It's difficult to pinpoint exactly who first definitively determined the moon was not a source of light but reflected light. This understanding likely developed gradually over time through a combination of observations and philosophical reasoning.

However, we can point to several key figures and developments:

* Ancient Greek philosophers: Some, like Anaxagoras (500-428 BC), proposed the moon did not produce its own light, but rather reflected the sun's light.

* Early astronomers: Aristarchus of Samos (310-230 BC) used geometry to calculate the relative sizes of the sun and moon, suggesting the moon was much smaller and therefore unlikely to be a source of light.

* Medieval scholars: Alhazen (965-1039 AD) in his book "Book of Optics" provided detailed observations of the moon's phases and how they changed with the sun's position. He argued that the changing phases were consistent with the moon reflecting sunlight.

* Telescopic observations: Galileo Galilei's use of the telescope in the early 17th century provided further evidence. He observed craters and mountains on the moon, indicating a solid surface that couldn't generate light.

Therefore, it's more accurate to say that our understanding of the moon's light source was a collective effort over centuries, built upon observations, reasoning, and technological advancements.