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When the moon comes between the sun and the earth, its shadow falls on the earth. People who are inside this shadow then see the sun either partially, or not at all. If the umbra of the moon's shadow reaches the earth, people in this region do not see the sun at all. They then observe a total eclipse. On the other hand, people in the penumbral region get light from some part of the sun. In other words, they can see a part of the sun or a partial eclipse.

We see the moon only by the sunlight reflected from its surface. Hence, when it lies between the sun and the earth, it cannot be seen at all. This is called the new moon or amavasya. A solar eclipse can, thus, occur only on the day of a new moon.

 

When the earth comes between the sun and the moon, the moon may pass through the umbra of the earth's shadow. When this happens, the moon becomes dark and there is a total lunar eclipse. If only a part of the moon passes through the umbra, there is a partial eclipse. And if the moon passes through the penumbra, it looks a little dull, but hardly dull enough for us to make out. We know that when the moon lies opposite the sun we see its full face or there is a full moon.A lunar eclipse can,thus,occur only when the moon is full.