Physics
Planets do not twinkle because

Their distance from the earth does not change with time
They are nearer to earth and hence we receive a greater amount of light.
They are very far away from the earth resulting in decrease in intensity of light
They emit light of a constant intensity

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Physics
A standing wave occurs when

The speed of the wave is zero or near zero
Two waves overlap
A wave reflects upon itself
The amplitude of a wave exceeds its wavelength

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