First, a Lunar eclipse occurs when the earth comes between the sun and the moon and the earth's shadow crosses the face of the moon. This only can happen when the moon is on the opposite side of the earth from the sun. This is the normal time of the month when the Full Moon would occur. We observe a full moon on the earth when the sun lights the entire face of the moon that we see from earth. For us to see the moon fully illuminated from earth (a Full Moon), the moon would have to be on the opposite side of the earth from the sun. With the moon in any other position, we would only see a partially lit moon and the earth's shadow would not cross the face of the moon causing an eclipse.