Why is the sky blue?

Look up on a sunny day: the sky is blue. But at sunset it turns orange and red and, at night, black. Why does it change color? The answer has to do with sunlight and the air around us.

White light hides colors

Sunlight looks white, but it is really made of all the colors of the rainbow mixed together: red, orange, yellow, green, blue, indigo and violet. Each color travels as waves and each has a different wavelength: blue is short and red is long.

Air scatters light

The atmosphere is full of tiny gas molecules. When sunlight enters it, it bumps into those molecules and scatters in all directions. Short waves, like blue, scatter much more than long ones, like red. This is called Rayleigh scattering.

Blue arrives from everywhere

Because blue scatters so much, it reaches our eyes from every part of the sky, not only from the Sun. That is why, wherever we look, the sky looks blue.

Why isn't it violet?

Violet scatters even more than blue, but the Sun gives off less violet light, the atmosphere absorbs some of it and our eyes are less sensitive to that color. What we perceive is blue.

The colors of sunset

At sunset, the Sun is near the horizon and its light travels a much longer path through the atmosphere. On the way almost all the blue scatters away and the long-wave colors reach us: oranges and reds. That is why sunsets are so colorful.

And on the Moon?

The Moon has almost no atmosphere, so there is no air to scatter light and its sky looks black even during the day.

Experiment: a sunset in a glass

Fill a clear glass with water and add a tiny drop of milk. In a dark room, shine a flashlight on the glass from the side. You will see bluish water from the side and orangey light when you look through the glass toward the flashlight: you have recreated a sky and a sunset.