When someone understands something, actually gets it—not as a fact memorized, but as a truth uncovered—for the first time, their eyes widen and glow with enlightenment. I've heard teachers say that being able to watch such a moment is one of their greatest rewards. I recently discovered exactly what they mean.
I was giving a sky tour during an astronomy event last year, and during the process of pointing out the constellations that were in the sky, I explained the ecliptic and the zodiacal constellations.
The ecliptic is an imaginary line in space, however, imaginary doesn't mean "unreal." For example, the equator is an imaginary line on the earth but the boundary of the northern and southern hemispheres is real. Lines of longitude and latitude are also imaginary lines, but they are still quite useful in finding a real position.
The ecliptic is imaginary in the same way. It's the line created by the sun's apparent path across the background of stars during the course of a year. Because the earth moves around the sun, from our perspective the sun blocks our view of different constellations at different times of the year. The specific 12 constellations it moves through are well known. They're the familiar signs of the zodiac.
On this particular night, when I started pointing out the constellations, Gemini was standing over the western horizon preparing to set. Cancer was obscured by light pollution. Leo arched high overhead, trailed by Virgo. Behind Virgo, Libra had risen, and I pointed out its wonderfully named brightest stars: Zubenelgenubi and Zubeneschalmali. I explained that these traditional names meant "northern claw" and "southern claw," designations linked to the separate constellation Scorpius. At that moment, however, I couldn't point out Scorpius because the only portion of it above the horizon at that time was obscured by trees.
I continued giving the tour. We looked at the constellations in greater detail. I provided tips for using pointer stars to hop from one spot to the next, and I told stories about the myths associated with the constellations. About an hour later I had circled back to Zubenelgenubi, and I invited people to look through my tripod-mounted binoculars to see that it was actually two stars, a binary system.
"Wait a minute." One woman was confused. The Zubenelgenubi I was now pointing to with my laser pointer was much higher in the sky than the one I had pointed to before.
"That's correct," I affirmed. "In fact, thanks for mentioning this because now I can point out the constellation Scorpius." And, I pointed my laser to the top of Scorpius, which was now hovering above the trees, visible all the way down to Antares.
She was astounded. "The stars moved?"
Well yes, but actually it was the Earth. And I did a quick demonstration by having her pretend to be a star while I was the earth. I spun on my axis and watched her rise and set.
"The stars rise and set like the sun," she said, making a connection. And I saw that light in her eye. "The earth rotates on its axis." She repeated the phrase she'd learned. Of course everyone knows that, but now for her it was an epiphany. A revelation. The earth rotates. I think she even felt it move. I know I did.