Stars twinkle because they are so far away from Earth that they appear as point sources even when seen through powerful telescopes. The light rays coming from them are refracted multiple times, making them look as if they were blinking. The sun and other planets, however, are quite close to us (relative to stars), and thus appear like disks.
Twinkle twinkle, little star, how I wonder what you are!
Some rhyme, huh?
Everyone must be familiar with that famous rhyme from Jane Taylor’s poem “The Star”. It was a big part of childhood for many people.
Well, it’s not just a song for children; it’s actually referring to a particular scientific phenomenon that has fascinated observers on Earth for thousands of years. As we grew up, we were taught that stars may not be like diamonds, but they do twinkle, right?
They certainly appear to do so, but what about our closest star? The Sun is a star, the heart of our solar system, but we don’t see the sun twinkling. Why do all the other stars in the night sky appear to twinkle, but our Sun never does?
Why do stars twinkle, but the sun and planets do not?
Short answer: Stars twinkle because they’re so far away from Earth that when light from the stars passes through the atmosphere, it is bent countless times due to refraction, making it look like as if they were blinking. The sun doesn’t twinkle because it’s too close to Earth compared to other stars.
Astronomers have provided a scientific name for the twinkling of stars: astronomical scintillation.
That also explains the reason why astronomers don’t write songs for young children; it would have been a lot harder to remember songs about twinkling stars with words like scintillation thrown in there…
Why the Sun Does Not Twinkle
The other stars that we see in the sky are located very far away from Earth. The Sun is definitely the closest star to Earth, sitting approximately 93 million miles (0.0000158 light years) away. The next closest star, however, is located about 4.22 light years away from Earth, so that gives you some impression of how far away those other stars truly are from Earth.
The reason the Sun doesn’t twinkle is because it’s too close to the Earth, as compared to other stars. Due to this, unlike stars, the sun appears much bigger than a small point in the sky, and therefore, doesn’t seem to twinkle. You see, Earth’s atmosphere is so turbulent that images of all objects in the sky tend to ‘swim’. Consequently, a tiny point in space gets mapped to 2 or more points in space, which imparts that blinking effect to the point source. (Source)
In other words, stars appear so small (thanks to their distance from us) that due to atmospheric refraction, it appears as if they are at more than one place, which, in turn, makes them look like as if they were blinking. The sun, however, is quite close and therefore looks more like a disk as opposed to a tiny point in the vastness of space. That’s why atmospheric refraction doesn’t play that big a role in how we perceive it from Earth, and hence, doesn’t appear to blink.
Why Do Other Stars Twinkle?
Light from other stars crosses a very long distance to reach us and also passes through pockets of Earth’s atmosphere, which vary in temperature and density. Our atmosphere is very turbulent, with streams and eddies forming, churning around, and dispersing all the time. Every layer of Earth’s atmosphere has air moving in different directions at different intensities. When light from stars passes through the atmosphere, it is bent due to refraction, which is why stars seem to twinkle when we stare at them. If viewed from outer space, you would not see the stars twinkling.
This process is similar to watching a coin appear to dance at the bottom of the swimming pool. This optical illusion occurs because the water in the pool bends the path of light emanating from the coin.
Why Planets Do Not Twinkle
Planets, just like the Sun, do not twinkle. Planets are also closer to the Earth than those distant stars, so planets appear larger in comparison. Due to the planets’ closeness to Earth, the light coming from these celestial bodies does not bend much due to Earth’s atmosphere. Therefore, the light coming from our solar system’s planets does not appear to twinkle like stars.
We should be very thankful to the Earth’s atmosphere; aside from protecting us from harmful UV rays that could fry us in an instant, the atmosphere is why that sky full of stars appears to twinkle at night. Thankfully, this means that we can continue teaching that famous rhyme from Jane Taylor to future generations. It’s a good poem… it would be a shame to waste it!
- Stars Twinkle – University of California, riverside Department of Mathmatics
- Ask an Astronomer
- Why do Star Twinkle – Harvard-Smithsonian Center for Astrophysics