Illuminated Fraction of a Full Moon

The orbit of the Moon is tilted 5.145° to Earth’s orbit around the Sun. As a result, a lunar eclipse does not occur every month, and a full moon is never 100% illuminated.

Flat-Earthers claim a full moon should not be possible due to the tilt of its orbit, incorrectly assuming a full moon means 100% illumination. In reality, a maximum full moon is at least 99.8% illuminated, and we cannot tell a 98% illuminated moon from a 100% illuminated one. We call it a full moon, even if it is never 100% illuminated.

We ran a simulation spanning 40 centuries from year 1 to the year 4000 to calculate the illuminated fraction of a full moon on every occurrence. The result is that a full moon is at least 99.8% illuminated.

A full moon occurs when the Moon is at maximum opposition from the Sun in the corresponding lunar phase cycle. A full moon is full only by appearance, but it is never actually 100% full. We cannot even tell a 98% illuminated Moon from a 100% illuminated one, let alone a 99.8% illuminated one.