code: function cosEq (α, δ, α0, δ0) {
    const [sδ, cδ] = base.sincos(δ)
    const [sδ0, cδ0] = base.sincos(δ0)
    return sδ0 * sδ + cδ0 * cδ * Math.cos(α0 - α)
  }
docs: cos elongation from equatorial coordinates
  @private