If you live in the mid-latitude regions of the Northern Hemisphere of the Earth – and there’s a good chance you do – you may have already noticed that days are becoming shorter and nights are becoming longer over the past few weeks. This process began at the time of the summer solstice in June, was fastest during the equinox in September, and culminated at 10:27 PM Eastern Time on December 21 (3:27 AM Universal Time on December 22).
Effects of the Winter Solstice
There are two main effects that we feel in the Northern Hemisphere. The first is that the sun’s path across the sky is the lowest of the year. The sun does not rise high in the sky, so its rays do not warm the ground efficiently, making our hemisphere colder. The second is that the time the sun is above the horizon – daylight length – is the shortest, so there is less time to warm us as well. This cools our hemisphere, hence winter occurs.
Sunrise and Sunset Timing
You might expect that if the winter solstice is the shortest day, December 21 would have the latest sunrise and the earliest sunset. But – as is often the case in the real world – things are more complicated than that.
If you check the sunrise and sunset times in Washington, D.C., for example, you’ll find that the latest sunrise around the time of the winter solstice is not on December 21 but actually on January 5, 2024 (at 7:27 AM), and the earliest sunset actually occurred two weeks earlier on December 7 (at 4:45 PM)! This is surprising.
Effect of Earth’s Orbit
The reason for this distortion in timing is Earth’s orbit – or more precisely, the shape of its orbit. It is not a circle but an oval – in other words, an ellipse.
It was not known that our planet’s orbit is not circular until the early 17th century. About 60 years before the beginning of that century, Nicolaus Copernicus worked to establish that the sun, not the Earth, is the center of the solar system and that all the planets revolve around it. However, he still assumed that those orbits were circular. Thus, while his heliocentric model works better than the Earth-centered model, it did not accurately predict the positions of the planets. Johannes Kepler, using carefully selected observations made by his mentor Tycho Brahe, was the one who realized that those orbits are, in fact, ellipses – a revolutionary discovery that ultimately allowed astronomers to accurately predict the locations of the planets and gain a better understanding of our local cosmic neighborhood.
The Earth’s orbit is indeed elliptical but still very close to being circular. The difference between the closest point and the farthest point of our planet from the sun during the year is about five million kilometers, which is only about 3 percent of the average of 150 million kilometers. What Kepler understood was that this slight difference means that Earth’s speed in space also changes. It moves at its fastest pace when it is closest to the sun (a point called perihelion) and slowest when it is farthest (a point called aphelion).
Speed Effect on Sunrise and Sunset
At this time of year, when the Earth approaches perihelion and is moving faster around the sun, our planet must rotate a little more to catch up with our home star, making the day a little longer – about 30 seconds longer. It takes a bit more time for the sun to appear in the sky once, and thus this means that solar noon – when the sun is due south in the sky – is a little later in the day, according to an approximation of mean time. Sunrise and sunset are symmetrical on either side of noon, which means they also occur later in the day.
And this,
In turn, this means that the sunset time on the winter solstice is later in the day compared to the previous day. We have already experienced the earliest sunset. That was on December 7. Conversely, it also means that sunrise occurred slightly later compared to the previous day and will continue to do so until around the closest time; the latest sunrise won’t be until January 5.
Complexity of the Universe
If you find it difficult to grasp, Henry Reich from Minute physics has an animated explanatory video for you.
I know, I know. This is still confusing and strange. But it leads to an important point, one I refer to all the time: the universe is not obliged to be simple. Often, it seems to be so until you start to dig a little, and then many complexities emerge. We might wish that our daily experiences, like the passage of time, were simple, but nature has other plans.
And once you see how the celestial clock works, you can truly appreciate its beauty and how it profoundly affects everything in our lives.
Copyright and Permissions: Phil Plait is a professional astronomer and science communicator in Colorado. He writes Bad Astronomy. Follow him on Subscribe.
Source: https://www.scientificamerican.com/article/the-winter-solstice-has-a-surprising-secret/
Leave a Reply