# What is the range of the zenith angle between velocity and position vectors?

I'm coding on a space simulator and I want to draw a predictable orbit for a satellite. I have read about orbits on http://www.braeunig.us/space/ and under the section "Launch of a Space Vehicle" they describe how I can obtain the orbital elements like eccentricity, true anomaly etc. given that you have the position vector, velocity vector and the zenith angle between them.

I have two questions that I need to understand:

1. What is the range of the zenith angle (0-180 degrees or 0-360 degrees etc.) ?
2. Can the angle tell which "half side" of the orbit the satellite is? I.e. if it's onwards to periapsis or apoapsis?

## 1 Answer

Partial answer:

What Braeunig calls gamma or zenith angle is actually beta!

According to @TomSpilker's answer

This is a problem that has plagued groups of people very knowledgeable about orbital dynamics but who learned using different textbooks: there are two different definitions of "flight path angle"!!

In addition to $$\gamma$$, the angle between the tangential direction and the velocity vector, there is $$\beta$$, the angle between the radial direction and the velocity vector. People often say "flight path angle" without saying which definition they're using. Confusing! (I just noticed that the diagram in Julio's answer also shows $$\beta$$)

If you work with $$\beta$$ instead of $$\gamma$$, $$\beta$$ is given by

$$\arccos\left(\frac{\mathbf{r \centerdot v}}{|\mathbf{r}| \ |\mathbf{v}|} \right) \tag{1}$$

So Braeunig's gamma is NASA's beta. It's positive from peri to apo, and negative from apo to peri. Because I'm not smart enough to do it with math, I'll do it with Python.

I'm doing this in dimensionless units, so the period is 2π and the semi-major axis is unity. You can see beta's behavior over one orbit, positive moving towards apoapsis, then negative moving towards periapsis.

def deriv (X, t):
x, v = X.reshape(2, -1)
acc = -x * ((x**2).sum())**-1.5
return np.hstack((v, acc))

import numpy as np
import matplotlib.pyplot as plt
from scipy.integrate import odeint as ODEint

halfpi, pi, twopi = [f*np.pi for f in (0.5, 1, 2)]
degs, rads = 180/pi, pi/180

time = np.linspace(0, twopi, 361)

r0 = 1.8
v0 = np.sqrt(2./r0 - 1)  # vis-viva

X0 = np.hstack([r0, 0, 0, v0])

answer, info = ODEint(deriv, X0, time, full_output=True)

x,    v      = answer.T.reshape(2, 2, -1)
rn = x/np.sqrt((x**2).sum(axis=0))
vn = v/np.sqrt((v**2).sum(axis=0))

x, y, vx, vy = answer.T

beta = np.arccos((rn*vn).sum(axis=0))

if True:
plt.figure()
plt.plot(x, y)
plt.plot([0], [0], 'ok', markersize=8)
x0, y0 = 0.5*(x.min() + x.max()), 0.5*(y.min() + y.max())
hwx, hwy = (np.abs(x-x0)).max(), (np.abs(y-y0)).max()
hw = 1.05*max(hwx, hwy)
plt.xlim(x0-hw, x0+hw)
plt.ylim(y0-hw, y0+hw)
plt.show()

if True:
plt.figure()
plt.subplot(3, 1, 1)
plt.plot(time, x)
plt.plot(time, y)
plt.plot(time, np.zeros_like(time), '-k')
plt.title('x and y vs time')
plt.subplot(3, 1, 2)
plt.plot(time, vx)
plt.plot(time, vy)
plt.plot(time, np.zeros_like(time), '-k')
plt.title('vx and vy vs time')
plt.subplot(3, 1, 3)
plt.plot(time, beta)
plt.plot(time, halfpi*np.ones_like(time), '-k')
plt.title('beta vs time')
plt.show()

• "It's positive from peri to apo, and negative from apo to peri.". I can't reproduce this when using NASA's beta. Only when I'm using NASA's gamma. Is it supposed to be true for both beta and gamma? – mrmclovin Dec 28 '18 at 20:30
• @mrmclovin okay I am brewing morning coffee now, will look as soon as I can get two neurons firing... – uhoh Dec 29 '18 at 0:20
• @mrmclovin apparently I never got the two neurons to fire properly, and lost track of this answer. It looks like I have do a little more here, taking a look now... – uhoh May 25 '19 at 22:54