Stars Twinkling Explanation

Overview

This lecture explains why stars twinkle due to atmospheric refraction and clarifies why planets do not twinkle when observed in the night sky.

Stars are light-emitting celestial bodies that appear point-sized due to their great distance from Earth.
Planets reflect sunlight, appear larger than stars, and generally do not twinkle.
Twinkling objects in the night sky are stars, while non-twinkling objects are planets.

Earth's atmosphere is made of air layers with varying density, highest at the surface and decreasing upward.
Starlight passes through these atmospheric layers and bends (refracts) towards the normal as it enters denser layers.
Because stars are distant, their light appears to come from point sources and is affected strongly by atmospheric refraction.
Random and continuous changes in air layers cause fluctuation in refractive indices.
These fluctuations shift the apparent position and brightness of a star, resulting in the twinkling effect.

Planets are closer to Earth and are not point sources, but rather appear as disk-shaped objects.
Multiple light rays from planets pass through the atmosphere and experience random refraction.
The atmospheric effects on the many rays from a planet cancel each other, so there is no visible change in position or brightness.
Therefore, planets do not twinkle, and their light appears steady.

Atmospheric Refraction — The bending of light as it passes through Earth's atmosphere with layers of varying density.
Twinkling (of Stars) — The apparent change in brightness and position of a star due to atmospheric refraction.
Refractive Index — A measure of how much light bends as it passes through a medium.
Point Source — An object that is so small compared to the observer's distance that it appears as a single point of light.

Review the differences between stars and planets in appearance and behavior in the night sky.
Understand and memorize the explanation of atmospheric refraction and its effect on starlight.