Why does lunar ECLIPSE not occur every full moon night?
Simply speaking, the Sun follows a path through the sky which we call the ecliptic. This line is actually the plane of the Earths orbit as seen from the surface of the Earth. Now the Moons orbit around the Earth is inclined to the ecliptic by about 5 degrees. Now 5 degrees in the sky represents about 10 Moon or Sun diameters and this is rather greater than the diameter of the Earths shadow at the distance of the Moon. Therefore the full Moon often passes above (north) or below (south) the Earths umbral and penumbral shadow and hence there is no lunar eclipse. Only if the full Moon passes sufficiently close to the ecliptic to pass through the Earths umbral or penumbral shadow will there be a lunar eclipse.
