MadSci Network: Physics

Re: Meteors and snowflakes seem to radiate from a central point. Why?

Date: Sun Apr 8 13:13:39 2001
Posted By: Harry Adam, Research Associate, Research Division, Kodak Limited
Area of science: Physics
ID: 985911587.Ph

Hi, Ernest – you asked why snowflakes appear to radiate from a point in 
front of you as you drive, but the effect disappears when you stop. This 
optical illusion is based on the property of perspective. Think about 
looking down a very long square corridor. As you know, perspective makes 
the walls converge – eventually to a point. But of course, the walls are 
actually parallel. If small lights are placed along the walls and floor 
and ceiling at even intervals they would appear to radiate from a point – 
but in this case, that effect occurs while you are stationary – you don’t 
have to move forward to see the effect. 
Now, in a snowstorm, the flakes are falling, but do so fairly slowly – 
compared say, to raindrops. If you are driving fast into them, they fall a 
lot more slowly than you take to reach them from first sighting them. 
Assume that the flakes are more or less evenly distributed – i.e. are 
equidistant from each other. Then imagine a small cell of snowflakes some 
distance in front of you. They will seem much closer to each other than 
they actually are, because of perspective. As you rapidly bear down on 
them they will appear to separate, as the distance between them becomes 
visually larger, again, due to perspective. This applies to all groups of 
flakes in your forward vision and so they all appear to radiate from your 
point of focus. If you stop the car, all you see is the flakes falling 
vertically – unlike a corridor there is no fixed structure to see 
converging to a point.  Meteor showers or stars flashing by at “Warp 
Speed” would behave just like snowdrops. The effect does not depend on 
persistence of vision.

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