I was watching a Fehnman lecture on YouTube, where he used Kepler's second law as an example of something he was explaining.
He was showing geometrically why a line joining a planet and a Sun sweeps out equal areas during equal intervals of time.
I followed his explanation for when the Sun has no pull on the planet. Then, I'm just solving the area of obtuse angles.
His explanation begins here and he loses me when he says that the triangular area swept out when the Sun has a force has the same height.
I understand that both triangles have the same base, but I don't understand how to see that they're the same height.
His explanation was, "...and do they have the same altitude? Sure, because they're included between parallel lines and so they have the same altitude."
That doesn't explain it for me, and so can someone show me how it's done?