There isn't a specific word for when the Sun lights up only the Moon's far side. This is because the Sun always illuminates one side of the Moon, while the other side remains in darkness.

Here's why:

* The Moon's rotation: The Moon is tidally locked to Earth, meaning it rotates at the same rate that it orbits our planet. This results in the same side of the Moon always facing Earth.

* Sun's light: The Sun's light is constantly shining on the Moon, but because of the Moon's rotation, only one side of the Moon receives direct sunlight at a time.

Therefore, the Sun never illuminates only the Moon's far side; it always illuminates one side of the Moon, while the other side is in darkness.

However, you could describe this situation using phrases like:

* "The Sun is illuminating the Moon's far side."

* "The Moon's far side is currently bathed in sunlight."

* "The Moon is experiencing its far side day."