system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How did humans discover that the earth tilts on it's axis?

How did we go from establishing that the earth is spherical to knowing that it tilts  and even figuring out the degree at which it tilts?
gpt
When sundials were first used, 5000 years ago, it was quite apparent that the gnomon casts a shadow in a different place in the summer than the winter.  By measuring the angle between the summer and winter value, and a little arithmetic, you can calculate the tilt angle.