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Skywatching

Can we believe our eyes?

In some way or other, our eyes have been strongly involved in almost every scientific discovery we have ever made. 

Despite this they are pretty awful measuring instruments. They are not good at judging colour, angles, brightness or sizes. The only way we can measure with them is to compare what we see with calibration standards. Of course, they are not intended to do that. Our eye/brain combination is supposed to give us optical information about our environments under a wide range of light levels and light colour balances.

We can get a good indication of what our eyes can do on these dull, grey winter afternoons. We can work outside, seeing quite well, and it is only when we go into our well-lit houses, and then look out through the windows, we see it is really dark out there. Our eyes can adapt to large differences in light level. 

On those same afternoons, when we are sitting in our warm houses, lit by incandescent lights, our rooms look comfortably lit but the outside world looks bluish. 

Then, after spending some time outside on those afternoons, shovelling snow or doing some other activity, the light outdoors looks perfectly normal and the light coming out of the windows looks yellow. This is because the light outside is more white, and our eyes adapt to that. 

Then, inside, the light from our incandescent light bulbs is mainly yellow, without much red or blue, so our eyes and brains compensate for that. 

No matter what colour the light, for most of our history, our survival depended on being able to see objects of all colours.

We have trouble judging sizes, too. During a long-ago visit to the Ontario Science Centre, I saw an exhibit where one looked through a window into a room. That room was badly distorted, with one corner maybe 1.5 metres high and the opposite corner maybe three or even four metres high. However, firmly programmed in my brain, and almost certainly yours too, is an idea of what rooms are shaped like, and that is what we see. I saw a normal room, the usual shape. My brain was so keen on rationalizing the shape of the room it was happy to accept that the toy soldier trundling back and forth across the room was changing between maybe one and three metres high! 

One of these illusions that we can see is how large the moon looks when rising or setting, compared with how small it looks when high in the sky. This is not due to refraction or some other atmospheric process. It is something our brain is up to. You can check this by looking at the moon through a drinking straw, where you can only see the moon and nothing much around it. Compare the size of the moon with the disc of sky you can see at moonrise and when the moon is high in the sky. It is not changing size. 

Seeing the horizon with the moon close to it makes our brains change the way they process the image. This could be because through most of our history, threats have come over the horizon much more often than they have come from above, so our brains apply more processing to the areas of potential interest or threat. 

A similar thing seems to happen when things in the sky are very bright. Often people report bright objects in the sky, satellites, planets and so on, and describe them as “big” – although they are not big, they are bright. Once again our brains are up to something.

On a starry night where there are a few clouds, take a long look at the stars. After a while, they seem as though they are moving. It's quite a convincing effect, even though we know that is not happening. This is why in most scientific research we prefer to use our eyes to read a dial, showing a measurement made using a brainless instrument.

  • Venus lies low in the southwest after sunset and Mars rises in the early hours, Jupiter a bit later, and Saturn low in the dawn glow. 
  • Mercury might be visible low in the southeast just before sunrise. 
  • The noon will be new on the 24th.


More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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