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Sun Dials

Sun Dials. What time is it ?. Sun was only way to tell time until ~1000AD in the West (Chinese had water clocks). Garden Sun Dials - old fashion - frumpy and they don’t work !. Why don’t Garden Sundials Work ?. The Sun reaches its Zenith (Peak) at High Noon … NOT!.

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Sun Dials

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  1. Sun Dials What time is it ? Sun was only way to tell time until ~1000AD in the West (Chinese had water clocks) Garden Sun Dials - old fashion - frumpy and they don’t work ! Why don’t Garden Sundials Work ?

  2. The Sun reaches its Zenith (Peak) at High Noon… NOT! The Sun in Columbus Peaks at 12:45 PM EST on Feb 23rd Corrections: 1) Daylight Savings 2) Time Zones 3) Earth’s 23.5o Axis Tilt 4) Earth’s Elliptical Orbit

  3. Time Zones Trains Made Time Zones Necessary Divide Earth into 24 15o Time Zones Centered on Greenwich, UK 24x15o =360oMean Sun peaks at Noon Center of Zone New York Center EST Mansfield Western Edge EST Columbus in Wrong Time Zone 7.90o or 33 minutes west of center Predict Sun Peaks at 12:33 EST or 1:33 EDT

  4. 82o 30’ Western Edge Theoretical EST But we are still 12 minutes off ! With time zone: 12:33 EST Actual Feb 23 12:45 EST

  5. Analemma Take a snapshot of the sun every day at 12:33 (Don’t Actually Do This! Can Cause Eye Damage) Time Sun Reaches Zenith Varies by 31 minutes Earth’s 23.5o Axis Tilt Earth’s Elliptical Orbit Earth’s 23.5o Axis Tilt

  6. Earth’s Axis is Tilted - length of day varies - sun sets far to south in winter and far to the north in summer - sun is low in sky in winter and high in sky in summer - sun reaches its zenith as slightly different times 80 15 70 14 60 13 50 12 40 11 30 10 20 9 Average Columbus Temperature Number of Daylight Hours DJFMAMJJASONDJFMAMJJASOND Month of Year

  7. Earth’s Elliptical Orbit On Jan 3rd (perihelion) is 3 % closer to the sun than it is 6 months later When the earth is closer to the sun it move faster than when it is far away Thus the Sun seem to speed up and slow as it crosses the sky The Analemma is Explained

  8. One can Calculate a Perfect Sundial for any Give Date and City Summer Solstice Columbus Winter Solstice These sundials are very different. At best a garden sundial is a compromise! Let’s go outside and make chalk sundials accurate to better than 5 minutes.

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