code: function meanphase (sdate, k) {
    // Time in Julian centuries from 1900 January 12 noon UTC
    const delta_t = (sdate - -2208945600000.0) / 86400000.0
    const t = delta_t / 36525
    return 2415020.75933 +
      SYNODIC_MONTH * k +
      (0.0001178 - 0.000000155 * t) * t * t +
      0.00033 * dsin(166.56 + (132.87 - 0.009173 * t) * t)
  }
docs: Calculates time of the mean new Moon for a given base date.
  This argument K to this function is the precomputed synodic month
  index, given by:
  K = (year - 1900) * 12.3685
  where year is expressed as a year and fractional year.
  @param  {Date} sdate   Start date
  @param  {[type]} k     [description]
  @return {[type]}       [description]