Physics
Planets do not twinkle because

They are nearer to earth and hence we receive a greater amount of light.
Their distance from the earth does not change with time
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
Specific gravity is defined as the ratio of

Density of the substance to the density of water
Density of the substance to the density of water at 0°C
Density of the substance to the density of water at 4°C
Density of water at 4°C to the density of the substance

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