Sun-rise-set

As per Varahamira -

उदयास्त मनाख्यं हि दर्शनादर्शनं रवेः।
“visibility of sun’s limb is the rising time and complete invisibility is the setting time.”

Dharmashastras -

रेखामात्रन्तु दृश्येत
रश्मिभिश्च समन्वितं। उदयन् तद् विजानीयात्
होमं कूय्यात् विचक्षणः॥

“when even a portion of the sun with it’s ray is visible it is to be considered as sunrise and at that time oblation (homa) is to be performed”