单词tropical year
na.: 【天】太阳年
网络: 回归年
【天文学】太阳年,回归年
In terms of how we keep track of time, it is easiest to go by the solar day and the tropical year.
说到我们怎么计算时间，最简单的方法使用太阳日和回归年。
Earth spins around its axis approximately 365. 24219 times annually. This rate is known as the mean tropical year.
地球每年绕其轴线自传365.24219次，也就是我们所知的太阳年。
So, the tropical year is a little bit shorter than the sidereal year.
因而回归年要比恒星年要短一些。
Why would this tropical year be different than the solar year?
那为什么回归年会和恒星年不同呢？
Here is the key point: the solar day and the tropical year aren"t locked in anyway.
关键在于：太阳日和回归年相互并无关联。
Xing Yunlu"s Determination of the Length of Tropical Year Revisited
邢云路测算回归年长度问题之再研究