Equinox means “equal night,” an appropriate description of the time of the year when nights and days are approximately 12 hours long everywhere on the Earth.

This morning, the Sun rose directly due east. At noon it occupies an altitude that is halfway in the sky all year. This evening, the Sun will set precisely in the west.

As a benchmark in the flow of time, the vernal equinox has long been a recipient of honor and homage. An ancient starting post for the beginning of the year, this day was greeted by sun worshipers around the world with awe and wonder.

Also rich in religious significance, the vernal equinox heralded the reawakening of the land and was commemorated in rituals, ceremonies, and prophesies.

Down in Mexico: Nowhere is this more evident than in Mexico's Yucatan peninsula. Against the magnificent backdrop of the great Maya pyramid of El Casatillo, a natural phenomena of the seasons becomes a magical and breathtaking rite of spring.

Other than the ancient Maya, and, until recently, their local descendants, few people have ever seen this spectacular sight.

This writer and 600 other adventurous travelers are sailing aboard Sun Line Cruises' Stella Solaris to witness today's interplay of light and shadow that signals the beginning of spring.

El Castillo - meaning “The Castle” - was so named by the first Europeans to reach the Maya ceremonial city of Chichen Itza. This great pyramid is a complex calendar wrought in stone - built to precise astronomical lines related directly to the spring and fall equinoxes.

Two of the pyramid's four staircases have been restored from the ravages of time and jungle. There are 91 steps in each staircase, for a total of 364 steps. They lead to an upper platform - the 365th step - and together represent the number of days in a solar year.

Nine terraces are divided by the stairs into 18 sections - one for each month in the Maya year.

Above the terraces are 52 panels, each one marking one year in the Maya cycle of years that is comparable to our century.

At the foot of each stairway is the stone sculpture of a plumed serpent's head. These feathered rattlesnakes are a manifestation of the Maya deity Kukulcan.

The yearly appearance of the stairdcase of a “moving” snake of light and shadow represented and reaffirmed the belief in life after death and the return of the season of growth.

The shadows flow: Tens of thousands of people have traveled to Chichen Itza to marvel at El Castillo's dance with the Sun.

As the afternoon sunlight bathes the structure's western face, the Sun itself slowly sinks westward and the stepped sides of the pyramid begin to form shadows.

An hour before the Sun sets, the shadows take on a distinctive zig-zag pattern that turns into a sequence of seven triangles descending the pyramid. At sunset, the stone sculpture at the foot of the stairs completes the transformation.

Light, shadow and stone have combined into a feathered serpent - a solar representation of Kukulcan.

The construction of this magnificent monument is a lasting testament to the importance of the movement of the Sun and other celestial bodies to Maya religious and daily life.

This writer, having marvelled at this dazzling interplay of light and shadow on many past spring equinoxes, is coordinating yet another 12-day sea voyage to view the next Maya equinox show in March 1995.