Crescent moon is NOT a religious Islamic symbol

One often sees the image of a fine crescent moon above the minaret of a mosque in pictures.  This has caused some very absurd and ignorant misinterpretations by non-Muslims claiming that Muslims "worship the moon." 

The crescent moon has NO religious significance whatsoever for Muslims.   Muslims follow the lunar calendar.  As stated in the Glorious Quran, the importance of the moon is only for calculating the number of days in every month.   Similarly the different positions of the sun help to assert the time.

"He it is Who appointed the sun a splendour and the moon a light, and measured for her stages, that ye might know the number of the years, and the reckoning. Allah created not (all) that save in truth. He detaileth the revelations for people who have knowledge."  (10:5)

"They ask thee concerning the New Moons. Say: They are but signs to mark fixed periods of time in (the affairs of) men, and for Pilgrimage...."  (2:189)

At the dawn of Islam there were no printed calendars, watches or clocks.   The dates were often counted by the 'year of elephant,' the months were determined by observing the moon and the time was calculated by the movement of the sun.  The concept of hours and minutes developed some 500 years later.   In modern times the practical or non-religious reason for occasionally showing the crescent moon is to make known that Muslims use the lunar calendar to calculate the months, unlike the Christians, who use the solar calendar.

During the era of Prophet Muhammad (SAAW) and the early Muslim communities, there was no concept of the crescent even though the Muslims used the lunar calendar.   Islamic armies and caravans flew simple solid-colored flags (generally black, green, or white) for identification purposes.  In later generations, the Muslim leaders continued to use a simple black, white, or green flag with no markings, writings, nor symbols for the same purpose of identification.

The crescent was a non-religious symbol of the Ottoman Empire, not of Islam.  It was the Turkish Ottoman Empire that began using the crescent as a symbol of their dynasty.

Osman Ghazi, the founder of the Ottoman Empire was a devout Muslim.  There's a story about him that's well-known in history and sounds interesting to listeners.  It's known as the 'sleepless night of Osman' which he spent before taking the throne.

One day, when Osman was 19 years old, his father Ertugrul went to visit a friend with his family where they would stay overnight. Osman went along too. After dinner, the host showed Osman his room and everyone retired for the night. As he was preparing to sleep, Osman noticed the Noble Quran hanging on the wall. His respect for the Holy Book of Islam prevented him from lying down, and as he was a guest, he could not take the Quran out of the room. So, he decided not to sleep until morning and sat down beside the sofa. But he was unable to stay awake all night and dozed off for a while before Fajr. As he fell asleep, he had a strange dream that left him feeling a bit uneasy. He saw a huge crescent moon rising from his chest and stretching upto the chest of his mentor, Sheikh Edebali. When Osman and his family returned to their village, he recounted this dream to Sheikh Edebali. Edebali smiled and tried to interpret the dream. He told Osman that Allah Almighty would grant him and his descendants an enormous empire, and he would get married to Sheikh Edebali's daughter. It so happened that Osman did get married to the Sheikh's daughter, Malhoon, and he did become the first ruler of the Ottoman dynasty which ruled over large portions of the Middle East, Turkey and Southern Europe for 600 years. However, this isn't supposed to mean that Sheikh Edebali knew future events. What Osman saw was a random dream and what his mentor said was only a story but happened to come true by the Will of Allah. This episode is widely known to be the reason why the Ottomans chose the crescent to represent their Empire as a harbinger of future conquests.

The Ottomans ruled successfully for hundreds of years and fought many victorious battles against Christian Europe. Thus, the symbol of the crescent moon became linked in the minds of many Europeans as a symbol of Islamic Faith. But this notion is totally incorrect, just as they carry plenty of other incorrect notions of Islam based on their superficial observation and lack of information.

In modern times too, the crescent has no other significance for Muslims except the calculation of months by observing the appearance of the moon.  The commencement of the month of Ramadan, the start of the Muslim new year of Moharram, the month of Zil-Hajj and the start and finish of the 12 months of the Hijri calendar are determined by the appearance of the moon.  This lunar cycle, from new moon to new moon, takes 29 to 30 days to complete. When the crescent moon appears in the sky at its finest, that's the first day of every Islamic month.