code: function eq2 (year, planet, lon, c) {
    let J0 = base.horner(year * 0.001, c)
  
    for (;;) {
      const a = solar.apparentVSOP87(planet, J0)
      const c = 58 * sin(lon - a.lon) // (27.1) p. 180
      J0 += c
      if (abs(c) < 0.000005) {
        break
      }
    }
  
    return J0
  }
docs: Accurate calculation of solstices/ equinoxes
  Result is accurate to one second of time.
  
  @param {Number} year - (int) year
  @param {planetposition.Planet} planet - vsop87 planet
  @param {Number} lon - longitude in radians
  @param {Array} c - term from table 27.a / 27.b
  @returns {Number} JDE