Sunrise and sunset, cycling endlessly — this is the law of nature.
Tips
history
From 《汉书·礼乐志》: '精健日月,星辰度理,阴阳五行,周而复始' — in describing the cosmos, the Han historians used this phrase for the turning of yin-yang and the five elements. Originally cosmological, now applied to anything cyclical.
grammar
Note the required apostrophe in pinyin: zhōu'érfùshǐ, because 而 begins with a vowel after 周.