Planetarium

"Why is the vernal equinox called the "First Point of Aries" when the Sun is actually in Pisces on this date?"

 

The Sun used to be "in" the constellation Aries on the first day of Spring, otherwise known as the vernal equinox.   We should explain that as Earth revolves around the Sun, the latter will appear to travel through thirteen constellations comprising the "zodiac."  Greek astronomer Hipparchus of Nicea (190-120 BCE) introduced the term "First Point of Aries" (or "Cusp of Aries.") when he observed that the Sun was within the constellation Aries during the vernal equinox.     However, the Sun's apparent vernal equinox position has continuously shifted along the ecliptic by about 1 degree every 73 years due to precessional wobbling.   The wobbling, caused primarily by interactions with the Sun and Moon, causes our planet's pole to describe a 47 degree circle through the sky every 26,000 years.   Consequently, the thirteen zodiac constellations will all "host" the vernal equinox point during this 26,000 year cycle.        According to astronomer Jean Meeus, the vernal equinox point crossed the Aries-Pisces border in 68 BCE.  Ironically, this shift occurred less than a century after Hipparchus' death. The vernal equinox point has been moving westward through Pisces ever since.  In AD 2597, the vernal equinox will move into Aquarius the Water Bearer.  Or, to be more specific, it will cross into the rectilinear region that the International Astronomical Union has designated as the Aqarius "region." Perhaps even then, astronomers will continue to refer to the vernal equinox as the "First Point of Aries."

 Zodiac precession

Earth's precessional wobbling causes the vernal equinox point to migrate around the ecliptic once every 26,000 years.     This point was once in Aries the Ram, hence the term "First Point of Aries."   The vernal equinox point is now in Pisces and in the late 26th century will shift into the Aquarius region.  Note that the symbol designating the vernal equinox point is the astrological sign of Aries.