In Surah Yunus (10:5), the Quran declares: "It is He who made the sun a shining light and the moon a derived light and determined for it phases - that you may know the number of years and account [of time]."
"هُوَ الَّذِي جَعَلَ الشَّمْسَ ضِيَاءً وَالْقَمَرَ نُورًا وَقَدَّرَهُ مَنَازِلَ لِتَعْلَمُوا عَدَدَ السِّنِينَ وَالْحِسَابَ" — "It is He who made the sun a shining light and the moon a derived light and determined for it phases - that you may know the number of years and account [of time]." (10:5)
The Quran clearly distinguishes the sun (ضِيَاءً, "shining light" — a source of its own light) from the moon (نُورًا, "derived light" — reflected light). This distinction, confirmed by modern astronomy, was unknown at the time of revelation.
Verse 2:189 specifies: "They ask you, [O Muhammad], about the new moons. Say: They are measurements of time for the people and for the Hajj." (يَسْأَلُونَكَ عَنِ الْأَهِلَّةِ ۖ قُلْ هِيَ مَوَاقِيتُ لِلنَّاسِ وَالْحَجِّ).
The Islamic lunar calendar is based on the actual phases of the moon. The duration of a synodic lunar month is 29.53 days on average. This value, measured with precision by modern astronomy, is the basis of the Hijri calendar.
Unlike the solar Gregorian calendar, the Islamic lunar calendar is directly observable: each month begins with the observation of the lunar crescent (hilal). This method, prescribed by the Quran and the Sunnah, depends not on theoretical calculation but on direct observation of the sky.
The lunar year of 354 days is about 11 days shorter than the solar year. This is why Islamic months (such as Ramadan) advance by about 11 days each year relative to the Gregorian calendar, passing through all seasons over a cycle of about 33 years.
This Quranic precision about the role of the moon as a measure of time, revealed more than 1400 years ago, is confirmed by modern astronomy.