Astronomy Picture of the Day——土星在昼夜平分点
Saturn at equinox
Image Credit:NASA
Saturn is famous for its bright, glorious rings but in this picture, taken during Saturn's 2009 equinox, the rings are cast in a different light as sunlight hits the rings edge-on.
The equinox is a point in a planet's orbit where the Sun shines directly overhead at the equator. It occurs twice per orbit and on Earth it happens in March and September. At the equinox, day and night are almost equal and the Sun rises due east and sets due west. This year, for northern hemisphere dwellers, the spring equinox occurs on 20 March.
Further afield, the international Cassini mission captured a Saturnian equinox for the first time on 12 August 2009. Saturn's equinoxes occur approximately every 15 Earth years and the next one will take place on 6 May 2025.
土星在昼夜平分点
土星以其光彩夺目的光环而闻名，这张照片是在2009年土星春分由卡西尼号拍摄的，阳光照射在土星环的边缘。
昼夜平分点是行星轨道上的一个点，太阳直射在赤道上方。每个公转周期发生两次，地球发生在三月和九月。在昼夜平分点，白天和黑夜几乎相等。
卡西尼号于2009年8月12日首次捕获了土星昼夜平分点。土星的昼夜平分点大约每15个地球年出现，下一次将于2025年5月6日发生。
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近期文章：
Astronomy Picture of the Day——超新星大炮驱逐脉冲星J0002