james.hartzell@gmail.com> wrote:
Dear Colleagues

I’ve come across two references in the Brāhmaṇas to the Sun as ‘the twenty-first’ –

ŚB 6.7.1.1: …” It (the plate) is round, for he (the Sun) is round. It has twenty-one knobs, for he is the twenty-first. He wears it with the knobs outside, for the knobs are his (the Sun's) rays, and his rays are outside." (Eggeling 1894:265),”
and
AB 4.18: "They perform the ceremonies of the Ekaviṃśa day, which is the equator, dividing the year (into two equal parts). By means of the performance of this day, the gods had raised the Sun up to the heavens. This Ekaviṃśa day on which the Divākīrtya mantra (was produced) is preceded by ten days, and followed by ten days, and is in the midst (of both periods). On both sides it is thus put in a Virāṭ: (the number ten). Being thus put in a Virāṭ (in the number ten) on both sides, this (Ekaviṃśa, i.e. the Sun) becomes not disturbed in his course through these worlds." (Haug 1977:288-289).

Does anyone have other references to the Sun as the 21st, and any other explanations for this other than these two Brahmana explanations?

Cheers

James Hartzell, PhD(2x)
Center for Mind/Brain Sciences (CIMeC)
The University of Trento, Italy

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