Operation Chou Li Episode II

Over 3,000 years ago a Chinese astronomer name Chou Li was the first to measured the accurate Earth's axis tilt. 3,000 years later his methodology is repeated with an ancient tool in Sakon Nakhon Province northeast of Thailand

 

 

Find the Earth's axis tilt with one stick

 

Location of Sakon Nakhon Province northeast of Thailand about 600 Km from Bangkok 

 

We know that Earth orbiting the sun with approximately 23.5 degree tilt but my question is how can you proof it by yourself 

 

       How to do

          Today it is very easy to find out the angle of Earth's axis tilt with the help of Google we all know that it is about 23.5 but my question is without any texts and internet How-Can-You-Do-It ?

           Looking back to the ancient days our ancesters did not have any sophisticated tools ..... wondering how they found the accurate Earth's axis tilt with the help of simple tools and repeated several astronomical observations.

           As an astronomy teacher in English Program at Tad Naraiwittaya School Sakon Nakhon Province I asked my students ..... can you measure Earth's axis tilt with one stick? ...... forget your internet and all of the modernized tools and get back to 3,000 years old scenario here you are

 

           Step 1

           Apply Pythagoras theorem, the Earth's axis tilt can be measured by this equation

 

To determine the Earth's axis tilt at your local latitude

 

 

To find out angle of your local latitude, therefor the angle of incidence at solar noon on equinox and angle of local latitude are the same

         

            Step 2

            With a stick find the angle of incidence at solar noon on summer solstice 21 June 2017 at Tad Naraiwittaya School Amphur Muang, Sakon Nakhon Province

 

Find the sun's shadow at solar noon (sun is in the highest point) 

 

  

Measure the lenght of shadow (casted by the stick) at solar noon 

 

 

Calculate the angle of incidence by tangent and here we got 6.37 

 

 

          Step 3

          Find angle of incidence at solar noon on Autumnal Equinox 23 September 2017 at Amphur Muang Sakon Nakhon by using shadow plot with a 12 Cm stick 

 

Shadow path from morning to afternoon 

 

Shadow's lenght at solar noon is the shortest line among all of the shadow path

 

 Shadow's path on autumanl equinox is exactly straight line for the whole day

 

W - E is shadow's path on autumnal equinox and N - S is alighment of solar noon 

 

Measure the shadow's lenght of solar noon

 

we got shadow's lenght = 3.7 Cm

 

 

To find the angle of incidence at solar noon on autumnal equinox 

 

          Step 4

          From the Pythagoras theorem

Earth's axis tilt = angle of incidence on summer solstice + angle of incidence on autumnal equinox

 

 

Conclusion

Earth's axis tilt from Operation Chou Li = 23.506

Actual Earth's axis tilt = 23.43698

Error = +0.069