It is He Who appointed the sun to give radiance, and the moon to give light, assigning it phases so you would know the number of years and the reckoning of time. Allah did not create these things except with truth. We make the Signs clear for people who know. (Qur'an, 10:5)
And We have decreed set phases for the moon, until it ends up looking like an old date branch. (Qur'an, 36:39)
In the first of the above verses, Allah has clearly revealed that the Moon will be a means of measurement for people to calculate the year. Furthermore, our attention is also drawn to the fact that these calculations will be performed according to the positions of the Moon as it revolves in its orbit. Since the angles between the Earth and Moon and the Moon and Sun constantly change, we see the Moon in different forms at different times. Furthermore, our ability to see the Moon is made possible by the fact that it is illuminated by the Sun. The amount of the lighted half of the Moon we see from Earth changes. Bearing in mind these changes, a number of calculations can be made, making it possible for human beings to measure the year.
In former times a month was calculated as the time between two full moons, or the time it took the Moon to travel around the Earth. According to this, one month was equal to 29 days, 12 hours and 44 minutes. This is known as the "lunar month." Twelve lunar months represent one year, according to the Hijri calendar. However, there is a difference of eleven days between the Hijri calendar and the Gregorian calendar, in which a year is the time it takes the Earth to orbit the Sun. Indeed, attention is drawn to this difference in another verse:
They stayed in their Cave for three hundred years and added nine. (Qur'an, 18:25)
We can clarify the time referred to in the verse thus: 300 years x 11 days (the difference which forms every year) = 3,300 days. Bearing in mind that one solar year lasts 365 days, 5 hours, 48 minutes and 45.5 seconds, 3,300 days/365.24 days = 9 years. To put it another way, 300 years according to the Gregorian calendar is equal to 300+9 years according to the Hijri calendar. As we can see, the verse refers to this finely calculated difference of 9 years. (Allah knows best.) There is no doubt that the Qur'an, which contains such pieces of information, which transcended the everyday knowledge of the time, is a miraculous revelation.
