A comet on a Kepler orbit sheds dust that feels sunlight pressure as a fraction beta of solar gravity. Small grains sweep far back and curve; the ion tail follows the solar wind. Looking down on the orbital plane.
The comet follows a Kepler ellipse of perihelion q and eccentricity e, semi-major axis a = q/(1 - e), period T = 2π√(a³/μ) with μ = GMsun = 2.959×10-4 AU³/day².
A dust grain feels solar gravity and the radiation force, both falling as 1/r². Their ratio β = Frad/Fgrav ≈ 5.7×10-4 Qpr/(ρg s) (density ρg in g/cm³, radius s in cm) depends on grain size, so after release the grain moves on an orbit around the Sun with the reduced strength μ(1 - β). Grains with β > 0.5 from a near-parabolic comet end up unbound. The locus of grains of one β released at different times is a syndyne; the locus of grains of all β released at one time is a synchrone. This page integrates every grain with a leapfrog scheme and draws syndynes through tracer grains.
The ion tail is made of plasma carried by the solar wind. Seen from the comet, the wind arrives with velocity vsw r̂ - vc, so the tail points along that vector; its aberration from the anti-solar line is ≈ atan(vc,⊥/vsw), a few degrees.
Simplifications: grains are non-interacting and do not fragment or sublimate, β is constant per grain, the ejection velocity is small and random, planetary perturbations and gas drag are ignored, and the ion tail is drawn as a straight guide.