Do you know why the sky is blue?

Prof. Know Why explains:

The atoms of nitrogen and oxygen in the atmosphere separate the sun’s white light into its many colors and scatter them throughout the atmosphere. The wavelength of the blue light scatters better than the rest, predominating over other colors in the light spectrum. This makes the sky appear blue to us on a clear day.

The scientific name for this phenomenon is ‘Tyndall Effect’, more commonly known as ‘Rayleigh Scattering’. This phenomenon describes the way in which light physically scatters, when it passes through particles in the earths atmosphere that are 1/10th in diameter of the color of light. The light spectrum ranges in wavelength from red to violet and since the wavelength of the blue light passes through the particles with greater ease than the wavelengths of other colors of light, the sky appears blue to the naked eye.

The human eye has three types of light receptors, known as cones, located in the retina. The cones are either considered to be red or blue or green, based upon their strong response to light at these wavelengths. As light stimulates these receptors, our vision translates the signals into the colors we see. The skylight stimulates the red and green cones almost equally, while stimulating the blue cones more strongly, resulting in the blue colour of the sky.

Do you know why stars twinkle?

Prof. Know Why explains:

On a clear, dark night, our eyes can see about 6,000 or so stars in the sky. They seem to twinkle, or change their brightness, all the time. The scientific name for this twinkling of stars is stellar scintillation or astronomical scintillation. Stars seem to twinkle when we see them from the Earth's surface, because we are viewing them through thick layers of turbulent (moving) air in the Earth’s atmosphere.

The Earth's atmosphere comprises of layers of gases surrounding the Earth. It’s composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and other gases. These gaseous layers insulate the Earth from extreme temperatures and block the Earth from much of the Sun’s incoming ultraviolet radiation.

As light of a star travels through these layers of the Earth's atmosphere, it is bent or refracted many times and in random directions (it happens whenever it hits a change in density - like a pocket of cold air or hot air). This random refraction results in the star appear to our eyes as twinkling.

Stars would not appear to twinkle if viewed from outer space or from a planet that doesn't have an atmosphere.