I often return from camping trips with a sore neck. For a while, I couldn’t figure out what caused this ailment. “Maybe I slept on it weird,” I think to myself. Then I think back to my last trip into the wilderness, and remember specifically the cloudless, moonless nights. The inky black sky. The stars. The universe over my head.
When the lights of day go out and the wonders of the terrestrial earth begin to fade, another world begins to appear above the horizon. One by one, burning white bulbs of gas appear overhead, first on a background of deep blue, then onwards to indigo. On a moonless night, the sky never really turns black. Rather, it becomes a canvas of the darkest blue, sprinkled with the shimmering white glitter of stars.
But why do the stars shimmer? As in the lullaby "Twinkle Twinkle Little Star", stars almost seem to dance around in the sky, moving slightly to the left and right. I know that the stars themselves are not moving up there. So, if they’re not sporadically jittering through space, it must be something to do with their light reaching our eyes.
The twinkling of a star is caused by what astronomers call stellar scintillation. With head tilted upwards, jaw flapped open, you might think there is simply nothing above your head in the night sky until you reach another planet or star. It sure feels like a void, some vacuum empty of all graspable matter. But this is not actually the case. Earth’s atmosphere is full of matter; the myriad molecules and atoms suspended above our heads create a thick blanket that covers the globe. And to reach our eyes, starlight from space must travel through it.
As each photon of starlight careens through our atmosphere, it gets refracted, or bent, by the various pockets of air that it must travel through. It’s similar to sticking your canoe paddle in a lake, and, when you look down through the water, the paddle appears to be bent because the light bouncing off of it must travel through a heap of molecules before it reaches our eyes.
The photons bump into different densities of matter, which refract and bend the light headed upwards out of the water. And if the water is moving, the paddle even seems to wiggle underneath the surface, emulating stellar scintillation.
Now imagine light passing through miles of turbulent atmosphere instead of just a foot of water. The pin-pricks of starlight race down through different pockets of thick and thin air, slightly bending this way and that, shifting around in one giant blender of atmospheric molecules.
Planets appear to twinkle as well, but their proximity to earth significantly mitigates the scintillation effect. The light reflected off of a planet travels through far less space than even our most nearby star, Alpha Centauri, whose light takes four and a half years to reach our eyes.
I can’t help but stay up late when the weather is nice. Warm and cozy in my sleeping bag, the collage of the cosmos is mesmerizing, so much so that it keeps me up at night. Wonder always gets the best of me. Ahh. Now I know. This is why my neck hurts after my camping trips. But a sore neck is a small price to pay for spending a night gazing at the stars..
"Field Notes" is produced by the Montana Natural History Center.
(Broadcast: "Fieldnotes," 10/04/15. Listen on air or online Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., and Fridays at 4:54 p.m., or via podcast.)
