Science Questions

Why do comets look green in photos?

Mon, 20th Apr 2015

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Tad Davison asked:

Just been watching the BBC's monthly 'Sky at Night' programme on iPlayer and they were talking about a comet that is now visible with a low-powered telescope or even binoculars. One of the show's presenters helpfully gave us some tips on how to take photographic images of the comet, but also stated that the photos showed the comet's trailing gasses to have a green tint, whereas the naked eye looking through a telescope can't pick that up.


We put Tad's question to astronomer Carolin Crawford...comet mcnaught

Carolin - Again, this is such a good question. You look at these wonderful pictures in coffee table astronomy books and you have these multi-coloured swirls, nebulae, comets. They all look fantastically coloured. But when you look at them through the telescope with your naked eye, itís often disappointing - they're sort of grey and whitish. The problem is not with the comet or the nebula. Itís with your eye, because the part of the eye that detects faint diffuse substances, these are the rods. They're around the outside of your eye. They're not colour-sensitive. Itís the cones at the centre of your eye that are colour-sensitive. And so, if you can see something faint and fuzzy through a telescope, you won't be picking up the colour. And thatís why the camera of course doesnít have any of these problems, it picks up all the colours that there are. So itís to do with your eye rather than anything to do with the comet.

Kat - Great question.

Chris - I have noticed that myself when you look at the night sky. If you look out of the corner of your eye the stars are much brighter, and then you look at them and they seem to almost disappear. Especially if itís a star thatís a little bit dim in the first place. You can see the star and you look at it and then itís gone. Thatís why, because you're then focusing it on the centre of your retina where youíve got these cones which are pretty good at seeing text on a bright screen, but they're not very good at seeing dark things.

Carolin - Yeah, you're exactly right.



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