可能有人认为，冬至之日，是北半球全年夜长最长的一天，理应也是日出最迟、日落最早的一天。实则 不然。这是因为地球的自转轴倾斜约23.5度，并以椭圆轨道绕太阳公转，导致每日的正午时间略有偏 差，称为均时差。 新华社天津12月19日电（记者周润健）12月21日迎来冬至，这是北半球白昼时间最短、夜晚最长的一 天。这一天我国各地的夜晚到底有多长？这一天是日出最晚且日落最早的时候吗？听天文科普专家一一 道来。 冬至是二十四节气中的第二十二个节气，也是冬季的第四个节气。中国天文学会会员、天津科学技术馆 天文科普专家刘仲利介绍，冬至这天，各地的夜长时间从南到北呈递增趋势。越是靠北，黑夜越长；越 是靠南，黑夜越短。这是因为地球围绕太阳公转，地轴是倾斜的，导致太阳直射点在地球的南北回归线 之间往复运动，进而表现为各地的昼夜长短不一。 昼短夜长的这种变化，纬度越高的地区越明显。福建省天文学会会员、天文科普专家许长榕通过不同城 市在今年冬至日的日出、日落时间计算出各城市当天的黑夜长度，可以明显看到这种效应，例如哈尔滨 夜长约15时20分、北京夜长约14小时40分、南京夜长约13小时57分、海口夜长约13小时05分。 "以此推算，我国 ...

Group 1 - The summer solstice occurs on June 21, marking the longest day of the year in the Northern Hemisphere, with daylight duration increasing as one moves north [1][4] - The term "summer solstice" refers to the moment when the sun reaches its northernmost point in the sky, directly overhead at the Tropic of Cancer, resulting in the shortest shadows at noon [4][10] - Despite the longest daylight duration, the latest sunset does not typically occur on the summer solstice due to the difference between true solar time and mean solar time, with the latest sunset usually happening in late June [7][10] Group 2 - The difference between true solar time and mean solar time is known as the equation of time, which varies throughout the year, ranging from -14 minutes to +16 minutes [7] - After the summer solstice, the ground continues to absorb more heat than it releases, leading to rising temperatures until the end of summer [10] - Ancient Chinese observed shadow lengths to determine seasonal changes, with the phenomenon of "no shadow" occurring at noon on the summer solstice for regions near the Tropic of Cancer [10]