Question

A lunar eclipse occurs on a full moon day, but not on every full moon day. Why?
I mean...
Why does not a lunar eclipse occur on every full moon day?

Answer 1

a lunar eclipse can only occur the night of a full moon. The type and length of an eclipse depend upon the Moon's location relative to its orbital nodes.
Even though the moon’s orbit is inclined to that of Earth – and even though there’s not an eclipse with every full and new moon – there are more eclipses than you might think. There are from four to seven eclipses every year. Some are lunar, some are solar, some are total, and some are partial. All are marvelous to behold – a reminder that we live on a planet – a chance to experience falling in line with great worlds in space!

Answer 2

A Full Moon occurs when the Moon is opposite the Sun as seen from the Earth, in other words the Earth is between the Moon and the Sun. As a result we see the full sunlit portion of the Moon at that time. The plane of the orbit is tilted about 5 degrees with respect to the ecliptic, so the lunar and solar eclipse happen only 2 times in a year when sun, moon and earth come in straight line and the tilted plane of moon just passes from earths ecliptic, then at that time the moon blocks sun light (in case of solar eclipse) and in case of lunar eclipse the earth cast shadow on moon as when it passes from the ecliptic of earth. but on an ordinary full moon the plane of moon never coincides with the earth's ecliptic and so no shadow is cast over it and hence no lunar eclipse is formed. and on ordinary new moon moon does come between sun and earth but due to it's tilted plane do not coincides with earth's ecliptic at that time no solar eclipse is cause as it do not come directly in the way of sun and earth. 

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