When we go out on a clear summer night and look at the stars, we notice that each one of them tends to twinkle and pulsate. Furthermore, the brightest stars seem to constantly change color, shifting from white to blue and sometimes to orange. The light emitted by stars, which travels a long distance to Earth, is not subject to any deviation. Travel in a straight line. Its path changes only as it passes through the Earth’s atmosphere.
The atmosphere, although transparent, is not a layer of uniform density. The part closest to the surface is denser than the upper layers. Moreover, during the day warm air currents are generated that rise, being denser than cold air currents. All of this causes the atmosphere to turn into a turbulent gas. In simple words, those pulsations we see are nothing more than atmospheric turbulence, like the one we feel when we are on an airplane.
The color of the stars changes
When starlight is about to reach us, it must pass through the atmosphere. Every time it encounters a layer of air of a different density, it experiences a slight deflection. It is subject to refraction, changes from one density to another, and so on continuously. Since the air is constantly moving, the stars also appear to be constantly moving, giving us the sensation that they are flickering. These small deviations can also cause stars to change color, as the Sun does when it hovers above the horizon. But in reality it is just a sensation that can be seen with the human eye.
Why doesn’t the moon beat?
This effect only affects stars because they are very small points of light. In the case of the planets and the Moon, since these bright spots are larger, they are more difficult to observe. If we look at the Moon with a prism or small telescope, we can observe the turbulence on its surface. In fact, these causes are caused by the atmosphere.
Those who live in major cities can see the same phenomenon by looking at the lights on the horizon. For example, on the strait, when looking at the lights of Reggio and Messina from the sea, at a certain distance you can notice this pulsation of lights. These pulsations become more intense on very windy days, when greater turbulence is created within individual filaments of air driven by the pressure difference. Also in this case we have the feeling that it is flashing. This is especially noticeable in the summer, when asphalt and concrete continue to give off heat (through infrared radiation) that has built up during the day.
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