In our earth, the blue sky only happen on the day. There are some reason, why the sky is blue on that time. Our earth is covered by air layers that called atmosphere. Although it can't seen with eyes, the air actually contain with the particles.
On the day, the sun emitting it's light to the earth. The light from the sun, is scattered by the particles on the atmosphere. As we know, colour of the light containing with a combine colour such as the red, yellow, green, blue till purple. Each colour has different frequency. The red colour has smaller frequency than yellow, yellow has smaller frequency than green, green has smaller than blue, and blue has smaller than purple. The higher frequency of the light, more stronger the light is scattered.
The colour of the sky is a part of scattered sunlight. Because the scattered color is much from the high frequency (green, blue and purple), than the sky colour is a combining from that colours. Their mixing produce the light-blue. That makes our sky, coloured with blue. Near the source (the sun), the light isn't much scattered, so the colour around it is still pure, like the real colour of light.
In the evening, the sun and sky colour changes into red. That is happened because in the evening the sun change it's position to the sideways. Because of that the sunlight takes more length to reach our eyes, and the more sunlight is scattered. When the light reach our eyes, it's only contain the low frequency of colour (red), which has longer wavelength than the other colours. That's why the colour of the sky we seen in the evening is red.
In the moon and in the other planet that doesn't have an atmosphere, the sunlight will not scattered. So in that place, the sky is always dark. Although it is on the day.
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On the day, the sun emitting it's light to the earth. The light from the sun, is scattered by the particles on the atmosphere. As we know, colour of the light containing with a combine colour such as the red, yellow, green, blue till purple. Each colour has different frequency. The red colour has smaller frequency than yellow, yellow has smaller frequency than green, green has smaller than blue, and blue has smaller than purple. The higher frequency of the light, more stronger the light is scattered.
The colour of the sky is a part of scattered sunlight. Because the scattered color is much from the high frequency (green, blue and purple), than the sky colour is a combining from that colours. Their mixing produce the light-blue. That makes our sky, coloured with blue. Near the source (the sun), the light isn't much scattered, so the colour around it is still pure, like the real colour of light.
In the evening, the sun and sky colour changes into red. That is happened because in the evening the sun change it's position to the sideways. Because of that the sunlight takes more length to reach our eyes, and the more sunlight is scattered. When the light reach our eyes, it's only contain the low frequency of colour (red), which has longer wavelength than the other colours. That's why the colour of the sky we seen in the evening is red.
In the moon and in the other planet that doesn't have an atmosphere, the sunlight will not scattered. So in that place, the sky is always dark. Although it is on the day.
