The Grey Moon is an astronomical phenomenon that occurs when the unlit part of the Moon appears slightly visible in the night sky. This happens because the Earth reflects the light from the Sun and reaches the lunar surface, creating a faint brightness. The Grey Moon is more evident when the Moon is in a crescent or waning phase because the difference in brightness between the lit and the “grey” part is greater.The Grey Moon has fascinated and intrigued humanity since ancient times, giving rise to different legends and beliefs. In Greek mythology, the unlit part of the Moon was called Selasphoros, the son of Helios, the god of the Sun, and Selene, the goddess of the Moon. In Chinese mythology, the unlit part of the Moon was called Yin, the feminine, passive, cold, and dark principle, as opposed to Yang, the masculine, active, hot, and bright principle. In Indian culture, the unlit part of the Moon was called Chandra, the god of the Moon, who drove his chariot across the sky every night.To better understand the phenomenon of the Grey Moon, we can use a simple mathematical model that describes the position and orientation of the Moon with respect to the Earth and the Sun. The model is based on two angles: the phase angle α and the position angle β. The phase angle α is the angle between the direction of the sunlight and the direction of the view from the Earth. The position angle β is the angle between the direction of the full Moon and the direction of the current Moon, seen from the Earth.To illustrate the phenomenon of the Grey Moon, I took a photo of the crescent Moon of 18/11/2023, using a monochrome ZWO ASI2600MM Pro camera mounted on a Tecnosky AG70 f/5 telescope and RGB Deep Sky Astronomik filters. I captured two sequences of 6 shots per filter, respectively 1ms for the lit part and 1 sec. for the “grey” part. I then aligned the subframes with PIPP, Autostakkert!, and Registax and composed the two final images with Pixinsight and Corel Paint Shop Pro.