Here's why:

* Lunar Phases: The moon doesn't produce its own light. It reflects sunlight. As the moon orbits Earth, the amount of sunlight reflecting back to us changes, creating different phases.

* Full Moon: During a full moon, the entire face of the moon is illuminated by the sun because the Earth is positioned between the Sun and the Moon.

* Waning Gibbous: After the full moon, the illuminated portion of the moon starts to shrink, appearing as a waning gibbous – more than half illuminated but less than a full circle.

* First Quarter: About a week after the full moon, the illuminated portion of the moon will be half-lit, appearing as a semi-circle. This is called the first quarter moon.

So, yes, a week after the full moon, the moon will indeed look like a semi-circle again.