The Atmospheric Cause of Twinkling
Stars twinkle because their light passes through Earth's turbulent atmosphere. As light from a distant star travels towards us, it encounters different layers of air with varying temperatures and densities. These atmospheric layers act like tiny, constantly moving lenses and prisms, bending and distorting the light path slightly and continuously.
Stellar Scintillation: The Key Principles
This bending of light is called refraction. Because the atmosphere is constantly moving due to air currents and temperature fluctuations, these refractions change rapidly from one moment to the next. This rapid and continuous change in the light's direction causes the star's apparent brightness to fluctuate and its position to subtly shift, which is precisely what we perceive as twinkling or 'scintillation'.
A Practical Comparison
To visualize this, imagine looking at an object at the bottom of a swimming pool on a hot day. The water above it shimmers and distorts the view due to temperature differences and convection currents, making the object appear to wiggle. Similarly, the Earth's atmosphere, especially closer to the ground, acts like this turbulent body of air for starlight, causing its perceived instability.
Why Planets Don't Twinkle and Its Importance
Planets, being much closer to Earth than stars, appear as small discs of light rather than pinpoints. Their larger apparent size means light from different parts of their disc passes through many atmospheric distortions simultaneously. These numerous light paths average out the twinkling effect, making planets appear to shine steadily. Astronomers use techniques like adaptive optics to counteract atmospheric turbulence, allowing for clearer observations of both stars and planets.