Happy autumnal equinox!
Fall is here once again! Today, September 22, the entire surface of the Earth will experience equal amounts of darkness and light. People standing on the north and south poles today would see the sun stay pegged on the horizon for 24 hours. You might call it a day-long sunset or sunrise, depending on which pole you were standing on.

After today at the north pole, the sun dips below the horizon, and is not seen again for six months, until the vernal equinox. At the south pole, it's the reverse; after the autumnal equinox, the sun rises a little higher above the horizon every day, until the vernal equinox.

In the northern hemisphere, the days have been getting shorter (meaning the hours of daylight have been decreasing) since the summer solstice, which took place on June 20. The days will continue to get shorter until the winter solstice, on December 21. After that, the days will begin to get longer again. It's the same celestial ballet our pale blue dot of a planet has twirled through for the last 4.6 billion years.

The path of Earth's orbit around the sun is an ellipse. Consequently, the planet's speed varies slightly as it travels around the sun. When the Earth passes closest to the sun (a geometric state called perihelion), its orbital speed reaches its maximum of 30.29 kilometers per second. When Earth is farthest from the sun (aphelion), the orbital speed slows to 29.29 kilometers per second. The total variation in speed is only 1 kilometer per second, or about 3%. In other words, Earth's orbit around the sun is almost, but not quite, a perfect circle.

The variation in orbital speed has very little to do with the variation in hours of daylight and darkness. And perihelion and aphelion DO NOT coincide with the equinoxes or solstices - at least not most of the time. Right now, perihelion, when Earth is closest to the sun, occurs in January. But this is not permanent. The date on which perihelion occurs is constantly shifting. This change, called orbital precession, is quite slow. Perihelion occurs about one day later every 58 years. Four thousand years in the future, in the year 6430, perihelion and the vernal equinox will occur on the same day.

Not only do the hours of daylight and darkness change throughout the year, but the RATE of change also varies a lot. On the days closely preceding or following the summer and winter solstices, the amounts of daylight and darkness change by about 20 seconds per day. But the change is much faster in the days near the autumnal and vernal equinoxes. Near the autumnal equinox, in the northern hemisphere, we are losing about 3 minutes of daylight per day. That loss of daylight will slow down as we approach the winter solstice, when the variance will once again be about 20 seconds a day.

The reason behind the seasonal changes is that Earth's axis of rotation is tilted 23½ degrees with respect to the ecliptic. The ecliptic is the plane in which planet Earth orbits the sun.

If Earth's axis of rotation were perpendicular to the ecliptic, then there would be no seasonal changes anywhere on Earth. There would be no spring, no summer, no fall, no winter. No matter where you were on Earth, every day would have 12 hours of daylight and 12 hours of darkness.

And If the axis were tilted even more than 23½ degrees, then the summers and winters would be longer, and more extreme. Winters would be colder, and summers hotter. Falls and springs would be shorter and the transitions from winter to summer, or vice versa, would happen faster. The greater the tilt, the more pronounced this pattern would be.

Lucky for us, this is not the case on Earth. But it could have been different. The planet Mercury's axis of rotation is almost perfectly perpendicular to the ecliptic. Mercury experiences no seasonal changes whatsoever. (Mercury is a hot little rock, due to the planet's close proximity to the sun.)

At the other extreme, the planet Uranus' axis lies almost parallel to the plane of the ecliptic. (Uranus is a big, cold, ball of gas. It's so far away from the sun that it's always sub-freezing, no matter the season.)

Seasonal change affects every aspect of life on Earth. Animal migrations are driven by seasonal change. Some of these are truly epic. For instance, a little shore bird called the red knot flies north over 9,000 miles every spring, from Patagonia, in South America, to their nesting grounds above the Arctic circle. The seasonal bloom of plants and insects in the Arctic spring provides the abundant food the birds need to raise their chicks. Without this seasonal explosion of life in the Arctic, the red knot would not, could not bring forth new generations.

