On a moonlit night, grandpa told his grandson a story.

At night, stars are scattered in the night sky like countless pearls. A child is sitting in the yard, leaning against his grandmother and the back of his chair, counting the stars in the sky. One, two, count to one hundred.

Grandma smiled and said, "Silly boy, count the stars again. So many stars, flashing and moving, can you count them clearly? "

The child said, "Grandma, if you can see it, you can count it clearly. The stars are moving, not moving. You see, this star and that star are always so far away. "

Grandpa came over and said, "Son, you look very carefully. Stars in the sky are moving, but the distance between them is constant. Our ancestors divided them into groups and gave them names. " Grandpa paused, pointed to the northern sky and said, "Look, those seven stars are connected together like spoons, and they are called the Big Dipper. Not far from them, there is the brightest star called Polaris. The Big Dipper always goes around the North Star. "

Is it true what Grandpa said? The child didn't sleep well all night and got up several times to look at the stars. He can see clearly that the Big Dipper slowly turns around the North Star.

The child who counts stars is called Zhang Heng, a native of Han Dynasty. When he grew up, he studied hard and invented the armillary sphere.