وَالقَمَرِ إِذا تَلاها
By the moon when he follows her.
EXEGESIS
Talāhā (following her) is from the verb talaya, which, according to Rāghib, means to follow something in such a manner that no external object comes between the two.
Most exegetes agree that her refers to the sun. However, it has been attributed to Ibn Abbas that it is referring to the day, and thus the verse would be understood as: By the moon when it follows the day.
EXPOSITION
There are two possibilities as to what is meant by the moon following the sun: 1. The moon receives light from the sun, in which case the state of it following the sun is a permanent state, because the moon constantly receives light from the sun. 2. The rising of the moon is after the setting of the sun, in which case the oath is not about a permanent state, but it is about the moon in two states: when the moon is in crescent, and the other is when the whole disk is lit up.
Another possibility is that this is referring only to the full moon which rises as the sun sets, and God is thus swearing by the moon following the sun in rising.
The moon is the only source of light for those engulfed in the darkness of night in the deserts and oceans (10:5) and is also the source for calculating time (2:189). More importantly, many religious rites are linked to the sighting of the crescent. For example, in 2:189, the stations of the moon are specifically linked to performing the hajj pilgrimage. The moon has been explicitly mentioned as a sign of God (41:37), and man is constantly reminded that the moon has been made subservient for him by the all-merciful God (7:54, 13:2, 14:33, 16:12, 29:61, 31:29, 35:13, 39:5).
REVIEW OF TAFSĪR LITERATURE
One other possible understanding of how the moon follows the sun is that the crescent of every month is only visible after the setting of the sun, whereby it only remains visible for a few minutes.
[1] Raghib, p. 271.
[2] Ibn Kathir, 8/398.
[3] Mizan, 20/296.
[4] Munyah, 30/377.
[5] Nasr, p. 1943.
[6] Munyah, 30/377; Ibn Ashur, 30/323.