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Astro Bob: Star or planet? A fun experiment to tell them apart

Here's a little experiment you can perform to help you see the dramatic difference between a star and planet.

Twinkling mine with atmosphere Sirius better S.jpg
Earth's atmosphere is comprised of layers and pockets of air of varying temperature and density. Each acts like a separate prism or lens and redirects light rays arriving from a star in different directions. We catch more or less of this light which we perceive as twinkling.
Contributed / Bob King
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Most people know the difference in appearance between a star and a planet. They'll tell you that stars twinkle while planets don't. And that's largely correct. Stars are so far away that they look like pinpoints even through telescopes. Picture the light from a star as a spider-web-thin shaft of light. Before it gets to your eye it must pass through Earth's turbulent atmosphere, which is made up of layers of air of different temperature and density. Each layer or pocket refracts or shifts the light's path in slightly different directions, forcing it to zig and zag instead of passing straight through.

At one moment, a portion of the star's light might get shifted away from your eye, making it dim slightly. Other times, a different pocket shifts more of the light toward your eye, and the star brightens. Since the atmosphere is constantly in motion, we perceive all these momentary peaks and valleys in brightness (zigs and zags) as twinkling.

The effect is quite beautiful to see. All stars twinkle, but the brighter the star the more obvious the shimmer. It's even more striking when you examine a brilliant gem like Sirius close to the horizon, where we look through the bottommost layer of the atmosphere. There the pockets really add up along our line of sight, and the fluctuations take on a positively explosive aspect!

Mercury Austin Jarboe dispersion horizon prism.jpg
This is the planet Mercury, but it was so low in the sky at the time its light was spread out into a tiny rainbow.
Contributed / Austin Jarboe

Not only that, but when we look at a star low in the sky, the atmosphere acts like a lens or prism and spreads (refracts) its light into all the colors of the rainbow. The technical term for this fanning out is dispersion. If you observe a bright star through a telescope near the horizon it actually looks like a tiny spectrum. Every color gets the zigzag treatment on its way to our eyes, so we see flashes of yellow, blue, green, red, purple and orange. Binoculars bring out the hues even better.

The degree of twinkling varies night to night depending on the state of the atmosphere. Some nights are like the 4th of July, others serene. If you wear glasses for nearsightedness, take them off on a twinkly night. Stars will swell into disks, and the glitter effect becomes even more obvious. From the moon or the International Space Station all stars shine with a perfectly steady light because there's no atmosphere to rough them up. I'd love to see the stars at night from Earth orbit.


Planet vs star twinkling A_220 S.jpg
Jupiter and the other planets are close enough that we can see their shapes compared to the enormously distant stars which look like pinpoints even in a telescope.
Contributed / Stellarium, Bob King

Planets also twinkle, only we can't detect it with the naked eye. That's because planets aren't pinpoints but close enough to us to show actual disks. You can think of a disk as a dense concentration of individual stellar points — a multitude of beams all bundled into one. As the atmosphere zigs and zags, those points randomly flash and fade across the disk. When one spot brightens, another fades, and the whole averages out. This is why planets appear steady to the eye.

Not so in a telescope, where we can magnify their disks and easily see the atmosphere shimmer, blur and shake them about. But at the lower resolution of the human eye, these variations cancel one another out.

On occasion, I've seen planets twinkle but only when they're very close to the horizon, where they're filtered by so many pockets and layers that their disks finally succumb to turbulence. This usually happens when the planet is far from the Earth and appears smallest. That makes sense, right?

Twinkling Jupiter Sirius Jan 23 2022.jpg
Find a south-facing location the next clear evening around the end of twilight (6:30-7 p.m.), when Jupiter and Sirius stand at similar altitudes. This will make it easy to compare their appearances and distinguish planet from star by twinkling or its absence.
Contributed / Stellarium

So here's a little experiment you can perform to help you see the dramatic difference between a star and planet. For the next week or so, Jupiter will appear at nearly the same altitude in the southwestern sky at the end of evening twilight (around 6:30-7p.m.) as Sirius in the southeastern sky. They're also of similar brightness, Sirius, at magnitude -1.5, just a half-magnitude brighter than Jupiter (-2.0).

Find a location with an open view in both directions and flip back and forth between the two. Jupiter should appear unflappable, while Sirius will be a riot of light in comparison. Look closely at the star and bring binoculars. You should be able to see the color flashes we discussed earlier because its altitude at 7 p.m. local time will only be around 9-10°. For even more dramatic views of Sirius you can start earlier in twilight, when the star hovers just a few degrees above the horizon.

Drop a comment and let us know how it goes. But mostly, just enjoy the show!

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"Astro" Bob King is a freelance writer for the Duluth News Tribune.

"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune.
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