Twinkle Twinkle Little Planet

Sounds wrong because the extra syllable blemishes the aesthetic quality of the symmetry of the rest of the poem.


Planets do twinkle, in fact. Its just that we do not notice. And it is the hallmark of a truly good (and bored) scientist to correct a misconception, even if it is only superhuman vision that can salvage the fallacy.

The atmosphere is a sea of air. Air is a fluid. Which means it is not vacuum. Therefore it has a refractive index. The density of air changes with altitude, so the refractive index also changes. Any light that enters the atmosphere (except from the normal vector, obviously) is bent.

Stars are very far away. They are so far away that they appear as point sources of light. So if we were to draw a ray diagram for a star, we would only need a single line to denote star light. It so happens that due to the constant ‘flow’ of the atmosphere, some times that star light is refracted so that it momentarily does not reach our eyes. That is called twinkling.



Planets are not that far away. Since they are nearer they can be resolved as light sources with a dimension. So a ray diagram for a planet will have multiple lines denoting light coming from different points on the planet. That light is also bent the same way as star light. And individual rays also sometimes bend enough so that they do not reach our eyes. But there are enough rays that do reach our eyes that we do not notice the slight change in the planet’s brightness. Hence we do not notice the planet twinkling.

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7 thoughts on “Twinkle Twinkle Little Planet

  1. I was wondering about this, since it makes sense that the atmosphere causes stars to twinkle. I guess the book should have said planets are the ones that don’t appear to twinkle. Thanks for the scientific explination.

  2. This is a great explanation. It makes me wonder if the idea can be extrapolated further. For example, are stars that are farther away affected more by atmospheric influence resulting in a more twinkly star?

    • That’s an interesting idea. However I do not think that by itself this phenomenon can be used explicitly to relate to distances. There are too many unknowns you see. The twinkling is based on the projection of all space on a celestial sphere. Anything that “appears” small twinkles. So a star, say Sirius, will appear the same to us as another star that is twice the radius and four times as far (considering everything else is same). And they will both twinkle the same way. Mathematically this will give us a 2-parameter series of general solutions, with one parameter being size and the other being distance. If you don’t know the size then you cant find the distance.

  3. Great blog. I think its hard to come to terms with the idea that all stars are basically just pinpoints or point sources of light from earth. The way light interacts with the atmosphere is complicated- do you understand why stars are just point sources where we can’t see details of their surface?

    • Imagine that there is a spherical shell around you that contains everything that you see: the sky, the trees, your outstretched hands, anything. Now imagine that there is a circle that goes all the way around the sphere, just like the equator or any other latitude/longitude line. Divide the circle into almost 21,600 little arcs. If there is any object which appears to fit inside one of those little arcs, your eye will not be able to distinguish it as more than a single point. That is the case with stars, but not with planets in our solar system (or if it is, the effect is several orders of magnitude less potent). I hope that answers your question. 🙂

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