Shloka 21
आत्मदर्शन-उपदेशः (Ātma-darśana Upadeśa) — Mind, Senses, and the All-pervading Self
संध्या और संध्यांशोंसहित अन्य तीन युगोंमें यह (चार हजार आठ सौ वर्षोकी) संख्या क्रमश: एक-एक चौथाई घटती जाती है-
sandhyā ca sandhyāṁśaiḥ sahitā anyeṣu triṣu yugeṣu idaṁ (catuḥsahasra-aṣṭaśata-varṣāṇāṁ) saṅkhyā kramaśaḥ eka-eka-caturthāṁśena hīyate
毗耶娑说道：连同交界期（sandhyā）与交界之分（sandhyāṃśa）在内，其余三劫之总数——此处计为四千八百年——将依次每次递减四分之一。此教示表明：时间本身亦受道德之律动所统摄；诸劫推移之际，达摩之分量与吉祥秩序之延续，皆不断衰落。
व्यास उवाच
Educational Q&A
The verse teaches that the durations of the yugas (including their transitional dawn/dusk periods) follow an ordered pattern of decline: starting from a stated total (here 4,800 years), each subsequent yuga is reduced by one quarter. This numerical scheme expresses an ethical idea—dharma and stability diminish as time moves through the ages.
In Śānti Parva’s instructional setting, Vyāsa is explaining the structure of cosmic time. He specifies how the measure assigned to one yuga, together with its sandhyā and sandhyāṁśa, becomes progressively smaller in the remaining three yugas by a fixed fractional reduction.