# Rotated coordinates on a unit sphere

Given three points on a unit sphere, their coordinates are $$p_1 = [\phi_1, \theta_1]$$, $$p_2 = [\phi_2, \theta_2]$$, and $$p_3 = [\phi_3, \theta_3]$$, where $$\phi$$ and $$\theta$$ are azimuthal angle and polar angle, respectively.

Rotate about the sphere center so that $$p_1$$ locates at $$[0, 0]$$ and $$p_2$$ at $$[0, l]$$ (obviously, $$l$$ equals to the great-arc distance between $$p_1$$ and $$p_2$$).

On the rotated sphere, what is the new coordinates $$[\phi, \theta]$$ for $$p_3$$?

• Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer.
– Community Bot
Nov 23 at 22:38

Here is a brute force method to find the new coordinates of $$p_3$$.

Assuming that $$\phi$$ is the azimuthal angle measured counter clockwise from the positive $$x$$ axis, and $$\theta$$ is the polar angle measured from the positive $$z$$ axis, we can first convert the spherical coordinates into rectangular coordinates as follows:

$$p_1 = \begin{bmatrix} \sin \theta_1 \cos \phi_1 \\ \sin \theta_1 \sin \phi_1 \\ \cos \theta_1 \end{bmatrix} \hspace{40pt} p_2 = \begin{bmatrix} \sin \theta_2 \cos \phi_2 \\ \sin \theta_2 \sin \phi_2 \\ \cos \theta_2 \end{bmatrix} \hspace{40pt}p_3 = \begin{bmatrix} \sin \theta_3 \cos \phi_3 \\ \sin \theta_3 \sin \phi_3 \\ \cos \theta_3 \end{bmatrix}$$

In addition, let vectors $$q_1$$ and $$q_2$$ be defined as follows:

$$q_1 = \begin{bmatrix} 0\\ 0 \\ 1\end{bmatrix} \hspace{40pt} q_2 = \begin{bmatrix} \sin \psi \\ 0\\ \cos \psi \end{bmatrix}$$

where the angle $$\psi = \cos^{-1} \left(p_1 \cdot p_2\right)$$

Now consider the rotation that takes $$p_1$$ and sends it to $$q_1$$ and takes $$p_2$$ and sends it to $$q_2$$, this can expressed as

$$q_1 = R p_1, \hspace{40pt} q_2 = R p_2$$

We need a third vector, so we'll take the cross product and write

$$q_1 \times q_2 = R ( p_1 \times p_2 )$$

Hence, we now have the matrix equation,

$$\begin{bmatrix} q_1 , q_2 , q_1 \times q_2 \end{bmatrix} = R \begin{bmatrix} p_1 , p_2 , p_1 \times p_2 \end{bmatrix}$$

which is of the form

$$Q = R P$$

From which it follows that

$$R = Q P^{-1}$$

Finally, apply this rotation to $$p_3$$ to obtain $$q_3$$

$$q_3 = R p_3 = \begin{bmatrix} \sin \theta \cos \phi \\ \sin \theta \sin \theta \\ \cos \theta \end{bmatrix}$$

The new angles $$\theta$$ and $$\phi$$ can be found from the coordinates of $$q_3$$ as follows

$$\theta = \cos^{-1} q_{3z}$$

$$\phi = \text{ATAN2} ( q_{3x} , q_{3y} )$$

• Thanks. I've edited the solution to clarify the angles. Nov 25 at 14:18
• @DavidK I've re-edited the solution to use the standard convention of angles. Nov 26 at 11:48
• Yes, you're right. I'll change that shortly. Nov 26 at 14:21
• Already upvoted. I think this matches the question exactly now. Nov 26 at 14:52