Question: why is the sun yellow and while the moon is white ?

Ian Stephenson answered on 20 Jun 2014:

There’s a whole bunch of things in that question, which all have some kind of effect.

Nuclear fusion in the sun produces a whole load of energy. Some of that gets emitted as light. Light is made up of many frequencies, and the exact mix if frequencies gives us the colour that we see.

The sun does emit light which has more energy in the yellow part of the spectrum than some other light sources, such as fluorescent light builds which emit a lot of blue light.

However what we call white is REALLY subjective. If you take a white sheet of paper outside, the it will look white. However its reflecting the sun, so its actually got the same spectrum as the sun has. If you take the same piece of paper indoors the light coming off it will change because now its reflecting light from a light bulb. However we would still call that white. Our brain adjusts what we call white so that white is always the colour of the light from the dominant light source in the scene.

So when you’re outside the sun really is white. but it you’re indoors looking out its yellow. It’s this indirect colour which makes you say the sun is yellow – you’ve probably never looked directly at the sun very much, and you shouldn’t as its really bad for your eyes. The moon reflects the sun, and is neutral in colour so its white too. However as its not as bright we can observe it directly.

To confuse things a bit more, at night, when you’re most likely to see the moon you’re using a different part of your eyes to see with. Your eyes have Cones which are good at colour vision, but only work in daylight, and Rods which can only see brightness, but work much better in low light, so at night you can’t see colours anyway.

Colour is a really complex subject, as it mixes physics, biology, and psychology. Our brains make a lot of assumptions about colour, and it doesn’t help that a lot of what gets taught about colour isn’t really true – for example you’ve probably been taught there are three primarily colours, which is a bit of a simplification: after all if you can mix yellow and blue to make green, why do computer screens have red, green and blue pixels rather than red, yellow and blue?