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When I lay down in my room at night it's pretty much completely dark but I observe this strange phenomenon going on.

When I lay down and look at my ceiling I can see my white fluorescent white bulbs and white fan blades but when it's dark I can't. But, as soon as I avert my eyes from the fan and lightbulbs about 1 foot I can see them in the dark but why? If it helps there is a very slight illumination from the street lights. And, also I can't see anything not white on the ceiling.

Why can I see the objects only when I avert my eyes?

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    $\begingroup$ The center of vision is less sensitive (but discriminates color). The periferal field is more sensitive (but only grayscale). Rods and cones, I never remember which ones. $\endgroup$ – Pieter Feb 24 '18 at 22:52
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    $\begingroup$ @Pieter "I never remember which ones" ::chuckles:: That's been a problem for me, too. But I've been teaching general-education astronomy the last few semester, so I've had to hammer it into my head. $\endgroup$ – dmckee --- ex-moderator kitten Feb 24 '18 at 22:54
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    $\begingroup$ C ones = C olour $\endgroup$ – Farcher Feb 24 '18 at 23:42
  • $\begingroup$ @Farcher thanks! I came up with "you can pack more stuff into rods, so rods should be dark-adapted, hence rods=no-color" $\endgroup$ – aaaaa says reinstate Monica Feb 25 '18 at 4:02
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This is really a biology topic, rather than a physics one.

The light sensing cells in your eyes come in two (or four depending on how you count) types. The cones are color sensitive. The rods are sensitive over almost the whole visible spectrum and offer no color discrimination, but they are considerably more sensitive than the cones.

The rods and cones are not evenly distributed in the eye. Cones are concentrated in the central part of the retina and rods are more common in your peripheral visual region. Which makes your peripheral vision more sensitive to very dim sources than your central vision.

In fact, naked-eye star-gazers learn to look near-but-not-at very dim observing target exactly to take advantage of this effect. But it take a lot of will-power to do at first. You'll find your self having a "Ah-ha!" reaction and then looking right at the target every time you make the trick work. At which point you have to go around again.

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