Illustration: Diurnal tracks on the globe, photographed at the Globe Museum in Vienna.
When we say that the longest day of the year comes around the winter solstice each year, this is not daytime, but the interval from noon (the time when the sun is in the middle of the day) or from noon to the next noon, on which the Earth rotates about half a minute longer than usual relative to the noon sun.
This year, the winter solstice arrived on December 22, at 12:19 Beijing time. For those of us in the Northern Hemisphere, the winter solstice means the shortest days and the longest nights. If we look at the length of a day from another angle, the longest daytime day in the world is in December.
Remember: The clock on our wall is not a tool for measuring the true length of a day. To measure the length of one noonday sun to the next, we need a sundial. Sundials tell you exactly when the local noon sun is, the time of day when the sun reaches its highest point.
The photo at the top of this article shows a figure-8 curve that shows the deflection angle of the sun, and its angle comes from the difference (in minutes) between the latitude of the celestial globe and the time measured by the clock and the time measured by the sun.
Illustration: A daywalk taken outside Bell Labs in New Jersey, USA, from 1998 to 1999, consisting of multiple images taken on different dates. Since the day track is used to show the difference in the position of the sun in the sky at the same time on different days of the year, it takes about a year to photograph this phenomenon.
So, it's December, which means that one rotation of the Earth relative to the Sun — what we call solar days — is about half a minute longer than the global average of 24 hours.
From noon on one sun to noon on the next, the time of day around two to the hour is always longer than 24 hours, and the time of day before and after the equinox is always shorter than 24 hours.
Illustration: The observed heliotic tracks in the Northern Hemisphere of the Earth have the same proportion of horizon latitude and horizon longitude
The world is now at the longest time in a day – because we are closer to the sun on the winter solstice day than on the summer solstice day. Earth's perihelion (the point closest to the Sun) always arrives in early January. When we are closest to the Sun, our planet moves in its orbit a little faster than the average. This means that our planet travels a little farther in space every day than average. As a result, the Earth would have to rotate a little more around the axis before the sun could return to its noon position. As a result, solar days become longer.
Illustration: The Earth's Daily Movements
Half a minute extra doesn't sound like a lot, but the difference will get bigger and bigger. For example, in the first two weeks of the winter solstice day, the noon time is about seven minutes earlier than the winter solstice day. In the last two weeks of the winter solstice day, the noon time on the clock is about seven minutes later than the winter solstice day itself. Because the clock is now out of sync with the solar pole, at this time of year, some confusing phenomenon drives people crazy.
In the Northern Hemisphere, the earliest sunset of the year is before the December winter solstice, and the latest sunrise of the year is after the December winter solstice.
In the Southern Hemisphere, the earliest sunrise of the year is before the December solstice, and the latest sunset of the year is after the December solstice.
Although two solstices bring the shortest/longest daytime, the earliest sunsets/sunrises are always before the solstice and the latest sunrises/sunsets are always after the solstice.
In fact, at the beginning of January, when we are closest to the sun, it also means that the northern hemisphere winter (southern hemisphere summer) is the shortest of four seasons. meantime... It is also the long season of sun days.
Illustration: Sundial outside the Adler Planetarium in Chicago, Illinois. Sundials allow you to measure the time from noon one sun to noon the next. Image source: Ben+Sam
In short: from noon on one sun to noon on the next, December is the day with the longest day/night cycle on the entire planet.
Related knowledge
Winter solstice, also known as winter solstice, hedong, one of the twenty-four solar terms, one of the eight celestial solar terms, the opposite of the summer solstice. The winter solstice day is the shortest day of the year, the northern hemisphere winter solstice is generally between December 21 and December 23 in the Western calendar, and the Chinese calendar uses the month in which the winter solstice is located to define November.
Resources
1. WJ Encyclopedia
2. Astronomical terms
3. Yan Yufei: Su Xiaosu - earthsky
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