Thefirst point of Aries, also known as thecusp of Aries or thevernal point, is the location of theMarchequinox (thevernal equinox in the northern hemisphere, and theautumnal equinox in the southern), used as a reference point incelestial coordinate systems. In diagrams using such coordinate systems, it is often indicated with the symbol ♈︎. Named for theconstellation ofAries, it is one of the two points on thecelestial sphere at which thecelestial equator crosses theecliptic, the other being thefirst point of Libra, located exactly 180° from it. Due toprecession of the equinoxes since the positions were originally named in antiquity, theposition of the Sun when at the March equinox is now inPisces; when it is at theSeptember equinox, it is inVirgo (as ofJ2000).
Along its yearly path through thezodiac, theSun meets the celestial equator as it travels 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 whichright ascension is calculated.
The choice of starting position from which to measure the Sun's motion across the 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 ofHilaria, a time of optimism and beginnings where farmers began to sow or observed the first growth and blossoming of trees and summer crops.[1] The naming ofAries is late in theBabylonian zodiac where the equinox was in its earliest tradition marked as in the earlyMiddle Bronze Age by actual coincidence with thePleiades.[2] The time also corresponds to the time ofcastration of male calves, mules and donkeys,Sanguia on the vernal equinox and marked the start of spring proper.[3][4]
The first point of Aries is so called because, whenHipparchus defined it in 130 BCE, it was located in the western extreme of the constellation of Aries, near its border withPisces and the starγ Arietis. Due to theSun's eastward movement across the sky throughout the year, this western end of Aries was the point at which the Sun entered the constellation, hence the namefirst point of Aries.[citation needed]
Due to Earth'saxial precession, this point gradually moves westwards at a rate of about one degree every 72 years. This means that, since the time of Hipparchus, it has shifted across the sky by about 30°, and is currently located within Pisces, near its border withAquarius. The Sun now appears in Aries from late April until mid-May, though the constellation is still associated with the beginning of the northern spring.[3][4][5]
The first point of Aries is important to the fields of astronomy, nautical navigation and astrology.Navigational ephemeris tables record the geographic position of the first point of Aries as the reference for position ofnavigational stars. Due to the slowprecession of the equinoxes, thezenith view (above a location) of constellations at a time of year from a given location have slowly moved west (by using solarepochs the drift is known). Thetropical Zodiac is similarly affected and no longer corresponds with theconstellations (the Cusp of Libra today is located withinVirgo). Insidereal astrology, by contrast, the first point of Aries remains aligned with the Aries constellation.[citation needed]
