When does lunar eclipse / chandra grahan occur?
Lunar eclipse / chandra grahan takes place when the earth is in a straight line between the sun and the moon. Due to this, the earth’s shadow falls on the moon.
When does a penumbral eclipse occur?
Now, earth’s shadow has two parts: umbra and penumbra. There is no direct solar radiation within umbra. But in penumbra, the lighter part / outer portion of the earth's shadow, solar illumination is only partially blocked.
A penumbral eclipse takes place when the moon passes through the earth's penumbra. What happens as a result is that the moon’s surface appears a bit darker.