Darkness is Self-Shadow
Understand that darkness is self-shadow: the dark side of any sphere — including the Moon — is simply the side facing away from the light source, not a permanent feature but a consequence of geometry.
Key Concepts
A common misconception is that the dark part of the Moon is caused by Earth's shadow. This is incorrect (except during an eclipse).
Under normal conditions, the dark portion we see is the Moon's own shadow . It is simply the side of the sphere turned away from the Sun.
Common Questions
What causes the dark side of the Moon?
The Moon's dark side is simply the hemisphere facing away from the Sun. Since light travels in straight lines, it can only illuminate the half of the sphere directly facing the light source.
What is a self-shadow in Grade 8 astronomy?
A self-shadow is the dark region on the unlit side of a three-dimensional object. For any sphere illuminated by a distant light source, exactly half will be in self-shadow at any given moment.
How does understanding self-shadow help explain lunar phases?
Lunar phases result from viewing different fractions of the Moon's self-shadow from Earth as the Moon orbits. The self-shadow itself is constant; what changes is our viewing angle relative to the lit and dark hemispheres.