2025年11月22日
Dione and Rhea Ring Transit
Image Credit & Copyright: Christopher Go
Explanation: Seen to the left of Saturn’s banded planetary disk, small icy moons Dione and Rhea are caught passing in front of the gas giant’s extensive ring system in this sharp telescopic snapshot. The remarkable image was recorded on November 20, when Saturn’s rings were nearly edge-on when viewed from planet Earth. In fact, every 13 to 16 years the view from planet Earth aligns with Saturn’s ring plane to produce a series of ring plane crossings. During a ring plane crossing, the interplanetary edge-on perspective makes the thin but otherwise bright rings seem to disappear. By November 23rd Saturn’s rings will have reached a minimum angle for now, at their narrowest for viewing from planet Earth, but then start to widen again. Of course, Dione and Rhea orbit Saturn near the ring plane once every 2.7 and 4.5 days respectively, while the next series of Saturn ring plane crossings as seen from Earth will begin again in 2038.
Tomorrow’s picture: everything, everywhere, all at once
土卫四与土卫五的凌环现象
影像提供与版权: Christopher Go
说明: 在这张清晰的望远镜照片中，可以看到土星环状行星盘左侧的两颗小型冰卫星——土卫四（Dione）和土卫五（Rhea）——正从这颗气态巨行星庞大的环系统前方掠过。这张非凡的照片拍摄于11月20日，当时从地球上观察，土星环几乎呈现侧向视角。事实上，每隔13到16年，地球的视线就会与土星环平面对齐，产生一系列的环平面穿越现象。在环平面穿越期间，由于行星间的侧向视角，原本纤细但明亮的土星环看起来似乎消失了。到11月23日，土星的环将达到目前的最小角度，从地球观察时最狭窄，但之后将开始再次开始扩张。当然，土卫四和土卫五绕土星运行，分别每2.7天和4.5天经过环平面，而从地球观察的下一次土星环平面穿越系列将于2038年再次开始。
