Axial tilt — the reason for the season?

We celebrate the birth of Jesus on Christmas Day, but how do we know that Dec. 25 is his birthday? The Bible contains no record of the date when Christ was born. During the first three centuries following Jesus’ birth, Christmas was not celebrated at all. Instead, Jan. 6, the Feast of the Epiphany, was a more important date. It wasn’t until the 4th century A.D. that Dec. 25 was officially accepted as Jesus’ birthday.

There are many different arguments as to why this date was selected. Here are two of the most popular:

1. It was based on Jesus’ conception. The date of his conception is unknown. However, the Catholic Church selected March 25 so as to coincide with the date of

Jesus’ death, which was tied to the spring equinox and Jewish Passover. Hence, the selection of Dec. 25 nine months later to celebrate Jesus’ birth.

2. It was selected to compete with the immensely popular Roman celebration of the winter solstice known as Saturnalia.

Both of these arguments, as well as practically all the lesser ones, ultimately use the seasons to set the liturgical calendars of both

Hebrew and Christian faiths. The argument for Saturnalia as a backdrop for the birth of Christ is compelling. Saturnalia was celebrated in honor of the Roman god Saturn, who represented agriculture or the sowing of seeds. The winter solstice was recognized as a time of birth because the days following it began to get longer, allowing the sun to warm the earth, leading to new life.

Since that era, modern astronomy has determined with great accuracy when the winter solstice occurs. It is the moment the northern polar axis of the earth is at maximum tilt away from the sun. This year, that moment occurs on Dec. 21 at 3:48 p.m. MST. The way we can see this phenomenon is to observe the sunrise. The sun will rise at its farthest location south of east on this day.

Each sunrise after that will move farther north until the summer solstice. So we can say that the axial tilt of the earth plays a pivotal role in making up the reason for the season.

COURTESY MAURICE ROETHEL ASTROPHOTOGRAPHY This image was taken Oct. 28 in Sandia Park, N.M. It helps us visualize the earth’s axis with motion around it. Here we see a time exposure taken from a camera on a fixed mount pointed toward true north. Each arc of blue, white or red light represents a single star smeared out by the rotation of the earth during several minutes of exposure. Polaris, the North Star, is the little bright smudge nearest the center of this cosmic maelstrom. Lights along the bottom right are from Santa Fe. There is a faint almost horizontal line going across the star trails. If you zoom in, you’ll see the red and green wingtip lights from a passing airplane. There is also one short bright streak near the glow of the ground lights that’s not in line with the star trails. That streak is a meteor.