Because you can see the difference during a lunar eclipse.
I took this photo awhile back during a so-called blood moon event. The reason you can still see the whole moon, and not just the little part on the lower right that is still lit by the sun, is because of light reflected off the Earth illuminating it. The reason it is red is the same as why sunsets are red.
Notice how you can't see any stars?
Edit:
So what's happened to the moon in my photo? And what in the actual fuck is happening in this photo?
Is half the moon really far away while the other half is really close to us? And still no fucking stars? Whaaaaa?