Rayleigh scattering plays a crucial role in determining the sky's color by selectively scattering different wavelengths of light. This process occurs when sunlight interacts with particles in the atmosphere that are much smaller than the wavelength of visible light, such as air molecules1. The intensity of scattered light is inversely proportional to the fourth power of its wavelength, meaning that shorter wavelengths (blue and violet) are scattered much more efficiently than longer wavelengths (red and orange)12. As a result, blue light is scattered about 16 times more than red light, causing the sky to appear predominantly blue to our eyes3. However, the sky isn't purely blue but rather a bluish-white, as all colors are scattered to some degree4. This scattering effect is most pronounced in the upper atmosphere, where air molecules are sufficiently far apart to allow for effective Rayleigh scattering without destructive interference4.

While violet light has an even shorter wavelength than blue and is scattered more according to Rayleigh's Law, the sky does not appear violet due to a combination of factors related to sunlight composition and human color perception. The sun emits less violet light compared to other colors, and much of the violet and ultraviolet light is absorbed or scattered by the upper atmosphere before reaching our eyes12. Additionally, human eyes are more sensitive to blue light than violet, with blue-sensitive cones outnumbering violet-sensitive ones1. The perception of sky color is further influenced by how our visual system processes the mixture of scattered wavelengths. When we look at the sky, our red cones respond weakly to the scattered red, orange, and yellow light, while our blue cones are strongly stimulated by the scattered blue light3. This combination of factors results in our perception of a blue sky rather than a violet one, despite violet light being scattered more intensely in the atmosphere.

The composition of a planet's atmosphere significantly influences its sky color. Earth's atmosphere, consisting primarily of nitrogen (78%) and oxygen (21%), is responsible for our blue sky through Rayleigh scattering1. However, different atmospheric compositions can result in vastly different sky colors on other planets. For instance, Mars' thin atmosphere, containing fine dust particles, creates a butterscotch-colored sky during the day due to the absorption of blue light by dust2. Interestingly, Mars experiences blue sunsets, as the longer path of light through its atmosphere at these times allows for more blue light scattering2. On Venus, the dense atmosphere rich in carbon dioxide and sulfuric acid clouds would likely produce a yellow or orange sky3. The Moon, lacking a substantial atmosphere, has a black sky both day and night2. These variations demonstrate how atmospheric composition and density play crucial roles in determining the perceived color of a planet's sky.