code: def transitDurationCircular(P, R_s, R_p, a, i):
      r
  
      if i is nan:
          i = 90 * aq.deg
  
      i = i.rescale(aq.rad)
      k = R_p / R_s  # lit reference for eclipsing binaries
      b = (a * cos(i)) / R_s
  
      duration = (P / pi) * arcsin(((R_s * sqrt((1 + k) **
                                                2 - b ** 2)) / (a * sin(i))).simplified)
  
      return duration.rescale(aq.min)
docs: r"""Estimation of the primary transit time. Assumes a circular orbit.
  
      .. math::
          T_\text{dur} = \frac{P}{\pi}\sin^{-1}
          \left[\frac{R_\star}{a}\frac{\sqrt{(1+k)^2 + b^2}}{\sin{a}} \right]
  
      Where :math:`T_\text{dur}` transit duration, P orbital period,
      :math:`R_\star` radius of the star, a is the semi-major axis,
      k is :math:`\frac{R_p}{R_s}`, b is :math:`\frac{a}{R_*} \cos{i}`
  
      (Seager & Mallen-Ornelas 2003)