As the sun sets it changes color, sometimes first becoming orange, then red, for example.

The device typically used to measure color is a colorimeter, but all of the colorimeters I have seen only measure objects nearby or a physical sample like a swatch. How can we measure the color of distant objects, like the sun?

One idea I had is to use a telescope, then somehow attach a colorimeter to the eyepiece of the telescope, but am not sure if this is a feasible idea, hence the question.

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    $\begingroup$ Does your colorimeter have an internal light source that it shines on the object, or does it just measure ambient light reflected off the object? $\endgroup$ – John Rennie Aug 31 '17 at 15:44

I posted this before it occurred to me that your colorimeter might use an internal light source. If it does then the method I describe won't work.

I did this once during my PhD studies, though that was 35 years ago and I forget the details. My recollection is that we shone the light onto pressed sheets of titanium dioxide (at least I think it was $\text{TiO}_2$) then used the colorimeter to measure the colour of the light reflected from the titanium dioxide.

Titanium dioxide is white in reflected light because it has no absorption bands in the visible spectrum and its refractive index is constant to a good approximation across the visible spectrum. So the spectra obtained are the same as directly measuring the incident light. Unless you want ultra high accuracy you could simply use a piece of good quality paper.

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  • $\begingroup$ I am not asking about a particular colorimeter, but about the right technique to do the measurement. To your suggestion, the followup question would be how the light from the sun is "shone" onto reflector. Note that to get an accurate only the light directly from the sun can be allowed. If ambient light, such as from the sky is allowed to mix then it will contaminate the reading. So the problem is how do you isolate the light from the sun? $\endgroup$ – Ambrose Swasey Aug 31 '17 at 16:10
  • $\begingroup$ @AmbroseSwasey simply hold the paper normal to the light rays from the sun. Assuming there are no reflecting objects nearby the ambient light from the rest of the sky will be several orders of magnitude less intense. If you struggle to get a high enough intensity to measure then use a few mirrors to reflect the sunlight onto the paper. $\endgroup$ – John Rennie Aug 31 '17 at 16:15

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