Why do the stars twinkle


This video not only has a beautiful view but also shows a very common phenomenon knows as 'twinkling' (Try HD viewing here). This is very well known with the stars we see in the night sky but this video is showing the street lights twinkling as well. The further they are the stronger is the effect.

So what is twinkling anyway.. my Oxford dictionary of Astronomy defines it this way:

The rapid fluctuations in brightness of stars, more properly known as scintillation. Twinkling is caused by atmospheric turbulence distorting the incoming wavefront from a star. Twinkling is less noticeable in planets, which have an extended area. Excessive twinkling is a sign of bad seeing.

A light ray not only changes its speed but it also bends its path when it moves from one medium to a different medium, which have different densities. The vertical rising air and the fast moving horizontal air in the atmosphere are constantly creating different pockets of various densities. Which in turns varies the refractive indices of different regions in the air. Here when and where the wavefront is convex, the light rays converge and where the wavefront is concave, the rays diverge. This concentrates and de-concentrates the energy of the wave in certain patches. Twinkling is born.

These changes begins in the high layers of earth's atmosphere and extends all the way down to the earth or inside your telescope tube and even on the surface of the mirror or lens.

So to keep the observatories more thermally balanced they now have windows like features in the domes' walls which are opened when needed. In my observatory, which has a roll off roof structure, i have tried various ways to make seeing as good as i can. Since it is on the roof top of my house which has bricks all over, in hot season, i sprinkle water on the roof when the sun goes down. In about two hours when i am ready to image, the evaporating water has already taken a lot of heat from the brick roof.

I have also tried multiple times to turn on a pedestal fan at various speeds in my observatory in the hope of getting the convection layers as low as possible but honestly i have not noticed any difference in my seeing, perhaps the structure of my observatory does not allow convection currents in the air to minimize this way. The other problem is the mount, at these high wind speeds and such a long focal lengths the mount does shake and the image degrades so the fan is not helpful for me.

Adaptive optics technology in the professional observatories helps a lot in minimizing the bad seeing.. these 'rubber optics' which can adjust according to the incoming wavefront are one of the key technological marvel that astronomers can really feel proud to have. Though my SBIG AO-8 is marketed as adaptive optics but in reality it does not have a bending optical instrument, rather it has a lens that can move on two axis at quite high speeds. It actually is a good guider than a bad seeing corrector device.

Coming back to this video, the far off street lights are traveling such a long distance that the varying densities of air, caused by the movement of hot air, are making them twinkle. The nearest lights are much less effected and show quite a stable sources of lights.. great view!