To get a better idea of how the Earth moves around the Sun, work in groups of three people. One person is the Sun and stands in the middle, just standing still. The second person is the Earth, and slowly walks in a large circle around the Sun while spinning around like a top at the same time (this is pretty hard; take it slowly). The third person is the Moon, who walks in small circles around the Earth, always facing towards the Earth. Once you get some practice, trade places so everyone gets a chance to be the Earth.
An eclipse of the moon
You can see the curved shadow of the Earth.
You can see for yourself that the Earth is round like a ball. One way is to observe an eclipse of the moon - when you see the shadow of the Earth on the Moon, you will see that the shadow is curved, because the Earth itself is curved like a ball. Here's a video of an eclipse of the Moon, in case you've never seen one yourself:
Another way to see that the Earth is round is to go to a seaport and watch ships disappearing over the horizon: the bottom part of the ship vanishes first, and then the top. That's because the earth is curved. If the earth were flat, the whole ship would gradually get smaller until you couldn't see it anymore.
If you travel, and know how to recognize the constellations, you can also see that different stars appear in the sky from different parts of the earth. If the earth were flat, we should all see the same stars.
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