This full-disk image from NOAA’s GOES-13 satellite shows the Americas at the start of astronomical summer in the Northern Hemisphere on June 21, 2012.
NASA
This full-disk image from NOAA’s GOES-13 satellite was captured at 7:45 a.m. EDT (11:45 UTC) and shows the Americas on June 21, 2012, the start of astronomical summer – in the Northern Hemisphere – that year.
The first day of summer in 2025 is June 20; it is also the longest day of the year. In the Southern Hemisphere, it’s the shortest day of the year and the beginning of winter.
Earth orbits at an angle, so the Northern Hemisphere is tilted toward the Sun half of the year — this is summer in the Northern Hemisphere, and winter in the Southern Hemisphere. The other half of the year, the Northern Hemisphere is tilted away from the Sun, creating winter in the north and summer in the south. Solstices happen twice per year, at the points in Earth’s orbit where this tilt is most pronounced.
Image credit: NASA
这张由NOAA的GOES-13气象卫星拍摄的全盘影像，展示了2012年6月21日夏至当天的美洲大陆，那一天也是北半球天文意义上夏季的开始。
NASA
该图像拍摄于美国东部时间上午7点45分（协调世界时11点45分）。2025年的夏季将于6月20日正式开始，这一天也是全年白昼时间最长的一天。而在南半球，这一天则是全年白昼最短的一天，也标志着冬季的开始。
由于地球绕太阳公转时自转轴存在倾斜，北半球会有半年朝向太阳，这段时间便是北半球的夏季、南半球的冬季；而在另一半公转轨道上，北半球背向太阳，进入冬季，而南半球则迎来夏季。每年这种倾斜最为明显的时刻，便是夏至和冬至，分别出现在一年中的两次分至点。
图片来源: NASA
