The Almighty in the Quran has revealed that one of the purposes for the Creation of the Sun and the Moon, was for the determination of time. And fasting being a pillar of Islam, the basis of indicating the start and the end of Ramadan is also set by Islam, observing each lunar month marked by the crescent moon.

“They ask you ˹O Prophet˺ about the phases of the moon. Say, “They are a means for people to determine time and pilgrimage.” Righteousness is not in entering your houses from the back doors. Rather, righteousness is to be mindful ˹of Allah˺.
So enter your homes through their ˹proper˺ doors, and be mindful of Allah so you may be successful.”
– Surat Al-Baqarah, verse 189.

For moon sighting information, visit this site:
https://hilalcommittee.org/