2025年9月21日
Equinox Sunset
Image Credit: Luca Vanzella
Explanation: Does the Sun set in the same direction every day? No, the direction of sunset depends on the time of the year. Although the Sun always sets approximately toward the west, on an equinox like today the Sun sets directly toward the west. After tomorrow’s September equinox, the Sun will set increasingly toward the southwest, reaching its maximum displacement at the December solstice. Before today’s September equinox, the Sun had set toward the northwest, reaching its maximum displacement at the June solstice. The featured time-lapse image shows seven bands of the Sun setting one day each month from 2019 December through 2020 June. These image sequences were taken from Alberta, Canada — well north of the Earth’s equator — and feature the city of Edmonton in the foreground. The middle band shows the Sun setting during the last equinox — in March. From this location, the Sun will set along this same equinox band again tomorrow.
Tomorrow’s picture: equinox on Saturn
春分日落
影像提供: Luca Vanzella
说明: 太阳每天都在相同的方向落下吗？太阳每天都在同一个方向落下吗？并不是。日落方向会随着一年中的时节而变化。虽然太阳大致上都是向西方落下，但在像今天这样的春分或秋分时，太阳会正好落在正西方。从明天的九月秋分点开始，日落方向将逐渐偏向西南方，并在十二月冬至时达到最南的极限。而在今天的九月秋分点之前，太阳则是逐渐偏向西北方向落下，并在六月夏至时达到最北的位置。这张展示的缩时影像显示了七条日落轨迹，每一条分别对应自2019年12月至2020年6月间的某个月份。影像拍摄地位于加拿大 阿尔伯塔省（在地球赤道以北的高纬度地区），前景则是埃德蒙顿市。当中的中间轨迹便是春分（3月）时的日落位置。从这个地点观测，明天的日落将再次沿着这条「春分带」落下。
明日的图片: equinox on Saturn
