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! The Sun in Columbus Peaks at 12:45 PM EST on Feb 23rd Corrections: 1) Daylight Savings 2) Time Zones 3) Earth’s 23.5 o Axis Tilt 4) Earth’s Elliptical Orbit

Time Zones Trains Made Time Zones Necessary Divide Earth into o Time Zones Centered on Greenwich, UK 24x15 o = 360 o Mean Sun peaks at Noon Center of Zone New York Center EST Mansfield Western Edge EST Columbus in Wrong Time Zone 7.90 o or 33 minutes west of center Predict Sun Peaks at 12:33 EST or 1:33 EDT

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

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

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 DJFMAMJJASONDJFMAMJJASOND Month of Year Average Columbus Temperature Number of Daylight Hours

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

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.