泰坦的影子凌土

2025年7月24日

Titan Shadow Transit

Image Credit & Copyright: Volodymyr Andrienko

Explanation: Every 15 years or so, Saturn’s rings are tilted edge-on to our line of sight. As the bright, beautiful ring system grows narrower and fainter it becomes increasingly difficult to see for denizens of planet Earth. But it does provide the opportunity to watch transits of Saturn’s moons and their dark shadows across the ringed gas giant’s still bright disk. Of course Saturn’s largest moon Titan is the easiest to spot in transit. In this telescopic snapshot from July 18, Titan itself is at the upper left, casting a round dark shadow on Saturn’s banded cloudtops above the narrow rings. In fact Titan’s transit season is in full swing now with shadow transits every 16 days corresponding to the moon’s orbital period. Its final shadow transit will be on October 6, though Titan’s pale disk will continue to cross in front of Saturn as seen from telescopes on planet Earth every 16 days through January 25, 2026.

Tomorrow’s picture: pixels in space

泰坦的影子凌土

影像提供与版权: Volodymyr Andrienko

说明: 大约每隔15年，土星倾斜的环面会与地球的视线几乎重叠。当土星明亮壮丽的环系变得越来越纤薄暗淡时，对地球的观测者来说，土星环也会变得难以察觉。不过，这也提供了观赏土星的卫星及其影子凌越土星明亮盘面的机会。其中，土星最大的卫星-土卫六(Titan 泰坦)-无疑是最容易观察到凌土的天体。在这张摄于7月18日的望远镜主题影像里，泰坦位于影像左上角，其圆形影子投射在土星条纹状的云顶之上，而纤薄的土星环亦清晰可见。事实上，目前正值泰坦凌土季的高峰，每隔16天便会上演一次影子凌土现象，这与其绕行土星的轨道周期一致。尽管最后一次泰坦影子凌土将发生在10月6日，然而如用地面的望远镜观测，泰坦暗淡的盘面仍会继续以相同的周期掠过土星前方，直到2026年1月25日方止。

明日的图片: pixels in space