Seasonal change is also deeply embedded in human culture. Most, if not all, religions are based on or incorporate elements of seasonal change in their mythologies.

For instance, in Greek mythology, the god Boreas was associated with winter. His name meant "North Wind" or "Devouring One." It's from his name that we get the adjective "boreal," and why that shimmering spectacle known as the northern lights is also known as the aurora borealis.

And it's no accident that Easter, the annual celebration of the resurrection of Jesus Christ, occurs in the spring, when new life emerges after the cold and dark of winter. There is even evidence that the word "Easter" originated from the name of a Germanic/Anglo-Saxon goddess, Eostre, whose Phoenician/Canaanite precursor was called Astarte. Eostre and Astarte were goddesses of love, sex, and fertility; their celebrations were always rites of spring.

An example of a god associated with summer is Zephyrus. It was he, according to Greek mythology, who brought the early summer rains and breezes. And from his name we get the word "zephyr," which is the pleasant west wind that keeps the California/Oregon coast cool in summer.

The Greek god associated with fall was Bacchus. He was first associated with the vine harvest and then became the god of autumn. And from his name we get the word for a really great party: bacchanal!

The Earth itself has been represented in different cultures by various gods and goddesses. In ancient Sumeria, the Earth father god, Dumuzid, was depicted as a shepherd. In ancient Greek mythology, the Earth mother goddess, Gaia, was represented as a beautiful, matronly woman.

The Aztecs also had an Earth-mother goddess; they called her Coatlicue. Coatlicue was a creator/destroyer goddess. In stone sculpture, her face is that of a fanged serpent; instead of fingers and toes she has claws; her skirt is a writhing mass of interwoven snakes (because snakes symbolized fertility); having nourished multitudes, her breasts are flabby; the necklace she wears is hands, hearts, and a skull, a grisly reminder that all living things die and are consumed by the Earth.

After the Spanish conquest of Mexico, the Catholic Church was faced with the challenge of converting the king's new subjects to Christianity. According to legend, on December 12, 1531, an image of the Virgin Mary "miraculously" appeared on an Aztec peasant's tunic. (In reality, the image was probably created by Don Juan de Zumárraga y Arrazola, the first Bishop of Mexico, or someone working under his close supervision.)

In the image, which has been endlessly copied and is now ubiquitous in Mexico, Mary is depicted with hands together and head bowed, as if in prayer. Her head and shoulders are covered with a dark blue shawl sprinkled with bright stars. Emanating from behind Mary are the rays of the sun. And she is standing on a crescent moon, which is in turn held up by an angel.

To the conquered Aztecs, the symbolism in this image would have been unmistakable. With crescent moon riding low in the sky, the body eclipsing the sun could only be the Earth herself. Mary was, in essence, another manifestation of the Aztec Earth mother goddess, Coatlicue.

It gets better. In the Aztec tradition, Coatlicue's son was Huizilopochtli, the sun god. And isn't Mary's son, Jesus, the light of the world?

John 8:12: "I am the light of the world. Whoever follows me will never walk in darkness, but will have the light of life".

It was a master stroke of salesmanship and symbolic imagery on the part of the good Bishop. Prospective converts needed something they could relate to, and he gave it to them. The year was 1531. Today's big shots on Madison Avenue have got nothing on Juan de Zumárraga.

And so, on December 12, 1531, ten days before the winter solstice, one mythological tradition was subsumed into another. Just in time for Christmas, or Hanukkah, or Yaldā Night if you're Iranian, or
Dong Zhi if you're Chinese, or Inti Raymi if you're Incan, or Saint Lucy's Day if you're Swedish, or Saturnalia if you're Roman....

Left to right, top to bottom, the images are of Astarte, Boreas, Zephyrus, Bacchus throwing one of his famous parties, Dumuzid feeding his flock, a modern representation of Gaia, the stone sculpture of Coatlicue that resides in the National Museum of Anthropology in Mexico City, a modern version of the 1531 image of La Virgen Santísima María de Guadalupe, and a diagram of Earth's orbital path around the sun.