First point of Aries

The first point of Aries, also known as the cusp of Aries, is the location of the vernal equinox (March equinox), used as a reference point in celestial coordinate systems. In diagrams using such coordinate systems, it is often indicated with the symbol ♈︎. Named for the constellation of Aries, it is one of the two points on the celestial sphere at which the celestial equator crosses the ecliptic, the other being the first point of Libra, located exactly 180° from it. Due to precession of the equinoxes since the positions were originally named in antiquity, the position of the Sun when at the vernal equinox is now in Pisces; when it is at the Autumnal equinox (September equinox), it is in Virgo (as of J2000). Along its yearly path through the zodiac, the Sun meets the celestial equator from south to north at the first point of Aries, and from north to south at the first point of Libra. The first point of Aries is considered to be the celestial "prime meridian" from which right ascension is calculated. The choice of starting position from which to measure the Sun's motion across celestial sphere is arbitrary. The equinoxes are preferred as an equinox marks the point in time when the Sun has neither northern nor southern declination but is crossing the celestial equator. Of the two possible equinoxes the ancient Greeks chose the March equinox as the starting point. This coincided with the festival of Hilaria, a time of optimism and beginnings where farmers began to sow or observed the first growth and blossoming of trees and summer crops. The naming of Aries is late in the Babylonian zodiac where the equinox was in its earliest tradition marked as in the early Middle Bronze Age by actual coincidence with the Pleiades. The time also corresponds to the time of castration of male calves, mules and donkeys, Sanguia on the vernal equinox and marked the start of spring proper. The first point of Aries is so called because, when Hipparchus defined it in 130 BCE, it was located in the western extreme of the constellation of Aries, near its border with Pisces and the star γ Arietis.

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March equinox

The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the vernal equinox (spring equinox) in the Northern Hemisphere and as the autumnal equinox in the Southern Hemisphere. On the Gregorian calendar, the northward equinox can occur as early as 19 March or as late as 21 March at 0° longitude.

Plane of reference

In celestial mechanics, the plane of reference (or reference plane) is the plane used to define orbital elements (positions). The two main orbital elements that are measured with respect to the plane of reference are the inclination and the longitude of the ascending node. Depending on the type of body being described, there are four different kinds of reference planes that are typically used: The ecliptic or invariable plane for planets, asteroids, comets, etc.

First point of Aries

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