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How can an observer distinguish planets from stars?

In general, planets emit a constant light or shine, whereas stars appear to twinkle. The twinkling effect of stars is due to the refraction, or bending, of starlight as it passes through the Earth's atmosphere. And because stars are so far away and appear only as points of light, the twinkling is very noticeable.

Light from planets is also refracted by the Earth's atmosphere. However, because the planets are closer to Earth than the stars, the planets appear as disks. Their disklike shapes mask the twinkling effect. The exception to this rule occurs when planets are observed near the horizon. Near the horizon, the light from planets must travel through the greatest depth of Earth's atmosphere, which causes the planets to appear to twinkle.

Source: The Encyclopedia Americana, p. 594.

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