First Point of Aries

The First Point of Aries defines the coordinate (0°, 0°). It on is the celestial equator at both the left and right extremes of the celestial chart, with the ecliptic (the orange dotted sine curve) passing through it.

The First Point of Aries is the location of the vernal equinox, and is named for the constellation of Aries. It is one of the two points on the celestial sphere at which the celestial equator meets the ecliptic plane, the other being the First Point of Libra, located exactly 180° from it. Over its year-long journey through the constellations, the Sun crosses 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 ascensions are calculated.

The First Point of Aries (also known as the Cusp 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. Due to the Sun'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 name First Point of Aries.

Due to Earth's axial 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 with Aquarius. Currently, the closest major star to the First Point of Aries is λ Piscium, located at (22^h 52^m 37^s, −07° 34′ 47″).

The Sun now appears in Aries from late April through mid May, though the constellation is still associated with the beginning of spring.

The Cusp 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 of navigational stars. Due to the precession of the equinoxes, the astrological signs of the tropical zodiac are likewise identically affected and thus also no longer correspond with the actual constellations once ascribed to them (with the Cusp of Libra now actually located within Virgo), and is the basis for the concept of astrological ages. In sidereal astrology (notably Hindu astrology), by contrast, the first point of Aries remains aligned with Ras Hammel "the head of the ram", i.e. the Aries constellation.

See also

• Equinox#Names – other names for the First Point of Aries
• March equinox
• Equinox (celestial coordinates)
• Aries (constellation)
• Celestial coordinate system
• Right ascension