When does the moon appear completely lit from Earth?

The moon appears completely lit from Earth during a phase known as the full moon. This occurs approximately once a month. Here are the steps that explain this phenomenon:

1. The moon orbits the Earth approximately once every 27.3 days, which is known as a sidereal month. However, because the Earth is also moving in its orbit around the sun, it takes a little longer for the moon to return to the same phase, about 29.5 days. This is known as a synodic month or lunar month, and it's the basis for the moon's phases.

2. The moon does not have its own light. It shines because its surface reflects sunlight. The moon's phases describe how much of the moon's disk is illuminated by sunlight, as seen from Earth.

3. A full moon occurs when the moon is on the opposite side of the Earth from the sun. In this position, the entire face of the moon that we can see from Earth is fully illuminated by the sun's light. This is why it appears completely lit.

4. The exact moment of the full moon phase occurs when the moon is exactly 180 degrees away from the sun in ecliptic longitude. At this moment, if the moon is above the horizon in your location, you will see it as completely lit.

5. After the full moon phase, the moon starts to wane, meaning less and less of its surface will be illuminated over time until it reaches the new moon phase, where it's not visible at all. Then, the cycle starts again with the waxing phases leading up to the next full moon.

So, the moon appears completely lit from Earth when it is in the full moon phase of its orbit.