Dust-tail angular lag
= Dust-tail angular lag
{title2=$\Delta\theta=\theta_d-\theta_p$}
For unwrapped grain and parent azimuths, define a signed lag $\Delta\theta=\theta_d-\theta_p$ and a positive trailing distance $s=-\Delta\theta$. A grain released without a velocity kick from a circular parent orbit has $\Delta\theta=2\beta(\sin\theta_d-\theta_d)+O(\beta^2\theta_d)$. Keeping track of this sign avoids mistaking trailing motion for leading motion.