Yāska's Nirukta 77.1.4

77.1.4

yathā cāyaṃ saptabhiḥ sarpaṇairvā raśmibhiryujyate tathaiyamaparā ṛgetasminneva sūkte “sapta yuñjanti0-adhi tasthuḥ ( ṛ0 sa0 2, 3, 14, 2 )” -iti | “sapta” saṅkhyopetāḥ sarpaṇā vā “yuñjanti” yojayantyātmanā raśmayaḥ “rathaṃ” rahaṇamādityam, udyanneva hyasāvādityo raśmibhiḥ yujyate, rahaṇaścāsau muhūrttamapyanavasthāyitvāt | “ekacakram” eka eva hyasāvantarikṣe carati, itarāṇi jyotīṃṣi nāśayatyeva svena tejasā prakāśena| “eko'śvaḥ”eka evāśvaḥ, aśanā vyāpanā sarvabhūtānām “vahati”gacchati “saptanāmā”saptastutiḥ,saptaputraḥ-ityevamādyāḥ saptasaṃkhyāyuktāḥ stutayo'sya | athavā“saptāsmai raśmayo rasānabhisannāmayanti”-etasmin maṇḍale,iti saptanāmā | athavā “saptainamṛṣayaḥ [namanti] stuvanti iti vā” saptanāmā | “idamapītarat” śabdaḥ “nāma” “etasmādeva”-“abhisannāmāt” tadapi svamarthaṃ pratyāyayituṃ kriyāpadasya vā guṇabhāvenābhimukhyena sannamati | sa evalakṣaṇaḥ saptacārī vyāpanaḥ saptanāmā, sarpaṇaiḥ saptabhirvā raśmibhiryukto yadā vahati, udayādārabhya yāvadastamiti || tadaivaṃ vahan kimabhinirvarttayati ? iti, ucyate-“trinābhicakramajaramanarvam” trinābhisaṃyukta kālacakram , cakanadharmikam , cakanaṃ calanamucyate, caraṇadharmi, kramaṇadharmi vā | “ajaram” “ajaraṇadharmāṇam” | “anavam” “apratyṛtamanyasmin” anāśritamityarthaḥ | “trinābhicakraṃ” “tryṛtuḥ” hi “saṃvatsaraḥ” “grīṣmo varṣā hemanta iti” anena ṛtupravibhāgena etadevāstha trinābhitvamabhipretam| ṛtubhirhi asau saṃvatsaro nahyataṃ sannahyate ityarthaḥ || ‘saṃvatsarapradhāna uttaro'rddharcaḥ” nādityapradhānaḥ ||