Sparks Both Icy And Atmospheric

Ice crystals condensed from the air during last night's freeze coat the leaves of a small bush this morning. Photo: Bob King

This morning the lawn sparkled with millions of tiny ice crystals deposited as frost overnight. I like a thick frost. The way it turns grass into spaghetti and leaves into sugary confections. When the sun rose high enough to touch the scene with light, the icy bits shattered the entire spectrum into multicolored glints and sparkles.

Atmospheric turbulence causes twinkling

The stars appear to sparkle or twinkle, too, but the reason is totally atmospheric. Over the weekend, but especially on Saturday night, twinkling was intense. I noticed this while walking the dog around 11 o’clock, when Orion and his entourage of bright stars ascended in the east. Every star was pulsing, flashing, fading and then re-brightening many times a second. If you’re nearsighted like I am, try taking off your glasses to look at the twinkling stars. The out-of-focus disks show the flashing effect even more vividly. Orion’s suns looked positively frenetic without them.

Because of their great distance from us, stars are essentially points of light – delicate things. Turbulent cells of air in the upper atmosphere of different temperatures, humidity and density (some pockets of air are more rarefied than others) bend or refract the fragile beams of starlight differently moment to moment. As different cells move in and out of our line of sight, they not only shift a star’s position this way and that but also split up its light into separate colors, the beams of which arrive in our eyes in this or that fraction of a second. We might catch a yellow flash, followed by spark of red or even blue. Since the air is constantly in motion, stars can twinkle wildly, varying in brightness by a couple magnitudes as their light is literally yanked this way and that by petulant air currents.

Stars seen higher up in the sky twinkle less because their light passes through less of the denser air in the lower part of our atmosphere. Starlight near the horizon must travel many hundreds miles through thicker, denser air cells and currents. Its more tortuous path means it twinkles more. Illustration: Bob King

I set up my telescope that night and wasn’t surprised to see blurry images. The same turbulence that causes stars to dance also makes images look soft, shimmery and bouncy when magnified. It’s even worse nearer the horizon. There our line of sight takes us through the lower, thicker part of Earth’s atmosphere, where even more currents interfere. Although pretty to the eye, astronomers call these effects “bad seeing”. Oftentimes, beginning telescope users will think they need a better telescope because the one they’re using “never seems to focus.” In many cases, the problem is just bad seeing. That’s why the Hubble Space Telescope is where it is – above the atmosphere where the stars never twinkle.

Only the largest, most obvious features are visible on Jupiter on a night of bad seeing. Credit: Eric Norland

Planets are relatively immune from twinkling because they have actual disks. Although they look like bright stars to our eyes, the tiny twinkling effects happening over different parts of the disk average out and give planets a placid, steady light. But not always. On nights of great turbulence, I’ve seen Venus, Mercury and Jupiter scintillate when low in the sky. Across the vast ocean of air, the shifting currents have strong enough refracting (bending) power to move a whole disk to and fro.

It’s important to distinguish between “seeing” and transparency. Transparency has to do with how clear the air is and dark the night. Saturday night’s sky was one of exceptional transparency, but alas, the seeing was poor. When this happens, amateur astronomers stick to observing large objects at low power where the effects of bad seeing aren’t as obvious.

Last night the atmosphere was steadier though the stars still danced some. In good seeing, much detail is visible on planets and low contrast features like the spiral arms in galaxies stand out with greater clarity. On the other hand, there’s pleasure in staring at a bright, low star like Capella or Betelgeuse sparking like a shorted-out wire. What will tonight bring I wonder?