|Greetings, why is Christmas the shortest day of the year? |
|Question Date: 2020-09-08|
Christmas, a holiday observed by many people, has nothing to do with Earth's orbit around the sun, and hence day length. (That being said, I have no idea why Christmas is on Dec. 25th).
As to know, Earth is a giant spinning sphere that travels in an ellipse (oblong circle) around the sun. One spin of Earth equals one day. One orbit of the sun by Earth equals one year. There are roughly 365 days per one year.
The spin axis of Earth is tilted 23.5 degrees with respect to the plane of the ellipse in which Earth travels around the sun. This means that at different times of the year, different parts of Earth receive different amounts of light--which is to say that the length of the day (and night) varies depending on where you are.
For example, in winter (short days) in the northern hemisphere, the north pole tips away from the sun. Northern areas receive few hours of sunlight per day as a result. The shortest day of the year in the northern hemisphere is roughly December 21st (winter solstice), which is the day that Earth sits at one "end" of its elliptical path around the sun. On that day, in fact, the sun never rises above the horizon in areas north of 66.5 degrees north latitude (the arctic circle) because that part of the world sits in Earth's own shadow.
Next, let's look at what happens in the southern hemisphere on December 21st. It's the longest day of the year (summer solstice). The south pole is pointing toward the sun, meaning the south receives more light per day than the north. On Dec. 21st in fact, in areas south of 66.5 degrees south latitude the sun never sets.
Six months later, June 21st, Earth sits at the other "end" of its elliptical orbit: the north pole now tips toward the sun and the south pole away. This day is the longest day of the year in the northern hemisphere (summer solstice) and the shortest day of the year in the southern hemisphere (winter solstice).
So...sorry I don't know anything about the timing of Christmas (which occurs near the northern hemisphere's winter solstice--meaning that night is long in the northern hemisphere), but at least you know when the shortest day of the is!
Thinking about Earth orbit and how that affects our everyday lives is fun.
Happy equinox, which happens September 22nd this year. (That's when the north and south receive equal amounts of sun, and when day and night are equally long.)
Hi Khole! Unless you are talking about the winter solstice, where it is the darkest day of the year (least sunlight), which usually falls around Christmas time, I will assume that you just love Christmas! (and who doesn't?). I think what you mean is that you feel like the time "flies" on Christmas day. But of course, Christmas day has 24 hours, just like any other day of the year, so the reason is due to our perception of time. Basically, the busier you are during a time interval (in this case the Christmas day), the faster that time interval will feel like it passes. When you are cognitively busy, you are focused on each task you are performing (be it unwrapping the presents, singing Christmas carols, watching your favorite cartoons, drinking hot chocolate, and so on), you don't have the opportunity to notice the passage of time. As a result, you feel that time passes by so quickly. On the other hand, when you are bored, you don't have a lot to occupy your time, so you are more likely to keep thinking about the time and check the clock. So time feels like it passes so slowly. So that is why when you are bored in class (hopefully not in a science class) or in my case, a really long meeting, we just can't stop looking at the clock, but the clock does not seem to budge!
Christmas is one of the shortest days of the year if you live in the northern hemisphere--in North America, Europe, northern Africa, and most of Asia--because it is very close to the winter solstice (the shortest day of the year, December 21). But Christmas, December 25, is one of the longest days of the year in the southern hemisphere--in Australia, Antarctica, South America, southeast Asia, and southern Africa--because December 21 is the summer solstice.
For the northern hemisphere, Christmas occurs in the winter, but in the southern hemisphere, December 25th occurs during the summer.
Let's break that down. When we say that the winter solstice is the "shortest day of the year," we do not mean that the day has less than 24 hours. We mean that it has the fewest hours of sunlight of any day of the year: the sun rises at its latest time and sets as its earliest time. After the winter solstice, the days get "longer"--the sun rises earlier and sets later until the summer solstice, the longest day of the year.
The specific sunrise and sunset times depend on where you are--which timezone and which latitude. How can this be? The daylight hours vary for the same reason that the seasons on earth vary: the earth is tilted on its axis. This means that during some of the year, the northern hemisphere is tilted closer to the sun (this is our summer), and for some of the year, the southern hemisphere is closer to the sun (this is their summer, and it corresponds to our winter). During our winter, when the northern hemisphere is tilted away from the sun, the sun appears lower in the sky and makes a shorter arc across it during the day. This is why there are fewer hours of sunlight for us. At the same time, the southern hemisphere is tilted towards the sun, so it appears higher in the sky and takes a longer time between sunrise and sunset. It's all about the angle between you (the observer) and the sun. The closer you live to the equator, the less this angle varies throughout the year, and the more consistent the sunlight hours per day.
Encyclopedia Britannica has a nice diagram and description here if you'd like to see some helpful pictures.
Christmas is not the shortest day of the year.
In the northern hemisphere, the shortest day of the year is December 21st. In the southern hemisphere, the shortest day is June 21st.
Day length changes through the year as a result of the Earth being tilted relative to its orbit: the equator is at a 23.4 degree angle compared to the plane of the Earth's orbit. This means that in June, the north pole is leaning toward the sun, while the south pole is leaning away from it, and in December, the north pole is leaning away from the sun, while the south pole is leaning toward it. When the pole of the hemisphere that you're in is leaning toward the sun, less time is spent with the sun below the horizon, so the days are longer. The opposite happens when your pole is pointing away from the sun.
Christmas is just close to the shortest day of the year. The shortest day of the year is usually December 21 or 22. It's a good time to celebrate, when the days have gotten to their shortest, and people can look forward to the days getting longer. But in the Southern Hemisphere, Christmas is close to the longest day of the year, because the Earth is tilted to have long days on one part when there are short days on the other part, depending on which part is tilted toward the sun.
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