Why is it easier to see a star if you look slightly to the side? (Intermediate)

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Details: Monday, 16 February 2004 11:48; Answered by Christopher Springob; Views: 69904

I'm just a curious person who has never been able to find an answer to this question: Why is it sometimes easier to see a star with the naked eye if you look slightly to the side of it rather than directly at it? Is there a name for this effect?

There are two kinds of light-detecting cells in the retina of your eyes: "rods" and "cones". The cones are designed for high light-level observing, while the rods are designed for low light-level observing. The cones are more concentrated in the center of your eye, while the rods are somewhat more spread out. So by looking just off to the side, you rely more on your rods, which allows you to see dimmer objects better. This effect is called "averted vision".

Exactly how far off-center you should look to maximize this effect varies from person to person. For most people, it's between 5 and 20 degrees away from looking straight ahead.

This page updated on June 27, 2015

About the Author

Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies. He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.