Illuminated Fraction of a Full Moon

A lunar eclipse does not happen every month because the orbit of the Moon is tilted 5.145° to Earth’s orbit around the Sun. And thus, the Earth does not always cast its shadow on the Moon during maximum opposition. Instead, a full moon occurs.

Some flat-Earthers take this as ‘evidence’ that a full moon cannot possibly happen because the geometry does not allow for a 100% illuminated Moon. They are wrong. In reality, a full moon 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.

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