The sky appears blue during the daytime because of a phenomenon called Rayleigh scattering. This scattering is caused by particles in the atmosphere that are much smaller than the wavelength of visible light. When sunlight strikes these particles, the blue light is scattered in all directions, while the other colors of light are not scattered as much. This is why we see the sky as blue during the day.

The amount of scattering depends on the wavelength of light and the size of the particles. Blue light has a shorter wavelength than red light, so it is scattered more by the particles in the atmosphere. This is why we see the sky as blue during the day, even though the sun's light actually contains all of the colors of the rainbow.

The scattering of sunlight by particles in the atmosphere also causes the sky to appear brighter at the horizon. This is because there is more atmosphere between the observer and the horizon, so there is more opportunity for the sunlight to be scattered.

Rayleigh scattering also causes the sky to appear blue to astronauts in space. However, the sky appears much darker to astronauts because there is no atmosphere to scatter the sunlight.