Inside
the shadow.
A lunar eclipse is a meeting of three bodies and one very long shadow. Move the Moon through it and watch the geometry become visible.
A shadow has an inside.
Before eclipseAfter eclipse
The whole Moon lies inside the umbra. Earth blocks direct sunlight; light bent through our atmosphere gives the Moon its copper color.
UmbraThe inner shadow. From here, the Earth hides the whole disc of the Sun.
PenumbraThe outer shadow. Some sunlight still reaches the Moon, making the darkening more subtle.
Why not every month?The Moon’s orbit is tilted. Most full moons pass above or below Earth’s shadow.