The first time I really understood why the stars moved, I was ten years old. Dad and I were in Woodacre for Christmas vacation, and on Christmas night, Grandpa and I put together the reproduction of Galileo’s telescope. Then we took sleeping bags onto the roof, where we lay down in the chilly, clear night. Grandpa said it was important to let our eyes adjust to the dark before looking through the telescope, so at first we simply gazed up at the star-scattered sky. I saw a point of light traveling steadily across the dark expanse, and when I pointed it out, he told me it wasn’t a star but a satellite, something built by human hands that we had put in orbit with a rocket ship. He told me Aunt Tammy built things like that, and though I’d known that before, I’d never comprehended that something she created could fly overhead in the dark, impersonating a star.

“The marvelous thing,” Grandpa told me that night, “is that everything you see above you is moving. The satellite is moving around the earth, the earth is moving around the sun, and the sun and the planets in our solar system are moving around the center of our Milky Way galaxy.”

“But the stars don’t look like they’re moving,” I said.

“That’s a funny thing,” Grandpa said. He explained to me that the stars did move—they also orbited the center of the galaxy—but we were so far away from them that to us, they appeared to be still. However, if we watched them all night, we’d see that the stars seemed to rise in the east and set in the west, exactly like the sun and the moon. “That’s an illusion,” Grandpa said. “It’s not really happening that way. The stars only seem to rise and set because our planet is spinning, and as it spins, we see different parts of the sky.”

The idea that the earth was actually turning at over one thousand miles per hour was mind-blowing to me. I felt the hard surface of the roof beneath me through the sleeping bag, and although it was barely slanted, I had the sudden fear that I’d slide off the edge as the planet spun. But nothing happened; everything remained as it was. It was astonishing, and I remember it even today: the realization that the world was not as it seemed. That the stars that appeared to hang motionless above me both moved and did not move, because the earth below me was not actually still. That what I saw might not be reality, but that it was possible to understand it through careful observation, through the instruments that scientists had built to peer into space.