![]() Without as much blue/violet light removal, the sky takes on a darker blue or nearly black appearance rather than purple when looking straight up on a clear night. Overhead, there is less scattering between the viewer and the upper reaches of the atmosphere. Light coming in at a shallow angle travels through more atmosphere than light coming from directly overhead, resulting in stronger scattering at the blue and violet end of the spectrum that gives a purple tint when looking out towards the horizon. This effect occurs because there are more air molecules lower in the atmosphere to scatter the light at altitudes near the horizon. Overhead, the sky tends to transition to a darker blue color. The purple hue of the night sky is most pronounced closer to the horizon. With less red light to balance it out, the predominance of scattered blue and violet light makes the night sky take on a purplish tone. But this long wavelength light is not scattered very much compared to the remaining blue/violet light coming from scattering of moonlight and starlight. However, artificial lighting on the ground emits some longer wavelength orange and red light that manages to bounce around and contribute to the night sky. At night, there is very little light at the red end of the spectrum coming from sunlight. Even without direct sunlight, there is scattering of ambient light in the atmosphere. ![]() The extra scattering of blue/violet gives the daytime sky its familiar bright blue appearance.īut this scattering process doesn’t stop when the sun sets. During the day, sunlight enters the atmosphere and its component colors with shorter wavelengths, like blue and violet light, are scattered more strongly than colors with longer wavelengths, like red and orange. This refers to the scattering of light off of particles in the atmosphere that are smaller than the wavelengths of visible light. The primary reason that the night sky looks purple is due to a phenomenon called Rayleigh scattering. The reasons behind the purple night sky have to do with the physics of light and the way our atmosphere scatters different wavelengths of visible light. This phenomenon has sparked questions about why the sky takes on this colorful appearance after sunset. Many people have gazed up at the night sky and noticed that it often appears to have a purple or violet hue, especially on the horizon.
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