code: function eq (y, c) {
    const J0 = base.horner(y * 0.001, c)
    const T = base.J2000Century(J0)
    const W = 35999.373 * D2R * T - 2.47 * D2R
    const Δλ = 1 + 0.0334 * cos(W) + 0.0007 * cos(2 * W)
    let S = 0
    for (let i = terms.length - 1; i >= 0; i--) {
      const t = terms[i]
      S += t.a * cos((t.b + t.c * T) * D2R)
    }
    return J0 + 0.00001 * S / Δλ
  }
docs: Fast calculation of solstices/ equinoxes
  Accuracy is within one minute of time for the years 1951-2050.
  
  @param {Number} y - (int) year
  @param {Array} c - term from table 27.a / 27.b
  @returns {Number} JDE