Why does the Moon seem to change shape?

Start with what you see

Picture a ball lit from one side. Half receives light, but an observer does not always see that whole illuminated half. As the Moon moves around Earth, our view of its sunlit surface changes. The unlit part has not disappeared.

What is happening?

Sunlight illuminates half the Moon. Its orbit changes how much of that half we see. Ordinary phases are not Earth’s shadow.

Make a month in a moment

Move through one full orbit.

New Moon

The lit half faces away from Earth.

Full Moon

The lit half faces Earth.

New Moon again

The same geometry returns.

Schematic geometry; not to scale. The slider follows an ideal orbit, not today’s real Moon.

Try a concrete example

Try this thought experiment: put a lamp at one end of a room and hold a ball at arm’s length. Turn your body while keeping the lamp fixed. The ball stays round and half-lit, yet the bright shape facing you changes. That separates two ideas: how much light the ball receives and how much of its illuminated surface you can see. An eclipse is a different alignment, in which a shadow actually blocks sunlight.