# Tag Info

Since you have all three positions in the same frame (in this case ECI but it doesn't matter which one for the purposes of your question), you calculate the angle the same way I did in this answer. The cosine of the angle between any two unit vectors $\mathbf{\hat{a}}$ and $\mathbf{\hat{b}}$ is their dot product:  \cos(\theta)=\mathbf{\hat{a}} \cdot \...