sentences:  The period of oscillation of the nutation is 18.6 years.A Besselian epoch, named after German mathematician and astronomer Friedrich Bessel (1784–1846), is an epoch that is based on a "Besselian year" of 365.242198781 days, which is a tropical year measured at the point where the Sun's longitude is exactly 280°.Since 1984, Besselian equinoxes and epochs have been superseded by Julian equinoxes and epochs.