Planets don't twinkle??

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tfwthom

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From Phil Plait's Bad Astronomy:<br /><br />Twinkle Twinkle Little Star<br />Week of August 23, 1999 <br />It's semi-common knowledge that stars twinkle and planets don't. By semi-common, I mean that a lot of folks know that, but I also mean it isn't strictly true. <br /><br />Stars twinkle because we see them from the bottom of a sea of air. Little cells of air, which are about ten centimeters across and located many kilometers high, move across our vision as we watch the sky. These small bundles of air act like little lenses, bending light as it passes through them. This bending, called refraction, is familiar to anyone who drives on a hot day: hot air just above the road surface bends light more than the cooler air slightly above it. That's why you can sometimes see that shimmery veil of what looks like water on the road; it's really the air bending the light above it. Sometimes you can even see cars reflected in the road! <br /><br />Anyway, these parcels of air up high in the atmosphere travel to and fro, bending the light from a star in more or less random directions. Stars are big, but they are so far away that they appear to be very small, much smaller to our eyes than each of these air bundles. So when the light gets bent, the apparent movement of the star is larger than the size of the star in the sky, and we see the star shifting around. Our eye can't really detect that motion, because it's too small. What we see is the light from the star flickering. That's why stars twinkle! <br /><br />So why don't planets twinkle? It's because planets are bigger. Well, really, they're smaller than stars, but they are so much closer they appear bigger to us. They are much bigger in apparent size than the air bundles, so the smearing out of their light is much less relative to the size of the planet itself. Since the image doesn't jump around, they don't appear to twinkle. <br /><br />There's always an exception though. In very turbulent air, even planets can appear to twinkle. The <div class="Discussion_UserSignature"> <font size="1" color="#3366ff">www.siriuslookers.org</font> </div>
 
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MeteorWayne

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The reason is that stars are a point. If you magnify a star in a telescope a thousand times, it's still a point. A planet is a disk, in other words, it a covers a certain area. <br />So the light from a star is refracted (bent) and split into different colors, since the atmosphere acts like tiny moving prisms.<br />With a planet, since it's a disk, it acts like a whole lot of individual points right next to each other, and they all average out.<br /><br />BY the way, planets can and do twinkle sometimes.<br />When they are at their smallest, and near the horison, and on a turbulent day, the size of the atmo-prisms is sufficient to cause twinking, although I have never seen color shifts. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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yevaud

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An addendum to Phil's good explanation:<br /><br />There really aren't distinct "little cells of air" centimeters wide. It's an arbitrary distinction used in Atmospheric Physics for convenience's sake. We're taught to consider "dimensionless" parcels of air of any arbitrary but useful size. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
 
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