«فَإِنْ يَكُنْ أَفَلَ قَمَرُ الشِّعْرَى فَقَدْ طَلَعَ قَمَرُ الشِّعْرِ»
If the light associated with al-Shiʿrā has set, the moon of poetry has risen.
Al-Shiʿrā names the bright star Sirius; al-shiʿr is poetry. Their near-identical sound creates a pun, while afala / ṭalaʿa contrasts setting and rising.
The celestial comparison elevates Abū Zayd’s poetic skill, transferring the imagery of light from the night sky to the gathering’s new guest.