Christmas Countdown, Sunday #2

Day 10

Sunday’s theme is a reminder that there are more holidays than just Christmas in December — or in the winter in general, so we’ll be going to another continent for this one. Now, why are there so many holidays this time of year?

Simple. Astrophysics.

The very basic version of this is that the Earth rotates around its axis, which you can imagine as a stick that goes from its north to south pole. (Illustrated version available here.) The Earth is perfectly happy to rotate around this axis at a rate that gives us one revolution per day. While the Earth rotates around its axis, it also orbits the Sun, and this takes about 365.25 days (which is why Leap Years exist, but that’s not relevant here.)

Now if the axis were straight up and down — meaning that the equator was exactly level with the Earth’s orbit, we’d have no seasons and all days would be the same length. However, it’s not. It’s tilted about 23 degrees. This means that as the Earth goes around the Sun, the angle at which light hits it changes. On the first day of spring and first day of fall (in the Northern hemisphere), the axis is straight up and down relative to the orbit, so day and night are of equal length. As spring progresses into summer, the axis (in the north) tilts toward the Sun; from fall into winter, it tilts away. Tilting toward makes days longer; tilting away makes them shorter.

In winter, the days become the shortest of all, and the winter holidays, like Christmas, tend to happen right around that longest night of the year, which is the Winter Solstice, generally around December 22nd now, but a few thousand years ago it was closer to the 25th.

But the salient bit is this: Once the solstice comes and goes, the days after that start to get longer, light returns, and the world is eventually reborn in spring. All of these winter festivals are partly a way for communities to come together at the darkest and coldest parts of the year, and partly a way to remind them that it’s going to get better soon.

Which brings us to Diwali, which actually happened right around Thanksgiving (in America) this year, although it’s a holiday celebrated by Hindus, Sikhs, Jains and some Buddhists around the world. Basically, it celebrates the triumph of good over evil, and prominently features, well… lights, since it’s the festival of lights. But it definitely fits the winter theme, and again you can see how astronomical realities can dictate social conventions. When the year gets dark, we celebrate the fact that the light will always win and return.

 

Don’t forget to check out the previous post or watch the next.