每年公历12月21日或22日，太阳到达黄经270度时，“冬至”节气开始，它是二十四节气中第二十二个节气，也是冬季的第四个节气。“冬至日”这天，太阳几乎直射南回归线，北半球阳光最少，白天最短，“终藏之气，至此而极也”。此后虽夜渐短而昼渐长，但地表热量积蓄越来越少，一年中最冷的“数九寒天”到来，须经九九八十一天，方能春回大地。(新华网）
IN 2021, THE winter solstice for the Northern Hemisphere happens on December 21. While not as dramatic as 2020's conjunction of Jupiter and Saturn, the longest night of the year is a significant turning point. Astronomical winter begins, and the days gradually get longer again.
Curious why this happens? WIRED spoke with Tansu Daylan, a Transiting Exoplanet Survey Satellite (TESS) postdoctoral associate at MIT, to better understand the winter solstice and our planet’s relationship with the sun.
To picture what is happening in space during a winter solstice, start by thinking of a giant glass ball surrounding the Earth, and let’s ignore the planet’s rotation (which complicates everything). Daylan says, “If you look at the three-dimensional sphere that’s around us, known as the celestial sphere, the sun as well as all other solar system objects move through a plane in this celestial sphere, known as the ecliptic plane.”
“While doing so, the sun changes its declination,” he says. Declination and right ascension are the two primary axes of the celestial sphere. “In this reference frame, the sun is at the most southern location when it’s the winter solstice from our perspective in the Northern Hemisphere.”
A winter solstice in the Northern Hemisphere happens when the North Pole is tilted farthest away from the sun. The Arctic Circle is cloaked in darkness and experiences its longest night of the year. In the southern hemisphere, a summer solstice simultaneously happens with the South Pole angled towards the sun and the Antarctic Circle experiencing midnight sun. （wired.com）