Sample script

import numpy as np
import matplotlib
import matplotlib.pyplot as plt

r = 10.;
h = 2.;

x = np.linspace(-r, +r, 1000)
xlarge = np.linspace(-r * 1.5, r*1.5, 1000)

u = np.sqrt( r**2 - x**2)
l = -u

a = np.sqrt( (1+h/r)**2 - 1.);
right = r + h - a * xlarge;
left  = r + h + a * xlarge;

plt.fill(x, u, 'b', alpha=0.3)
plt.fill(x, l, 'b', alpha=0.3)
plt.plot(xlarge, right, color='g');
plt.plot(xlarge, left , color='g');
plt.xlabel("x")
plt.ylabel("y")
plt.title("Horizon; what is it?")
plt.xlim(-r * 1.5, r*1.5)
plt.ylim(-r * 1.5, r*1.5)
plt.show()