code: function pa (Δ, R, cψ) {
    const sψ = Math.sin(Math.acos(cψ))
    let i = Math.atan(R * sψ / (Δ - R * cψ))
    if (i < 0) {
      i += Math.PI
    }
    return i
  }
docs: phase angle from cos elongation and distances
  @private
  @param {number} Δ
  @param {number} R
  @param {number} cψ
  @returns {number}