Christmas is right around the corner. Christmas lights illuminate our streets and curtain some buildings with their sparkly glow. Cooler, dryer winds are starting to blow our way. Gifts (and our spending for them) are piling up. Our bellies are already getting their practice for the festivities to come. And of course there is the traffic of the holiday rush.
In all of this, it is sometimes easy to forget that Christmas is actually a religious celebration observed by many Christians.
Even easier to forget is the fact that even before Christians started celebrating the birth of Christ during the 25th of December, people from around the world have already been celebrating this time of the year because of what it represents in the yearly cycles of the sky. In other words, this holiday season is also about outer space.
Of course the people who celebrated Yule, Dongzhi, Ayan Parivartan, and other similar celebrations around the world probably did not understand the full correlation of their celebrations with a regular astronomical event. What they were aware of, however, was that they were celebrating an important point in the cycles of the sky, cycles that affected the way they lived in very intimate ways. Today we know that these cycles occur because of regularities dictated by the laws of gravity and motion. In other words, the equations of physics conduct the regularity of our holidays.
Two important set of laws are the conservation of angular momentum and the laws of gravity.
The first one says that the tilt of the Earth’s axis will change very slowly because of how big and massive the Earth is. In effect, over the course of hundreds of years the Earth tilts toward the same direction in outer space. One effect of this is that Polaris, our current North Star, has been our north star for centuries now and will continue to be so for the next few thousands of years.
The second set of physical laws, the laws of gravity, dictate the way the Earth moves around the Sun over the course of a year.
A combination of the effects stated above results in the fact that each of the Earth’s halves or hemispheres gets tilted away from or toward the Sun at specific times of the year. The Earth’s northern hemisphere, where the Philippines is located, is tilted toward the Sun in June and tilted away in December. The opposite holds for the southern hemisphere, where countries like Australia and South Africa are located; the Earth’s south pole is tilted away from the Sun in June and tilted toward in December.
As the Earth goes around the Sun in its yearly cycle, as appointed by the laws of physics, we here on Earth observed regularities in the way the Sun moves across our sky. Because the Sun is the main source of energy for most of the process here on Earth, these cycles also translate to changes in the way we humans live our lives.
The cycles, more or less, go like this. Starting from March, the days progressively get longer and longer. The Sun climbs higher and higher in the sky, giving us both the heat of summer and the energy that results in our rainy seasons from June onward. By June, the day gets as long as it can be, after which time it starts to get shorter, giving way to longer nights. By September, daytime and nighttime are almost equal. By October, the nights start to gain ground, progressively getting longer than the days, culminating in the shortest day of the year, the solstice of December.
The solstice of December, also called the “winter solstice” in parts of the northern hemisphere which experience winter, falls on December 21 or 22. (This year it falls on the 22nd.) This is the time of the year when, in the view of many pre-modern societies, the Sun gets reborn. The days, which have previously gotten shorter, begin to get longer once again. The longer nights begin to give way to the daytime.
People from around the world have observed this pattern and began celebrating this special day of the year in many forms. The Germanic people of Europe celebrate the Yule, the Romans celebrated Saturnalia, the Chinese celebrate Dongzhi, and so on. Some historians speculate that Pope Julius I declared Christmas to be on December 25th partly to make it coincide with the Roman celebration of Saturnalia, that way making celebrations seamless for new converts to Christianity.
So this holiday season, however you celebrate it, I hope you would consider adding thinking about our place in the universe to the mix. Happy holidays to one and all!