why do the stars twinkle at night

Stars twinkle, while planets (usually) shine steadily. Why? Stars twinkle because Б theyБre so far away from Earth that, even through large telescopes, they appear only as pinpoints. And itБs easy for EarthБs atmosphere to disturb the pinpoint light of a star. As a starБs light pierces our atmosphere, each single stream of starlight is refracted Б caused to change direction, slightly Б by the various temperature and density layers in EarthБs atmosphere. You might think of it as the light traveling a zig-zag path to our eyes, instead of the straight path the light would travel if Earth didnБt have an atmosphere. Planets shine more steadily because Б theyБre closer to Earth and so appear not as pinpoints, but as tiny disks in our sky. YouБd could see planets as disks if you looked through a telescope, while stars would remain pinpoints.


The light from these little disks is also refracted by EarthБs atmosphere, as it travels toward our eyes. But Б while the light from one edge of a planetБs disk might be forced to БzigБ one way Б light from the opposite edge of the disk might be БzaggingБ in an opposite way. The zigs and zags of light from a planetary disk cancel each other out, and thatБs why planets appear to shine steadily. You might see planets twinkling if you spot them low in the sky. ThatБs because, in the direction of any horizon, youБre looking through more atmosphere than when you look overhead. If you could see stars and planets from outer space, both would shine steadily. ThereБd be no atmosphere to disturb the steady streaming of their light. Can you figure out which objects are stars and which are planets just by looking for the twinklers vs the non-twinklers?


Experienced observers often can, but, at first, if you can recognize a planet in some other way, you might notice the steadiness of its light by contrasting it to a nearby star. Bottom line: Stars twinkle because they appear as tiny pinpoints as seen from Earth, even through telescopes. Planets donБt twinkle because they are closer, and thus appear larger in our sky, as tiny disks instead of pinpoints.
The songline goes "Twinkle twinkle little star". What is the cause of the "twinkling" of stars? Does light from planets "twinkle" as does light from stars? A young person of my acquaintance asked me this question, and I didn't have a good answer. Stars twinkle because of turbulence in the atmosphere of the Earth.


As the atmosphere churns, the light from the star is refracted in different directions. This causes the star's image to change slightly in brightness and position, hence "twinkle. " This is one of the reasons the Hubble telescope is so successful: in space, there is no atmosphere to make the stars twinkle, allowing a much better image to be obtained. Planets do not twinkle the way stars do. In fact, this is a good way of figuring out if a particular object you see in the sky is a planet or a star. The reason is that stars are so far away that they are essentially points of light on the sky, while planets actually have finite size. The size of a planet on the sky in a sense "averages out" the turbulent effects of the atmosphere, presenting a relatively stable image to the eye. This page updated on June 27, 2015

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