When a star’s light pierces our atmosphere, each single stream of starlight is refracted – caused to change direction, slightly – by the various temperature gradients, density layers and turbulence in the Earth’s atmosphere. You might think of it as the star light traveling a zig-zag path to our eyes, instead of the straight path the light would travel if Earth didn’t have an atmosphere.
Stellar scintillation* is described as the effect produced by the scattering of light from refractive index variations in the Earth’s atmosphere. The refractive index variations cause changes in the phase of the light and these, in turn, lead to intensity variations. The intensity may vary spatially and temporally forming what is often referred to as a shadow pattern at the Earth’s surface. The twinkling of stars arises from the motion of this shadow pattern across the eye as well as from fluctuations in the pattern itself. Since our eyes are rather slow detectors, the twinkling is a time-integrated observation of the complete scintillation process.
The apparent position of the star is different from the actual position. As the atmosphere bends the starlight, the star will appear slightly higher when seen from the near the horizon. This position of star keeps on changing with the changing atmosphere because of its dynamic physical conditions.
Refraction in physics means a change in the direction of a wave (Lightwave, sound wave, and water waves) when it passes from one medium to another. Therefore when the light from the star undergoes refraction before entering the earth’s atmosphere because of change in the medium. As a result the refractive index changes gradually.
The refractive index or the index of refraction is the ratio of the speed of light in a vacuum to the speed of light in the material:
Stars hugging the horizon will appear to twinkle more than other stars. This is because there are many more gradations of atmosphere between you and a star near the horizon than between you and a star higher in the sky.
bhmpics.com
A star doesn't twinkle when seen from the near perfect vacuum on the Moon's surface or when viewed from a port on the International Space Station (ISS).
Planets shine with a steady light because they’re much closer to Earth and so appear
not as pinpoints of light, but as tiny disks in our sky.
You can see planets as disks if you looked through a pair of good binoculars or a modest spotting telescope, a la Venus and its distinct phases (see blow), while stars still remain pinpoints. The light from these little planetary disks are also refracted by Earth’s atmosphere, as it travels toward our eyes. However – while the light from one edge of a planet’s disk might be forced to “zig” one way – light from the opposite edge of the disk might be “zagging” in an opposite way. The zigs and zags of light from a planetary disk cancel each other out, and that’s why planets appear to shine steadily.
skymania.com
See:
https://sciencequery.com/why-do-stars-twinkle/
* Scintillation: 1. the rapid fluctuations in the phase and amplitude of the wave. These are caused by local rapid variations in the
refractive index of the medium through which the wave is traversing. 2. The act of scintillating. ... scintillation - the twinkling of the stars caused when changes in the density of the earth's atmosphere produce uneven refraction of starlight.
See:
https://www.thefreedictionary.com/scintillations
Stars and planets and our friend, Old Mr Moon, are our nightly visitors. Some twinkle, while some burn with a steady reflected light, while the Moon is often bright enough to light our path.
Hartmann352