======Measure the Earth======


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**Alternative titles:** Eratosthenes Experiment
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====Summary====                   
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====Procedure====  
  - On a sunny day (ideally the equinox), place a straight stick vertically into flat ground.  
  - Measure the height of the stick.  
  - At local noon (when the Sun is highest), measure the length of the stick’s shadow.  
  - Use trigonometry to calculate the angle of the Sun above the horizon.  
  - Compare the angle measured at your location with data from another location at the same time (historically done between Alexandria and Syene by Eratosthenes).  
  - Use the difference in angles and the distance between the two locations to estimate the circumference of the Earth.



====Links==== 
Measure the Earth: An Easy, At-Home Science Experiment - Plateau Astro: 
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📄 The Eratosthenes experiment: calculating the Earth’s circumference - Science in School: [[https://scienceinschool.org/article/2023/calculating-earths-circumference/]]\\


====Variations====
  * Collaborate with another school in a different city or country to share shadow data.  
  * Try the experiment on different days to compare results with equinox measurements.  
  * Use different stick lengths to check for accuracy and consistency.



====Safety Precautions====
  * Do not look directly at the Sun during the experiment.  
  * Ensure the stick is firmly placed so it does not fall.  
  * Perform measurements in a safe, open area away from traffic.




====Questions to Consider====
  * Why is the equinox the best time to perform this experiment? (The Sun is positioned such that shadow angles are easier to compare between different locations.)  
  * What assumptions did Eratosthenes make about the Earth when performing his experiment? (That Earth is spherical and the Sun’s rays are parallel.)  
  * How accurate do you expect this method to be compared with modern measurements? (Eratosthenes’ original estimate was within a few percent of today’s accepted value.)  
  * What are some potential sources of error in this experiment? (Uneven ground, incorrect measurement of shadow length, imprecise timing of local noon.)