The Moon must be in the full phase during a lunar eclipse, which happens when the Earth's shadow falls on the Moon as the Sun, Earth, and Moon align with Earth in the middle.
The phase of the Moon must be full when the shadow of the Earth is falling upon it, which occurs during a lunar eclipse. This celestial event only happens when the Sun, Earth, and Moon are aligned, with the Earth situated between the Sun and the Moon. The full phase of the Moon signifies the opposite position of the Moon from the Sun, making it fully illuminated from our perspective just before it enters the Earth's shadow.
As the Moon begins to enter the Earth's dark shadow, called the umbra, a noticeable dimming occurs approximately 20 minutes prior, and the round shape of the Earth's shadow becomes visible on the Moon's surface.
In summary, for a lunar eclipse to take place, the Moon has to be in the full moon phase. This alignment allows Earth to cast its shadow over the entire face of the Moon, which we observe from Earth as a gradual darkening, eventually resulting in the eclipse. The process reveals the curved edge of Earth's shadow as it moves across the lunar surface.