Optical Illusions at Night

The human brain does a lot of things that we're not aware of. It takes shortcuts and most of the time that saves us time. However, occasionally the brain makes simple mistakes.

One of the shortcuts it likes to take is to judge distances by brightness. If our brain thinks two lights are equally bright, but one appears less bright to us, then the easiest solution is that the dimmer light must be further away. This shortcut works well, almost all of the time.

The problem comes when there happens to be a brighter light a bit further away than less bright one nearer. This trips our brain up and the further one can appear nearer to us.

This illusion can be a challenge for navigators at night. A lighthouse can appear much nearer than a buoy that is actually much closer. Fortunately, the solution is that we identify navigation lights by colour and flashing characteristics, not by brightness.

Incidentally, on land this shortcut is the reason that we often overestimate distances in poor visibility. Something glimpsed through mist is probably closer than we first think.


My thanks to Bob Ludlow for his email reminding me of this phenomenon and prompting me to add it to the website:

I've just finished "How to read water", and thoroughly enjoyed it. Thank you.

You mentioned in a couple of places how much we tend to see what we want or expect to see, rather than what is actually before us, and also described how difficult lights can be to resolve in a busy harbour. Tying these two points together, I always try to take my students out at night to a spot where there is a series of lateral lights to follow, IF ONLY to demonstrate how the brain wants to tell us that the brightest light is the closest! I'm sure that you've experienced that buoy hopping depending on light intensities can be disastrous!

All the best

Bob Ludlow



For those who enjoy cracking the code of navigation lights, see if you can decipher the picture above.


The easy bit: there is a green starboard light.

Red over White over Red - "a vessel restricted in her ability to manoeuvre" - it's dredging.

Either side of these are two bright deck working lights, just there to confuse things.

The white light 'in the distance' is actually only just behind this vessel. It appear further away because it is less bright than the lights on the boat.

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For a complete guide to Natural Navigation read Tristan’s books.