The winter solstice: a time of reflection, renewal, and rejoicing. It is the shortest day and longest night of the year, long celebrated by decorating trees and embracing family.
A practice that remains among many religions during this time of year, though not all celebrate in the same way as one another.
But what does this have to do with astronomy, you ask?
The solstice is a great teaching tool for understanding how our planet moves around the sun and how that movement causes seasons.
We’ll start at the beginning. One year is the amount of time it takes a given planet to orbit once around its sun.
For us, that journey takes roughly about 365 days. But most of you already know this. And likely, you also know that our distance from the sun varies during the year by a few billion miles. This means that sometimes we are closer to the sun, and sometimes we are further away.
Interestingly, our planet is also tilted. So, imagine a pole is driven straight down through the Earth from north to south. This imaginary pole is known as the Earth’s axis.
Now, imagine that someone grabs that pole by the top and just pulls it like a slot machine handle, only it gets stuck. That causes the entire planet to tilt.
But our tilted, little planet is still going around the sun, and it’s spinning on this imaginary pole, too. So now, as the Earth is orbiting the sun, there are times when the northern half is tilted toward the sun, and times when it is tilted away. This is also true for the southern half of the planet.
During the winter solstice in the northern hemisphere, the invisible pole is tilted away from the sun. This causes the northern half of the continents to experience a longer night because they are on the dark side of the Earth for a slightly longer time than at any other point in the year. Likewise, the southern half of the Earth will be experiencing their summer solstice (longest day and shortest night).
This tilting is what causes the seasons. As one half of the Earth is tilted toward the sun, that half is experiencing summer, and when it’s tilted away, it experiences winter. That’s why Australians get to go to the beach on Christmas.
Consider this: our entire measurement of time is based on the speed at which the Earth travels around the sun (year) and the rotation on its axis (day and night).
But going back to the spiritual aspect, if I may bend your ear a bit longer…winter solstice here in the north is seen as a time of renewal and rebirth. The longest night stretching out to embrace the world as things in nature pass on giving sustenance so that others can be born.
Plants that die turn to compost that feeds the microbes that fertilize the soil for seedlings to sprout. And even on a cosmic scale, stars that die in a supernova explosion expel their gases and debris that become nebula in which new stars and planets are formed. It’s truly a wonderous thing.
And, despite how we measure it, time marches ever forward, and the cycle of all things continues.