Space Telescope

There's an Easy Trick to Telling Stars and Planets Apart in the Sky

Look up in the night sky, and you'll see millions of stars and a handful of planets. How can you tell the difference? Just remember: The classic lullaby doesn't go "twinkle, twinkle, little planet." There's a reason for that.

Planet Light, Planet Bright, First Planet I See Tonight

Stars twinkle because of the massive distance between them and Earth. And we mean massive: Our sun's closest stellar neighbors are more than four light-years away. Because of that great distance, we essentially see each star as a single point of light — a shape with almost zero diameter. Once it hits Earth, that starlight is refracted by the various differences of temperature and density as it makes its way through our atmosphere. The refraction is greater than the star's tiny diameter, so it's easy to see — and to us, it looks like twinkling. The scientific term for this is astronomical scintillation.

Our sun and the planets in our solar system are much closer than the rest of the stars in the sky. Earth's atmosphere refracts light from those, too, but since they're a lot closer to us, they show up with a larger diameter than those faraway stars. This makes them look more like tiny disks than pinpoints — something you might not notice with the naked eye but that's easy to see with binoculars or a telescope. The light from one edge of that disk might be forced to "zig" in one direction, but light from the opposite side might "zag" in an opposite direction. Those opposing directions effectively cancel each other out, producing a steady shine that doesn't twinkle like a little star.

Lost in Space

Experienced stargazers can figure out which objects are stars and which are planets just by observing which ones twinkle and which ones don't. But keep in mind that sometimes planets twinkle, too, if you spot them low in the sky. That's because when you look toward the horizon, you're looking through more atmosphere than when you're looking straight up. This means more light refraction, which means more of that astronomical scintillation — aka twinkling.

If you ever get a chance to visit outer space, of course, then you can expect to see a distinct lack of twinkling to go along with that distinct lack of atmosphere. The lack of light refraction from the atmosphere is why we put telescopes up in space, helping behemoths like the Hubble Space Telescope produce the brilliant and crisp images of the universe that make it famous. But that's not the only difference you'll notice in space.

NASA astronaut James Reilly told SpaceFlight Insider that once your eyes adjust during a spacewalk, "you can start to notice that some of the stars have colors we don't see here on the ground. So you see these pastel colors — light yellows, light pinks, light oranges, even light red ones and light blues — there's all kinds of colors that you can see in these stars that you can't see here because it's filtered out by the atmosphere."

Hubble in orbit.

Twinkling Is in the Eye of the Beholder

One other quirk of stargazing is that bright objects in the sky look different to everyone — even different telescopes. The four points emanating from stars in images from the Hubble Telescope, for example, happen in any telescope that focuses light with a mirror rather than with a lens. The four points are known as diffraction spikes and are caused by the light's path being diffracted slightly as it passes by the cross-shaped struts that support the telescope's secondary mirror.

Distortion isn't just for telescopes. Remember, the human eye has a lens, too! Those lenses have subtle structural imperfections called suture lines that show up where the lens fibers meet together. These imperfections leave a particular imprint on light as it reaches your eyes, so even though stars actually appear as tiny round dots, our lenses have smeared the light into a star-like shape by the time the light reaches our retinas. Because we're all different, every eye on Earth will see a slightly different star-like smear depending on the exact nature of its suture lines; even your own left and right eyes will differ. But every eye sees the same shape for every single star. Try closing one eye the next time you're looking up at the sky and see what happens!

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For more stargazing tips, check out "100 Things to See in the Night Sky: From Planets and Satellites to Meteors and Constellations, Your Guide to Stargazing" by Dean Regas. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Cody Gough January 25, 2018

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