冬至的日期为什么会变化？

Winter Solstice is a solar term and a living festival, but why does its date move from December 21 to 23?

冬至既是节气，也是仍在生活中的节日，但它的日期为什么会在12月21日至23日前后变化？

It is not chosen by a fixed day in the Gregorian calendar.

它并不是由公历中的一个固定日期决定的。

Astronomers define it as the moment when the Sun's apparent longitude reaches 270 degrees.

天文学上，冬至是太阳视黄经到达270度的时刻。

Earth's tilted axis then makes the Northern Hemisphere lean farthest away from the Sun.

此时，倾斜的地轴使北半球最明显地背向太阳。

The noon Sun is lowest, so this hemisphere has its shortest daylight and longest night.

正午太阳到达全年最低位置，因此北半球迎来最短的白昼和最长的夜晚。

You can compare sunrise and sunset times, or measure a vertical stick's noon shadow for several days.

你可以比较日出日落时间，或连续几天测量一根竖直杆在正午的影子。

Near the solstice, daylight reaches its minimum and the low noon Sun makes a relatively long shadow.

在冬至前后，白昼长度达到最低值，较低的正午太阳也会形成相对较长的影子。

However, one cycle of the seasons does not fit exactly into 365 calendar days.

不过，一个季节循环年并不能正好装进365个公历日。

The Gregorian calendar uses common and leap years to stay close to this cycle of seasons.

公历用平年和闰年来接近这个季节循环。

So the solstice time shifts by several hours each year, and a leap day shifts it earlier on the calendar.

因此，冬至时刻每年会移动几个小时，而闰日又会把它向前调回。

Crossing midnight or changing time zones can also change the date written on a calendar.

如果这个时刻跨过午夜，或换到另一个时区，日历上写出的日期也可能改变。

That is why Winter Solstice usually appears around December 21 to 23, not on one fixed date.

这就是冬至通常出现在12月21日至23日前后，而不是固定在某一天的原因。

Ancient observers lacked modern longitude numbers, but they watched the Sun and measured shadows with a gnomon.

古代观察者没有现代黄经数字，但他们会观察太阳，并用圭表测量日影。

Records say Zhou-period observers used shadows, and the solstices were among the earliest solar terms identified.

资料记载，周代观察者已经使用日影，而二至也是最早被确定的节气之一。

By the Han period, Winter Solstice was an official winter festival with rest and court celebrations.

到汉代，冬至已经成为带有休息和朝廷庆贺活动的正式冬节。

Later, state ceremonies, paying respect to ancestors, family meals, and visits gave the day wider social meanings.

后来，国家礼仪、敬祖、家庭饮食和往来又给这一天增加了更广的社会意义。

Ming and Qing emperors held a major Heaven-worship ceremony at Beijing's southern altar on Winter Solstice.

明清皇帝会在冬至于北京南郊举行重要的祭天礼。

Food customs, however, never formed one rule for the whole country.

不过，冬至食俗从未形成一条全国统一的规则。

Dumplings or wontons are common in many northern communities, while tangyuan and other rice foods appear in many southern places.

许多北方地区常见饺子或馄饨，许多南方地区则可见汤圆和其他米食。

Local and family choices still differ, so a custom should be linked to a named place.

各地和各家的选择仍有差异，因此说明习俗时应点明具体地区。

Today, the festival continues through family meals, community lessons, heritage work, and seasonal food sales.

今天，冬至通过家庭饮食、社区讲解、遗产传承和节令食品销售继续存在。

To recognize it, check the exact solar-term time, observe the short daylight, and ask how local people mark the day.

要辨认冬至，可以查看当年节气的准确时刻，观察较短的白昼，并了解当地人如何度过这一天。

Its moving date comes from astronomy and the civil calendar, while its meanings come from history and local practice.

它会移动的日期来自天文周期与民用历法的配合，而它丰富的意义来自历史和地方实践。