See Earth’s Shadow in a Lunar Eclipse

Build a simple model

Use a lamp for the Sun, a larger ball for Earth and a small ball for the Moon. Put them roughly in a line in a dim room, with Earth between the lamp and Moon. Move the Moon behind Earth until Earth blocks the lamp and its shadow falls across the smaller ball. Never aim a bright lamp into anyone’s eyes. A matte ball can make the edge of the shadow easier to see.

1. Line up the lamp, Earth and Moon.
2. Move the Moon behind Earth until the shadow reaches it.
3. Slightly change the angle and watch the shadow shift.

What the shadow shows

During a lunar eclipse, Earth lies between the Sun and the Moon, and the Moon moves through Earth’s shadow. It is a direct demonstration that Earth casts a shadow into space. How much of the Moon darkens depends on how deeply it enters the shadow. During a total eclipse the Moon can look reddish: sunlight filtered and bent through Earth’s atmosphere reaches it, although the exact color varies.

Why not every full Moon?

A lunar eclipse can happen only at full Moon, but most full moons do not produce one. The Moon’s orbit is tilted relative to Earth’s orbit around the Sun, so the Moon usually passes above or below Earth’s shadow. An eclipse is possible when a full Moon occurs near where the two orbital planes cross.

Planning a real observation

The tabletop model explains the geometry; it does not tell you when the next eclipse will be visible from your location. Check a current astronomy schedule and local visibility before making plans. An eclipse may occur while the Moon is below your horizon. NASA’s linked guides explain eclipse types and the Moon’s orbit.