Brihat Parashara Hora Shastra
रवेस्तु मण्डलार्द्धास्तात् सायं संध्या त्रिनाडिका। तथैवार्द्धोदयात् पूर्वं प्रातः संध्या त्रिनाडिका॥ १४॥
Definition of Sandhya. 3 Ghatis before the sight of the semi disc (half) of the rising Sūrya and a similar duration, following Sūrya‟s set, are called, as morning twilight and evening twilight, respectively.
english translation
ravestu maNDalArddhAstAt sAyaM saMdhyA trinADikA। tathaivArddhodayAt pUrvaM prAtaH saMdhyA trinADikA॥ 14॥
hk transliteration by Sanscript