The vernal equinox -- the moment when Spring officially arrives -- "is defined in astronomy as that point in space where the plane of the Earth's orbit around the Sun, the ecliptic, intersects the plane of the Earth's equator extended into space." (From "The Astrologer's Handbook;" by Frances Sakoian and Lewis Acker.) This is where you need to construct that visual model in your imagination I mentioned earlier. Put the Sun in the center of the picture you're building in your mind and see the Earth rotating around it.
Now instead of seeing the rotating Earth as following the path of a ring around the Sun, see it as traveling on the outermost edge of a plane ... such as a phonograph record. If you follow the phonograph record analogy, the Sun is still in the center -- in the hole of the record -- and the Earth is still traveling in a circle around it, but there is a visually solid platform now between the two bodies instead of just blank, empty space.
Now, remember from your days of studying these subjects in school ... the Earth is not just rotating around the Sun, it's also spinning on its own axis ... moving like a spinning top in a circle around the Sun at the center of the Solar System.
The Earth is also tilted on its axis ... and as it both rotates on its axis and revolves around the Sun, there will come a point where the equator -- the imaginary circle around the middle of the earth that separates the Northern Hemisphere of the globe from the Southern Hemisphere lines up exactly with -- intersects -- the plane of the ecliptic ... the "phonograph record" that we've postulated bridges the space between the Earth and the Sun. (See why I said you'd be sorry you asked?)
The moment this happens marks either the vernal (Spring) equinox or the autumnal (Autumn) equinox -- the days of the year where daytime hours and nighttime hours are equal in length. When this occurs at the vernal equinox -- in the Spring, around March 19 - 21, depending on how "out of synch" calendar measurement is with solar / planetary movement -- the Sun moves from a position south of the equator ... as seen from Earth ... to a position north of the equator in its apparent motion along the ecliptic.
You may not have noticed this, but in the winter in the Northern Hemisphere, the Sun travels a line lower in the sky as it moves from horizon to horizon during the course of a day. This accounts in part for why winter days are shorter. The Sun travels a line higher in the sky -- and thus has a farther distance to go -- in the summer. When the Sun hits its lowest point in the sky -- in late December in the Northern Hemisphere -- we've reached the Winter Solstice point. When the Sun hits its highest point in the sky -- in late June in the Northern Hemisphere -- we've reached the Summer Solstice point.
Astronomy and mathematics are able to determine the exact moment in the these things happen Astrology marks its major beginning points based on those positions and calculations. Calendars, on the other hand, were build using much less precise, much more arbitrary standards, and so the slack between the two -- the discrepancies that seem so annoying and mysterious -- can be attributed to that.
The rock structures at Stonehenge in England and in Chaco Canyon in the United States seem to have been laboriously, meticulously constructed by ancient people to determine these essential points in yearly time -- the solstice points and equinoxes. We can assume there were once cultures that paid close attention to these moments for their social rhythms, spiritual practices and commercial / farming / planting practices. When you see what work they devoted to construct tools that would help them determine these precise moments in time, it's clear these were a big deal to them ... long ago.
In Astrology, and in the earth-based religions such as wicca, these days are still a big deal.
Spring, the vernal equinox ... the advent of the Sun into Aries -- day and night of equal length and getting longer
Summer, the Summer Solstice ... advent of the Sun into Cancer -- the longest day of the year
Autumn, the autumnal equinox ... the advent of the Sun into Libra -- day and night again of equal length and getting shorter
Winter, the Winter Solstice ... the advent of the Sun into Capricorn -- the shortest day of the year
Some pagan religions hold festivals on these days to celebrate their arrival -- in the same way Christians celebrate Christmas and Easter, and the Jews celebrate events like Passover and Hannukah in their culture, history and religious practices. And for Astrologers, these days mark the beginning of the Cardinal Signs ... signs whose energy marks a distinctive change of pace in the business of life and promotes action to acknowledge and support a new set of interests.
Aries, for instance, centers around the individual and the establishment of self-identity. Cancer centers around home and family interests. Libra centers around marriage and partnership concerns. Capricorn centers around public life efforts and community accomplishments. The two signs that follow each Cardinal sign supports the focus and agendas of the Cardinal sign that precedes it in a natural fashion that gives Astrology its logic and structure.
But that's a topic for another time. For now ... this is why Astrology signs and calendar time are not synchronized ... and why even the beginnings of the signs themselves will differ from one year to the next and certainly why the times when these occur are widely divergent.
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