vernal equinox

Vernal equinox

The Vernal equinox is the equinox on the earth when the Sun appears to cross the celestial equator, heading northward. The Vernal equinox is the vernal equinox in the Northern Hemisphere and the autumnal equinox in the Southern Hemisphere.

The Vernal equinox can be as early as March 19 or as late as March 22, the precise time being about 5 hours 49 minutes later in a common year, and about 18 hours 11 minutes earlier in a leap year, than in the previous year. It is the balance of common years and leap years that keeps the calendar date of the equinox from drifting more than a day from March 20 each year.

vernal equinox
Vernal equinox: Google Doodle

 

Northward equinox solar year

The March equinox is one point in time commonly used to determine the length of the tropical year. The length of Northward equinox solar year is relatively stable in the time from 6000 BCE to 10000 CE at 47:37 to 49:20 in excess of 365 days and 5 hours.

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:

Constellation

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).

The Northward equinox passed from Taurus into Aries in year −1865, passed into Pisces in year −67, will pass into Aquarius in year 2597, will pass into Capricornus in year 4312. It passed along (but not into) a ‘corner’ of Cetus on 0°10′ distance in year 1489.

Movement of the horizon in relation to the Sun

At the equinox, the sun’s disk crosses the horizon directly in the east at dawn and crosses directly in the west at dusk. However, because of refraction the sun will usually appear slightly above the horizon when its “true” middle is rising or setting. For viewers at the north or south poles, the sun moves steadily just above the horizon, not obviously rising or sinking apart from the movement in “declination” (and hence elevation) of a little under a half (0.39) degree per day.

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