diff --git a/src/parameterizations.jl b/src/parameterizations.jl
index ff7eabc6..88a28221 100644
--- a/src/parameterizations.jl
+++ b/src/parameterizations.jl
@@ -26,16 +26,16 @@ function θ_at_epoch_to_tperi(system,planet,theta_epoch)
             (;a) = planet
         elseif hasproperty(planet, :P)
             (; P) = planet
-            a = ∛(system.M * b.P^2)
+            a = ∛(system.M * b.P^2) # TODO
         else
             error("Your planet must specify either i, Ω, ω, and a/P, or B, G, A, F.")
         end
 
-        (;a, i, Ω, ω) = planet
-        A = a*( cos(Ω)*cos(ω)-sin(Ω)*sin(ω)*cos(i))
-        B = a*( sin(Ω)*cos(ω)+cos(Ω)*sin(ω)*cos(i))
-        F = a*(-cos(Ω)*sin(ω)-sin(Ω)*cos(ω)*cos(i))
-        G = a*(-sin(Ω)*sin(ω)+cos(Ω)*cos(ω)*cos(i))
+        (;i, Ω, ω) = planet
+        A = ( cos(Ω)*cos(ω)-sin(Ω)*sin(ω)*cos(i))
+        B = ( sin(Ω)*cos(ω)+cos(Ω)*sin(ω)*cos(i))
+        F = (-cos(Ω)*sin(ω)-sin(Ω)*cos(ω)*cos(i))
+        G = (-sin(Ω)*sin(ω)+cos(Ω)*cos(ω)*cos(i))
     else
         error("Your planet must specify either i, Ω, ω, and a/P, or B, G, A, F.")
     end
@@ -66,3 +66,61 @@ function θ_at_epoch_to_tperi(system,planet,theta_epoch)
     tₚ = theta_epoch - MA/n*PlanetOrbits.year2day_julian
     return tₚ
 end
+
+function θ_sep_at_epoch_to_tperi_sma(system,planet,theta_epoch)
+    (;M,plx) = system
+    (;e,i, Ω, ω, θ, sep) = planet
+    A = ( cos(Ω)*cos(ω)-sin(Ω)*sin(ω)*cos(i))
+    B = ( sin(Ω)*cos(ω)+cos(Ω)*sin(ω)*cos(i))
+    F = (-cos(Ω)*sin(ω)-sin(Ω)*cos(ω)*cos(i))
+    G = (-sin(Ω)*sin(ω)+cos(Ω)*cos(ω)*cos(i))
+
+    # Thiele-Innes transformation matrix
+    T = @SArray [
+        A F
+        B G
+    ]
+    x_over_r, y_over_r = T \ @SArray[
+        cos(θ)
+        sin(θ)
+    ]
+    # Note: this is missing the radius factor but we don't need it for true anomaly.
+    # r = sqrt((sol.x*sol.elem.B+sol.y*sol.elem.G)^2 + (sol.x*sol.elem.A + sol.y*sol.elem.F)^2 )
+    #  sqrt(r^2* (x*B+y*G)^2 + r^2 * (x*A + y*F)^2)
+    #  sqrt(r^2* (x*B+y*G)^2 + r^2 * (x*A + y*F)^2)
+    # r * sqrt(r^2* (x*B+y*G)^2 + r^2 * (x*A + y*F)^2)
+    # r_unit = sqrt(
+    #     (x_over_r*B+y_over_r*G)^2 + (x_over_r*A + y_over_r*F)^2
+    # )
+
+
+    # True anomaly is now straightforward to calculate in the deprojected coordinate space
+    ν = atan(y_over_r,x_over_r)
+
+    # Mean anomaly (see Wikipedia page)
+    MA = atan(-sqrt(1-e^2)*sin(ν), -e-cos(ν))+π-e*sqrt(1-e^2)*sin(ν)/(1+e*cos(ν))
+
+    # r = a*(1 - e^2)/(1 + ecosν)
+    # r_unit = (1 - e^2)/(1 + e*cos(ν))
+    r_unit = (1 - e^2)/(1 + e*cos(ν))
+
+    # unitless projection vectors for case of a=1
+    xcart_unit = r_unit*(cos(ν+ω)*sin(Ω) + sin(ν+ω)*cos(i)*cos(Ω))
+    ycart_unit = r_unit*(cos(ν+ω)*cos(Ω) - sin(ν+ω)*cos(i)*sin(Ω))
+    zcart_unit = r_unit*(sin(ν+ω)*sin(i))
+
+    
+    # scale a until projected sep matches variable
+    dist = 1000/plx * PlanetOrbits.pc2au
+    cart2angle = PlanetOrbits.rad2as*1e3/dist
+    sep_unit = sqrt(xcart_unit^2 + ycart_unit^2)*cart2angle
+    a = sep/sep_unit
+
+    period_days = √(a^3/M)*PlanetOrbits.kepler_year_to_julian_day_conversion_factor
+    period_yrs = period_days/PlanetOrbits.year2day_julian
+    n = 2π/period_yrs # mean motion
+
+    # Get epoch of periastron passage
+    tₚ = theta_epoch - MA/n*PlanetOrbits.year2day_julian
+    return tₚ, a
+end