It is the star that shines with a piercing brightness --
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Tafseer
'Abdullāh Ibn 'Abbās / Muḥammad al-Fīrūzabādī
تفسير : allah then explained by saying: (the piercing star!) the bright, piercing star; this refers to the planet saturn which shines at night and disappears during the day.
Jalāl al-Dīn al-Maḥallī
تفسير : the piercing, the brilliant, because of its piercing the darkness with its light, star!, meaning [the constellation] pleiades; or [it refers to] all stars (the response to the oath [follows]):
