Have you ever wondered why stars twinkle? It's caused by poor seeing conditions and is the bane of Earthbound astronomers. Some stars twinkle and change color so rapidly that they have been reported as UFOs.
Poor seeing conditions are caused by turbulent mixing in the Earth's atmosphere. Light from stars must go through our atmosphere and is perturbed as it goes through varying layers of air. The lower the elevation of the observer, the more air the light must travel through, the more perturbation. That is the reason observatories with powerful telescopes are built on top of mountains or launched into space.
Poor seeing is also called scintillation. Scintillation, or seeing effects, are always much more pronounced near the horizon than near the zenith (straight up) because light travels through more atmosphere at the horizon than when viewed from straight below.
The worst seeing conditions are found immediately after a cold front has gone through the area. This turbulence is cause by the mixing of different air masses along the surface of the Earth to around 180,000 feet or more. If you are looking at a celestial object such a planet or the moon during scintillation, study the image for several minutes and the object will come in and out of focus.
Recently, ground-based telescopes, including some advanced amateur equipment, can reduce the amount of scintillation with the use of state-of-the-art adaptive optics. This device keeps the image constant and is used for astrometry, lightcurve photometry, variable star studies and long-exposure astrophotography. The next time you are out on a clear dark night and you see stars twinkle and change color, it's not a UFO, but simply refracting starlight as it travels through our atmosphere.