The winter solstice has arrived. The coldest part of winter is coming. The winter solstice is not only a day of special significance in the year, it also contains a lot of interesting natural and humanistic knowledge. Let’s talk about 6 cold facts related to the winter solstice and see if you know them all?
Winter Solstice
The twenty-four solar terms are derived from Big Dipper
"Spring rain shocks spring and clears the valley, summer is full of awns and summer heat connects. Autumn dew, autumn cold frost falls , winter snows and winters are cold. ..." Have you all recited this twenty-four solar terms song at a very young age? The twenty-four solar terms are a splendid treasure of our Chinese civilization and are known as "China's fifth greatest invention".
The Big Dipper and seasonal changes
Did you know? The earliest twenty-four solar terms were formulated based on the changes in the stars. Ancient ancestors discovered that the changes in the stars in the sky are closely related to the changes in the four seasons. The Big Dipper in the sky is like a huge clock. Its bucket handle is the hands of the clock. When the bucket handle points due east, the spring equinox has arrived; when the bucket handle points due south, the summer solstice has arrived; when the bucket handle points to the west, the autumnal equinox has arrived. If we observe the Big Dipper on the night of the winter solstice, we will find that the end of the handle of the Big Dipper points to true north. In this way, the Big Dipper in the sky rotates once, and the four seasons on the earth change again, and a year has passed. So people formulated the twenty-four solar terms based on the directions of the handles of the Big Dipper.
The 24 solar terms on the earth's orbit
The current twenty-four solar terms are determined by the position of the sun on the ecliptic . It's like thinking of the earth as a train traveling around the sun. The 24 solar terms are 24 stations on the track. They bisect the earth's orbit , and each solar term is 15° apart. The earth's orbit is a circle, where is the starting point? People define the time when the sun hits the earth's equator directly as the starting point, which is the vernal equinox. Starting from the spring equinox, the winter solstice is the 18th station. On the day of the winter solstice, the earth has taken us around the sun exactly 270°
How far has the earth traveled
Earth revolution and four seasons
Time has arrived at the winter solstice. How far has the earth traveled in its own orbit? We think of the earth's orbit around the sun as a circle. Then the average distance between the sun and the earth is 2.1496 billion kilometers, which is the radius of this circle. In this way, we can easily conclude that the orbital circumference of the Earth is about 940 million kilometers. From this we further conclude that the length of the Earth's orbit between each solar term is approximately 39.17 million kilometers. When the time passes from the spring equinox to the winter solstice, the earth has already taken us in orbit around the sun for 705 million kilometers!
Have you ever thought that the passing of spring and autumn brings not only changes in time, but also in space?
The longest day of darkness
We all know that the day on the winter solstice is the shortest day of the year and the night is the longest. Taking Beijing as an example, on the winter solstice, the sun rises lazily from the southeast at 7:32, and clocks out early at 4:52 pm. The sun is only in the sky for 9 hours and 20 minutes. And the further north you go, the shorter the daylight hours become.
yellow red angle
Why is this? The reason is that the earth holds its head high. It turns out that there is an angle between the equatorial plane of the earth and the ecliptic plane of the earth's revolution around the sun, which is called the ecliptic angle. The yellow-red intersection angle is about 23°26′. Due to the existence of the yellow-red angle, the direct point of the sun moves back and forth between the earth's north and south latitudes 23°26′. This causes changes in the height of the sun at noon and the length of day and night.
The Earth at the Winter Solstice
When the winter solstice comes, the direct point of the sun is exactly on the Tropic of Capricorn. At this time, the earth seems to have its head raised, and the light from the northern hemisphere is slanting. And the further north you go, the stronger the sunlight is. At this time, the sun's position in the sky is at its lowest position of the year. The sun only walked a small distance in the sky before setting.What is even more exaggerated is that if we come within the Arctic Circle at 66.5° north latitude, we will find that the sun does not rise at all. This is the extreme night phenomenon. Therefore, the winter solstice is the day with the shortest day and the longest night of the year.
How long are the days there on the winter solstice?
The earth is closer to the sun
Winter Solstice and perihelion
After the winter solstice, we start to " count nine ". The really cold time of winter has arrived. Strangely, the Earth is closer to the sun at the winter solstice than at the summer solstice. It turns out that the Earth is about to reach the perihelion point of its orbit at the winter solstice, while the Earth at the summer solstice is near the aphelion. The Earth at the winter solstice is about 4.8 million kilometers closer to the sun than it is at the summer solstice.
The earth is closer to the sun during the winter solstice than during the summer solstice. Why does it become colder instead? If anyone knows the reason, please share it in the comment section!
The earth is running faster
The earth and the sun
The length of the earth’s orbit around the sun is about 940 million kilometers. The earth travels once a year, so we can calculate the earth's average revolution speed to be 29.783 kilometers/second. However, the speed at which the Earth orbits the Sun is not uniform but constantly changing. The reason is that the distance from the Earth to the Sun also changes at all times. At the winter solstice, the earth reaches near perihelion. At this time, the gravitational pull it receives from the sun will be greater, and its revolution speed will be faster. On the winter solstice, the earth's revolution speed is approximately 30.273 kilometers/second.
The Earth's Revolution
On the contrary, during the summer solstice, the Earth is located near the aphelion, so the gravitational pull it receives from the Sun will be smaller, and its revolution speed will be slower. Its revolution speed at the summer solstice is approximately 29.305 kilometers/second. By comparison, we found that the Earth's revolution speed at the winter solstice is about 1 km/s faster than that at the summer solstice. The earth is moving faster. Does the world that is quietly accelerating look like us who are rushing to achieve results at the end of the year?
The days will get longer and longer
The Earth at the Winter Solstice
There is a proverb that goes, "After eating the winter solstice noodles, one day will grow longer." This sentence means that after the winter solstice, the days will get longer and longer. How long is the first line? It refers to the time it took an ancient woman to use up one thread for needlework, which was about two minutes. This time estimate is quite accurate. I really admire the ancient people’s meticulous observation and wisdom! After the winter solstice, we will obviously feel that the days are getting longer and longer. During the summer solstice, the feeling that the days are getting shorter does not seem to be so obvious. Therefore, you will still hear the old saying " The summer solstice lasts for eighteen days, and you will return on the winter solstice. " What is going on?
Direct and oblique sunlight
In fact, this change is also caused by the change of the earth's revolution and its revolution speed. After the winter solstice, the direct point of the sun begins to move north as the earth rotates. The sun is getting higher and higher in the sky, and the days are naturally getting longer. At the same time, after the winter solstice, the Earth will reach perihelion in early January. During this period, the earth's revolution speed is getting faster and faster. Therefore, the sun's direct point moves northward faster. In this way, we will obviously feel that the days are getting longer and longer. At the summer solstice the situation is completely reversed. As the earth's revolution speed slows down, the speed of the earth's day and night changes also slows down.
The subtle changes in day length between the winter solstice and the summer solstice have brought us many benefits? This leaves the northern hemisphere with direct sunlight for eight days longer than the southern hemisphere. This means that winter in the Northern Hemisphere is slightly shorter than summer. After all, no one likes long and cold winters! Everyone says that’s the case, right?
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