Astronomy Picture of the Day——夜光云、倒影与剪影
Noctilucent Clouds, Reflections, andSilhouettes
Image Credit & Copyright: PeterSimmering
Explanation: Sometimes it's night on theground but day in the air. As the Earth rotates to eclipse the Sun, sunsetrises up from the ground. Therefore, at sunset on the ground, sunlight stillshines on clouds above. Under usual circumstances, a pretty sunset might bevisible, but unusual noctilucent clouds float so high up they can be seen wellafter dark. Normally too dim to be seen, they may become visible just aftersunset during the summer when illuminated by sunlight from below. Noctilucentclouds are the highest clouds known and thought to be part of polar mesosphericclouds. Featured here as they appeared two weeks ago, a network of noctilucentclouds was captured not only in the distant sky but in reflection from a smalllake just north of Zwolle, Netherlands, with trees in stark silhouette acrossthe horizon. Unusually bright noctilucent clouds continue to appear over muchof northern Europe. Much about noctilucent clouds has been discovered only overthe past decade, while how they form and evolve remains a topic of activeresearch.
夜光云、倒影与剪影
有时候地面上到了夜晚，但天空中仍是白昼。当地球因自转遮挡了太阳时，日落便从地面开始。因此，在地面上的日落时分，仍然能看到阳光照耀着天空中的云层。在通常情况下，我们可以看到美丽的落日，但在这个不同寻常的夜光下云漂浮得如此之高，以至于在天黑后还能看到。夜光云通常因太过黯淡而难以被发现，它们往往在夏季的日落之后，当阳光从下方照耀时，才能显现出来。夜光云是已知最高的云层，它被认为是极地中间层云的一部分。这幅特征图像拍摄于两周之前，它呈现了荷兰.兹沃勒市北部遥远天空中的网状夜光云和一片小湖中的夜光云倒影，以及地平线上显著的树木剪影。这种异常明亮的夜光云不断出现在北欧的大部分地区。关于夜光云的多数发现都来自于过去十年，而它们是如何形成和演化的仍然是一个有待研究的课题。
参考文献：
https://apod.nasa.gov/apod/image/1906/NoctilucentNetherlands_Simmering_5766.jpg
近期文章：
Astronomy Picture of the Day——夜空中最亮的25颗星