The phase of the moon where less and less of the lighted side becomes visible each night is called waning.

Here's how the lunar phases work:

* New Moon: The moon is not visible from Earth because it's between the Earth and the sun.

* Waxing Crescent: A sliver of the moon becomes visible, getting bigger each night.

* First Quarter: Half of the moon is visible.

* Waxing Gibbous: More than half of the moon is visible, getting bigger each night.

* Full Moon: The entire illuminated side of the moon is visible.

* Waning Gibbous: The moon is still more than half illuminated but decreasing in size.

* Last Quarter: Half of the moon is visible again.

* Waning Crescent: A sliver of the moon remains visible, getting smaller each night until it disappears completely.

So, as the moon transitions from full to new, it's said to be waning.