Why does stars twinkle but planets do not?
Stars emit their own light and they twinkle due to the atmospheric refraction of light. Stars are very far away from the earth. Hence, they are considered as point sources of light. When the light coming from stars enters the earth’s atmosphere, it gets refracted at different levels because of the variation in the air density at different levels of the atmosphere. When the star light refracted by the atmosphere comes more towards us, it appears brighter than when it comes less towards us. Therefore, it appears as if the stars are twinkling at night
Planets do not twinkle because they appear larger in size than the stars as they are relatively closer to earth. Planets can be considered as a collection of a large number of point-size sources of light. The different parts of these planets produce either brighter or dimmer effect in such a way that the average of brighter and dimmer effect is zero. Hence, the twinkling effects of the planets are nullified and they do not twinkle
As the light from a star enters the atmosphere, it hits gas molecules and scatters(reflection and interference).
Since the star is so far away, we only see a tiny beam of light from it.
This beam gets scattered away from our eyes and then back into them almost like it is blinking on and off.
It happens so fast that it just looks like it is twinkling.
(Planets are closer to us and send more light; if some of the light beams are scattered away, others still get through to us, so planets don't usually twinkle.)
Planets are much closer to the earth compared to stars and thus can be considered as extended sources. If we consider a planet as a collection of a large number of point sources of light the total variation in the amount of light entering our eye from all the independent point sources will average out to zero and hence the twinkling effect is cancelled.
Planets are much closer to earth as compared to stars and as such they are extended bigger source of light when observed from earth.A planet can be considered as a collection of a large number of point sized sources of light.Due to atmospheric refraction, each point source will send varying bright and faint light to the observer.If all the point size are taken together, the net variation in the amount of light entering eye average out to zero,nullifying the twinkling effect.Thus, the planets donot twinkle.
Compare the distance between a star and the Earth and a planet and the Earth.
A star is located far from the Earth. The light rays from the star has to suffer repeated refraction as it passes through each successive layer of the atmosphere. The upper layers of the atmosphere are rarer than the lower, hence, after each successive refraction, the light ray bends towards the normal. Now, the density of each layer continuously changes due to slight change in physical conditions like temperature and pressure, hence the apparent position of the Star continuously changes. Since the distance of the star is much, to our eyes, the change in position occurs twinkling; hence, we see stars twinkling.
On the other hand, planets are much closer to the Earth. Moreover, planets are not luminous by themselves, light from the Stars (like Sun) reflect from the planet. Thus, planets do not seem twinkling.
LIGHT FROM STARS-POINT SOURCES OF LIGHT
LIGHT FROM PLANETS-EXTENDED SOURCES OF LIGHT
Unlike stars, planets don't twinkle. Stars are so distant that they appear as pinpoints of light in the night sky, even when viewed through a telescope. Because all the light is coming from a single point, its path is highly susceptible to atmospheric interference (i.e. their light is easily diffracted).
The much closer planets appear instead as tiny disks in the sky (a distinction more easily discerned with a telescope than with the naked eye). Their apparent sizes are usually larger than the pockets of air that would distort their light, so the diffractions cancel out and the effects of astronomical scintillation are negligible.