The crescent moon is one of the oldest symbols known to humanity. In fact, it is the symbol of many ancient civilizations. In ancient Egypt, the crescent moon was a symbol of Mother goddess Hathor and Saitic Isis.
How did the crescent-moon symbol of Saitic Isis in ancient Egypt become the symbol of Islam today? I have a strong conviction that it was Al Ma'mun, the 7th Caliph of the Abbasid Caliphate, who reduced this ancient symbol to de facto symbol of Islam.
Al Ma'mun Ibn Harun (the 7th Caliph of Islam) was the founder of Abbasid library in Baghdad (I.e. House of Wisdom), where ancient Egyptian, Greek and Byzantine-Christian texts on philosophy and astronomy were translated into Arabic - many of these translated texts would later go into the formulation of the Quran (see the Mihna). An Avid astronomer, Al Ma'mun built astronomical observatories in Baghdad, and in recognition of his contributions to astronomy, the crater Almanon on the moon was named after him.
That Al Ma'mun was seriously commitment to astronomy is old and well-known in history. What is hidden, however, is how the knowledge of astronomy influenced Al Ma'mun, and the role astronomy played in the codification of the Quran. Let me attempt to bring what is hidden to light with scholarly facts; so, follow me.
To an average Muslim, the word "Quran" means "to recite" or "to read " in Arabic. The only problem with that meaning is that Arabic is not really a language but a script. What many people call Arabic is actually a dialect of middle Aramaic/Syriac. Prior to 7th-8th century CE, "Syriac was the medium of communication and cultural dissemination for Arabs and, to a lesser extent, Persians. Primarily a Christian medium of expression, Syriac had a fundamental cultural and literary influence on the development of Arabic." (Dave Johnson). In other words, most words in Arabic can best be understood in Aramaic/Syriac. So let's examine the word "Quran" in its original Syriac form. In Syriac, the word "Quran" is written "Qryn" (consonantal Alphabet), meaning "Pentateuchal reader."
I'm convinced beyond reasonable doubt that Arabic authorities reduced the ancient symbol of the crescent moon to de facto symbol of Islam because of their commitment to astronomy. In fact, the Abrahamic Triad scriptures can be seen as a compedium of astrological charts and messages, disguised in ancient fables!
References
The Hidden Origins of Islam, "Karl-Heinz Ohling," 2009
Walter Williams, "The Historical Origin of Islam," 2003
Lionel Pepper, "The Pentagram Through History," 2008
David Pingree "Carmen Astrologicum book 1," translated 1976.