Rcjp's Weblog

April 12, 2007

Circular Halo

Filed under: physics — rcjp @ 12:53 pm

circular-halo1

I hardly noticed this without my Polaroid sunglasses on, as the colours were quite faint (and I’m fairly colour-blind), but the camera seemed to pick it out reasonably well; though its not as impressive as this one.

I tried to estimate the angular measurement of the halo and got my measurements wrong by quite a bit. From the sun to the start of the arc was the distance from my thumb to little finger tips with my hand held spread out at arms length. That’s 2 feet away from my eye and 8 inches across (I’ve measure that before and its easy to remember – all nice round numbers; no metric system centimeters here for estimating thank you – those units are too small.) This gives the halo an angular radius of about… (using a handy Common Lisp prompt)


    CL-USER> (* (/ 180 pi) (atan 8/24))
    18.434948654789338d0 

that’s nowhere near the 22 degrees its supposed to be. But of course, holding your arm up at an angle reduces the distance between your eye and your hand that I’d measured before with my arm out horizontally. Re-measuring at around 45 degrees the hand-eye distance is down to 20″ and directly overhead 16″:


    CL-USER> (* (/ 180 pi) (atan 8/20))
    21.801408877810637d0 

much better, phew.

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