the half moon appears half-lit half dark from the surface of Earth on 2 days each month what causes this
The reason the half moon appears half-lit and half dark from the surface of Earth on two days each month is due to the changing position of the Moon in relation to the Sun and Earth.
The Moon orbits around the Earth, and as it does so, different portions of its illuminated side become visible from Earth. When the Moon is positioned between the Sun and Earth, with the illuminated side facing away from us, we observe a New Moon, which appears dark because the sunlight is not reaching the side of the Moon facing us.
About two weeks later, as the Moon continues its orbit, it reaches the position where it is on the opposite side of the Earth from the Sun. In this arrangement, the illuminated side of the Moon faces us directly, resulting in a Full Moon with the entire surface appearing bright.
The phase of the half moon occurs when the Moon is at a 90-degree angle relative to the Sun and Earth. This means that from our perspective on Earth, we can only see half of the illuminated side of the Moon, while the other half remains in darkness. This phase occurs twice a month, usually around two weeks after the Full Moon and two weeks before the next Full Moon.
Our perception of the Moon's phases is determined by the relative positions of the Sun, Earth, and Moon, causing the different amounts of illumination that we observe from Earth at specific times during the Moon's orbit.