= Stellar obliquity
{title2=$\psi$}
The stellar obliquity is the angle between a star's spin and a planet's orbital angular momentum. The <Rossiter-McLaughlin effect> constrains the angle $\lambda$ between the sky projections of those two axes, rather than a component of $\psi$. If the orbital and stellar-spin inclinations to the line of sight are $i_{\rm orb}$ and $i_*$, then $\cos\psi=\cos i_*\cos i_{\rm orb}+\sin i_*\sin i_{\rm orb}\cos\lambda$; $\lambda$ alone generally does not determine $\psi$. Large misalignments constrain formation and <planetary migration>, but can also reflect a tilted birth disc.
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