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Length of an arc - when will a person leave your view?

Two people 1.8m tall walk from each other until they can no longer see each other (due to the curvature of the earth which has a radius of 6378km).
Assuming nothing else blocks their view, how far do they have to walk?

Note. I cant get my head around how this relates to what we've learned about radius and arc length.

Solution Preview

Let S be the distance between the two people such that after walking through a distance S, they won't be able to see each other.
<br>Let R = 6378Km = 6378000 m be the radius of the earth.
<br>Ler angle ACB = 2Theta (see figure)
<br>We know that,
<br>arc length / radius = angle in ...

Solution Summary

With a simple and easy to understand explanation. A figure is included for a better understanding.