LearnMechanics & orbitsSome maths
Barycentre
The common centre of mass that two orbiting bodies both circle, rather than one circling the other.
Why it matters
It is how most exoplanets were found: the star wobbles around the barycentre it shares with its planets, and that wobble shows up as a periodic Doppler shift.
What people usually get wrong
The Moon orbits the Earth.
Both orbit their barycentre, which sits about 4,670 km from Earth's centre — inside the Earth, but well off-centre, so the Earth genuinely wobbles.
Leads to
Nothing here builds on it yet.