The Spring Equinox 2018 is one point in time commonly used to determine the length of the tropical year. The mean tropical year is the average of all the tropical years measured from every point along the earth's orbit.[2] In 1983 the mean March equinox fell at 1.48 AM GMT on 23 March. When tropical year measurements from several successive years are compared, variations are found which are due to nutation, and to the planetary perturbations acting on the Sun. Meeus and Savoie (1992, p. 41) provided the following examples of intervals between northward equinoxes.
The point where the horizon crosses the sun's disk at the celestial equator northwards is called the first point of Aries. However, due to the precession of the equinoxes, this point is no longer in the constellation Aries, but rather in Pisces. By the year 2600 it will be in Aquarius (some Archeoastronomers and Astrologers believe that will be the start of the approximate 2,150 years of "the Age of Aquarius", while others think it may have already started, and varying calculations in between).