What is the winter solstice, and why do we have it？
什么是冬至？为什么有冬至这个节气？
"Earth's axis is tilted 23.5 degrees relative to its orbit, and on December 21, Earth will be at the point in its orbit when the North Pole is tilted at its maximum away from the sun," Schneider told CBS News. "The effect of this in the Arctic is completely in darkness as Earth spins that day."
施奈德说：“地轴相对于自转轨道的倾角为23.5度。12月21日，地球的北极距离太阳最远。这会导致当天北极圈地区完全见不到太阳。”
For those of us in the Northern Hemisphere, it's the day with the shortest amount of daylight — less than 12 hours — and our longest night of the year. However, that's not the case for every**. While it's winter for those of us in the Northern Hemisphere, people in the Southern Hemisphere experience it as their summer solstice with the longest stretch of daylight.
对北半球的人们来说，冬至这一天白昼最短，不到12小时，夜晚最长。但并不是对所有人都这样。北半球经历寒冬时，南半球在这一天迎来白昼最长的夏至日。
When does the winter solstice occur？
冬至在哪天？
The Earth's North Pole will be tilted farthest from the sun at 11:27 am ET on Thursday, Dec. 21, 2017, according to the National Weather Service.
根据美国国家气象局的数据，今年的冬至发生在美国东部时间2017年12月21日上午11点27分(北京时间2017年12月22日零点27分)，此时北极距离太阳最远。
Why isn't the winter solstice on the same day each year？
为什么每年的冬至都不是同一天？
The date of the winter solstice varies from year to year for people in different time z**s. It typically occurs around Dec. 21 or 22, though on rare occasions it can be as early as Dec. 20 or as late as Dec. 23, according to the Weather Channel.
根据天气频道的数据，对不同时区的人们来说，冬至的日期每年会有变化。多数年份的冬至日都在12月21日或者22日，偶尔提早到20日，或者错后到23日。
That's because our calendars aren't a precise match to the solar year.
这是因为日历与阳历并非完全吻合。
Schneider writes: "Earth takes about 365 1/4 days to orbit the Sun. Next year, the moment of the solstice will be about 6 hours later at 5:23 pm ET. In 2019, it will be at 11:19 pm ET, so for people living in Pu**o Rico and other time z**s to the east, the date of the solstice will be on the 22nd."
施奈德写道：“地球围绕太阳公转周期为365又1/4天。明年的冬至比今年大约迟6个小时(美国东部时间下午5点23分)，后年又推后到美国东部时间晚上11点19分。所以对于居住在波多黎各及以东时区的人们来说，冬至日期是22日。”
Is the winter solstice the coldest day of the year？
冬至日是全年最冷的一天吗？
The winter solstice is not normally the coldest day of the year. There is actually a lag between the shortest day of the year and the coldest average temperatures, the National Weather Service reports.
美国国家气象局报告说，冬至日通常不是全年最冷的一天。白昼最短的一天与平均气温最低的一天之间其实有延迟。
"The coldest time usually comes a month or two later because even though the amount of solar heating is beginning to increase, it isn't yet enough to reverse the cooling," Schneider says. "This effect, called 'the lag of the seasons,' is similar to how when you turn down the heat while you're cooking — the pot won't immediately reach its coolest temperature."
施奈德说：“全年最冷的时候通常比冬至日晚一两个月，因为尽管日照**始变长，但仍不足以改变寒冷的趋势。这称为‘季节延迟’，就像你**饭时关火以后，锅的温度不会马上降低。”
Will shadows on the day of the solstice be longer？
冬至日这天影子会变长吗？
Noontime shadows on Thursday will be longest for those of us in the Northern Hemisphere because the sun doesn't rise as high in the sky. The closer you are to the Arctic, the longer the shadows (and the shorter your day) will be.
对北半球的人们来说，冬至这天中午时影子最长，因为太阳的高度最低。越靠近北极圈，影子越长，白昼时间越短。
Why doesn't the earliest sunset come on the shortest day？
为什么白昼最短的当天日落并非最早？
The winter solstice may be the shortest day of the year, but that doesn't mean you'll have an early sunset. The exact date of the earliest sunset depends on your latitude, so it's not the same for every**.
冬至日是全年白昼最短的一天，但并不意味着日落最早。日落最早在哪天取决于你所在的纬度，对每个地方的人来说都不同。