Photons are emitted by stars or reflected from planets in spherical wave fronts. This spread is smaller than an object which is optically resolved. A star appears on the retina as a diffraction вЂњhump,вЂќ called an Airy function, due to the circular aperture. The image disk is just spread out a little more on the retina than would be a star. Even on the eye they are imaged as disks, just very small ones. The planets are close to the Rayleigh criterion, but we image them as disks. As a result we can resolve an image of a planet readily, but we are not able to do so with a distant star So put this together and we have the size resolutionДЇor R_p the distance to a planet and R_s the distance to even a nearby star we have R_p/R_s ~ 10^ that of a star. If this object is far enough away, R > D, then we have sin? ~= ? for small angle theta and the arc length formula tells us that l = R?. Suppose the object we are watching has a size l. An example of this is driving at night on a long flat highway, where cars at a distance appear to have one light, but then as you get closer you resolve it into two headlights. The formula tells us that any object at a distance which has a size whose angular resolution is below ? will not appear. The 1.22 comes from the evaluation of a Bessel function and physically is due to some diffraction. The angular resolution is given byД¬alled the Rayleigh criterion. Suppose we are observing something with emits optical radiation of wavelength ?. The reason for twinkling is due to optical resolution. Twinkle, twinkle little star, now we know what you are (and why you are twinkling!) One of most famous telescope of them all, the Hubble Space Telescope doesnвЂ™t get affected at all by our atmosphere as it is in space, making the light from stars crystal clear. This optical illusion is a big pain for astronomers and some very large telescopes such as those in Chile and Hawaii use special equipment and techniques to reduce the effects of the atmosphere. Phil Plait, the Bad Astronomer explains it very well on his website.) The larger disks of light usually arenвЂ™t distorted however if you are looking through especially turbulent areas of our atmosphere, and even sometimes when looking at planets that are low in the thicker parts of the atmosphere, they will twinkle. ( Addition due to the questions in the comment section: planets donвЂ™t usually twinkle because they are closer and therefore bigger - they are disks of light instead of faraway points of light. This is also the cause of colourful sunrise and sunsets. The higher an observer is looking, the thinner the atmosphere. The lower towards the horizon an observer is looking, the thicker the atmosphere.

We are actually looking at it through a very dense part of the atmosphere which can be turbulent and contain many different particles and dust. It is very bright, which can amplify atmospheric effects and it is also very low down in the atmosphere for those in the northern hemisphere. Sirius appears to twinkle or shimmer more than other stars for some very simple reasons.

Twinkle in time