A dark sky is a sight to behold. We’re gazing millions of years into the past at beams of light that travelled untold distances to twinkle in our eyes at that precise moment.
But why do the stars twinkle at all? And why do they shimmer brighter in the winter sky than they do on a midsummer’s night?
The answer lies in our skies above.
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Looking at a cross-section of the sky over a city would reveal layers of warmer and colder air stacked from the surface all the way to the edge of the atmosphere. Weather balloons often reveal dozens of significant temperature changes over relatively short distances.
The density of air changes with its temperature. Light bends as it passes through materials with different densities—in this case, layers of warmer air and colder air.
A straw appears bent or broken in a glass of water for the same reason—water is much denser than air, so the light bends dramatically as it passes through the water on its way to your eye.
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It’s not quite that dramatic in the atmosphere, of course, but a similar phenomenon happens when light passes through all those different temperature layers above our heads.
Light from tiny specks of stars in the night sky passes through many different layers of air on its way to your eye. The light bends and turns as it passes through each layer’s slightly different density.
Here’s the catch: air is always on the move.
Those layers are always slightly shifting and moving around as powerful winds rush through the upper atmosphere. This turbulence means that each ray of light is taking a slightly different path to your eye, making the stars appear to twinkle and shimmer in the night sky.
Temperature inversions—a rapid change in temperature with height—are more common in the winter than they are in the summer. These extreme temperature gradients make the stars appear to twinkle more in the winter than at other times of the year.
The air is also clearer during the colder months. There’s usually less moisture and humidity in the air during the wintertime than we’d see in the heart of summer, allowing a clearer and less obstructed view of the night sky.
