Private Universe

As we head into December, people the world over prepare for some of the year’s most significant holidays. Christmas, Hanukkah and Kwanza all fall sometime in the darkest period of winter, reflecting the fact that, despite our religious or cultural differences, we are all human and humans crave warmth and light in this season of darkness. While the true meaning of these holidays has become obscured by the retail brouhaha that usually occurs, even these “true” meanings can trace their origins back to one thing we all share: the sky.

Simply put, all of these festivals occur in winter because it’s winter. During the winter months, the sun rises late, stays in the sky for a short period of time and sets early. Darkness reigns for nearly 16 hours at the latitude of Winnipeg, giving the Earth lots of time to shed the sun’s energy out into space.

This lack of sunlight hours is one of the reasons why it’s cold in winter. Contrary to popular belief, the Earth is not farther away from the sun in December — it’s actually closest to the sun on Jan. 4! But in December, the northern hemisphere of the Earth is tilted away from the sun, and so we’re getting less sunlight. In the southern hemisphere, they are tilted towards the sun in December, which adds to the “closest to the sun” factor and makes southern summers generally warmer than northern hemisphere ones.

The ancients observed the sun’s rising and setting, and found that it doesn’t always rise due east or set due west — in fact, it almost never does. The sun rose due east around Sept. 21, the autumnal equinox, but since then it has been rising and setting a little bit farther south of the east-west line every day. By the date of the winter solstice (around Dec. 21), the sun is at its farthest south when it rises and sets. The sun’s altitude above the horizon at high noon was also getting lower and lower each day. It didn’t take much imagination to see that if this trend continued, the sun would disappear entirely, and the world would be locked in an icy darkness forever.

So, around the world, various festivals and rituals were enacted to coax the sun back to a higher path across the sky. Great bonfires were lit on high hills to warm the sun. Candles, logs and lights were lit to drive back the darkness and encourage light to return. It always seemed to work, since after a few days around the solstice the sun would begin moving northward again and the days would grow longer. These festivals of light became a tradition that remained long after the sun’s motion was understood scientifically. These ancient traditions of light were later absorbed into the various religious festivals that came about more recently.
Christmas lights, the Yule log and candles all feature prominently in Christmas traditions, even though none of these symbols come from the biblical sources of the celebration. Similarly, Hanukkah’s central symbol of the lighting of the menorah evokes the idea of rekindling light from darkness, paralleling the earlier pagan solstice festivals.

Understanding the earlier roots of these modern holiday traditions in no way undermines their validity or significance — it merely shows that the peoples who made up the early churches and synagogues did not do so in a vacuum, but built upon traditions that were already in place in the world around them. In the case of Christmas, there is no historical record of the date of Jesus’ birth, but the date of Christmas was set in the fourth century AD to match the date of the winter solstice at that time. (Long-term changes in the Earth’s rotation actually alter the date of the solstices over thousands of years; this is why the solstice now falls on Dec. 22 rather than the 25th.) In the Gregorian calendar, the date of the solstice is Jan. 6, and this date is celebrated as “Ukrainian Christmas,” even though the two dates refer to the same instant of time.

No matter which winter festival you celebrate, its astronomical roots should be easy to understand. Step outside on a clear, dark December night and watch the thousands of sparkling stars slowly cycle overhead. The same precession occurred for our earliest ancestors, who were inspired to begin the traditions of light that have survived in many forms to the present day. What will the winter skies inspire in you?

Sky update
December provides a major meteor shower — the Geminids — on Dec. 13-14, a total lunar eclipse on Dec. 20-21 and a bright evening planet (Jupiter).

Scott Young is an astronomer and planetarium producer, and is a past president of the Royal Astronomical Society of Canada.