system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did Galileo manage to calculate the moon's mountains' height ?
gpt
Put a stick in the ground.  
As the day goes by the sun rises and sets.  
Now watch the shadows the stick casts. 
As the day goes by the shadows change.  
If you know the angle the sun is at , you can use that to calculate how tall the stick is from the changes in the shadow.  
  
Galileo did that with the mountains.  
As the month goes on the angle the sun is shining on the moon changes.  
Galileo knew that angle.
He then tracked how the shadows of the mountains and craters changed and calculated their relative height.