Tafsir of Ash-Shams 91:2

The second oath is taken by:

وَالْقَمَرِ‌ إِذَا تَلَاهَا (and by the moon when she follows him,...91:2). This signifies one of two things: [ 1] The moon following the sun is seen clearly during the middle of the lunar months when the full moon rises to dominate the sky with its radiance after sunset. [ 2] The phrase 'when she follows him' could signify just as the sun is seen in full in broad light, so does the moon, following the sun, becomes full. The third oath is taken by: