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Why the planets are not twinkling, while the stars are?
Asked by: Izrailsky Yuri
Since stars are so distant, they subtend a minute angle of the total visual field despite their enormous diameter. In other words, they approximate point sources of light. As this light travels through the atmosphere, it passes through areas of lower and higher density, and therefore lower and higher indices of refraction. These areas act as virtual lenses, refracting light and concentrating first greater amounts, and then lesser amounts of total radiant energy on any one point on the surface of the Earth. This is the source of the 'twinkling' of stars - i.e. the fluctuation of intensity.
Planets are much closer to the Earth and therefore subtend a larger angle within the total visual field. That is why we often speak of the planetary 'disk' in astronomy: even under slight magnification (e.g. through binoculars) the human eye interprets a planet as a 2 dimensional source of light and not a point source.
Most peoples' visual acuity is not sufficient to notice the discrepancy between the angle subtended by a star and that subtended by a planet. However when the planet's light is refracted through the atmospheric 'lenses' the spread of light from the disk is sufficient to blur the focused area and therefore prevent the light reaching the eye from ever dipping to as low a level as it does from a point source. In other words, the darkest parts of the twinkle cycle from one part of the disk may be overlapped by focused light from other regions of the disk, keeping the light intensity closer to some mean value and reducing fluctuation. In short, no twinkle!
Answered by: Rob Landolfi, Science Teacher, Washington, DC