Is the sun yellow?

Every four year old knows that the sun is yellow. Ask one to draw a house and there, smiling down on the four windows, door and smoking chimney is the sun, rendered in golden hues with a chipper expression on his face. It’s a good thing we don’t trust four year olds with major policy decisions because not only does the sun not have an expression — happy or otherwise — but it isn’t really yellow.

If you rely on the evidence of your eyes you may agree with the preschoolers. From down here, at the bottom of the thick atmospheric fug, the sun does look yellow, but looks can be deceiving. It’s often hard to pin down the colour of even everyday objects. We’ve all seen clothes that look one colour in daylight and another in the harsh glare of fluorescent lights. The perceived colour of an object depends not just on the object itself but also the viewing conditions. The sun is no different.

Clothes change colour because perceived colour is a combination of pigment and the colour of the illuminating light. Though the sun’s light comes from within rather than being reflected from some other source the viewing conditions can still alter our perception of its colour.

The sun glows with a white heat but looks yellow because of the atmosphere. Different coloured light is scattered differently in the air. Blue light is scattered a lot, and so ends up spread far from the direct beam of sunlight all over the sky. Red light, by contrast, is relatively unaffected by its journey through the air and streams straight down. The result is that most of the sky looks blue, while the sun’s disc turns a yellow orange. This is most pronounced at sunset and sunrise when the sun is a much deeper colour than at midday.

In space the sun looks white — perhaps a mere mention of yellow but nothing like we see from down here on the surface of the earth. Despite appearances and rumours to the contrary, the sun just ain’t yeller, y’hear?

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