Tafsir Al-Jalalayn: Al-Ma'aarij, Ayah 40

(But nay! I swear by the Lord of the rising places) the rising places of winter and summer (and the setting places of the planets) the setting places of winter and summer. There are two rising places and two setting places. The rising place of winter and summer has 177 phases and so do the setting places of winter and summer. The sun rises, in a year, two days in the same phase, and it also sets, in a year, two days in the same phase (that We are Able) this is the reason of the above oath