16-17 世纪是欧洲天文学发生重大变革的时期，主要体现在从古代数学天文学与物理天文学截然二分，到走向融合；从用天球的匀速圆周运动解释行星运动的不均等性的传统天文学，转变到探寻行星真实运动轨道的物理天文学。开普勒（Johannes Kepler,1571-1630）在这个转变过程中做出了重要的贡献。现代早期，欧洲绝大多数天文学家依然相信天球真实存在，作为动力携带行星做匀速圆周运动，但为了解释行星视运动的不均等性而构造的本轮等工具是否真实存在有一定的争议。持“工具主义”立场的天文学家将宇宙模型作为数学计算的工具，不考虑其真实性问题，而持“实在论”立场的天文学家则强调模型与天界实际情况相对应。在开普勒的年代，实体天球的观念开始发生变化，第谷通过自己精确的观测质疑了固体天球的存在。开普勒在 1596 年出版的《宇宙的奥秘》中已经将“天球（orbis）”视作行星运动的几何边界，而非真实存在于天界，但他在这本书中没有明确强调几何天球的观念。在 1600 年左右写作的当时未出版的《为第谷反驳乌尔苏斯辩护》一书中，他开始明确表示不存在实体天球。在这本书中，开普勒通过区分几何假说与天文学假说，尝试将物理学与数学天文学相结合，认为通过构建几何模型的方式解释行星运动的不规则性并非天文学家的任务，进而明确了天文学的任务是寻找行星运动的唯一真实的路径。他的宇宙体系不再以天球为基本观念来构造，而是使用轨道这一概念来表示自己的物理天文学诉求。根据第谷精确的观测数据，开普勒在《新天文学（1609）中完成了对火星轨道的研究，实现了自己提出的天文学任务，即找到行星在物理原因的驱动下围绕太阳运动的唯一路径，并用“轨道（orbita）”一词来表示这个与传统天文学完全不同的物理天文学概念。从《宇宙的奥秘》到《辩护》再到《新天文学》，开普勒实现了从天球到轨道观念的转变。在开普勒的文本中，对这两个词汇的使用有明显的语境区分，通过对这两个词汇的含义和其使用的分析，可以看出开普勒新天文学工作中的观念转变过程。
The 16th and 17th centuries were a period of great changes in astronomy, mainly from the separation of ancient mathematical astronomy and physical astronomy to the integration; from the traditional astronomy explaining the inequality of planetary motion with uniform-circular motion to the physical astronomy exploring the real orbit of planets. Johannes Kepler (1571-1630) has made an important contribution to this transition process. In the early modern Europe, most astronomers still believed that the solid orbs really exist and carry the planets in uniform circular motion, meanwhile, the epicycles and eccentrics which were constructed to explain the inequality of planetary motion were pure mathematical tools. "Instrumentalist" considered the astronomical model as a tool for mathematical calculation, regardless of its authenticity, while "realist" emphasize that the model corresponds to the actual situation of the heaven.In Kepler‘s years, the idea of the solid orbs began to change, and Tycho Brahe (1546-1601) questioned the reality of the solid orbs through its own precise observations. In his Mysterium Cosmographicum, Kepler considered the celestial orbs as the geometric boundary of planetary motion, rather than the real, solid entities, although he makes no explicit expression in this book. But in the subsequent unpublished book Kepler’s Defence of Tycho against Ursus，he believes that to explain the inequality of planetary motion by constructing geometric models are merely pure geometrical work, while finding out the true path of the planet is a proper astronomical work. He distinguished the geometric hypothesis from the astronomical hypothesis, trying to combine physics with mathematical astronomy, thus clarifying that the task of astronomy is to find the real path to planetary motion driven by physical causes. His cosmic system is no longer constructed with the basic concept of the celestial orbs, but uses the concept of orbit to express his own physical astronomy. Through the work of Astronomia nova, Kepler accomplished his own mission by finding the only path of the planets around the true sun, and using the word "orbit (orbita)" to represent this completely different concept from traditional astronomy. During the years of writing Mysterium Cosmographicum , Kepler’s Defence of Tycho against Ursus and Astronomia nova, Kepler finished the transition from the basic concept of celestial orbs to the concept of orbit in astronomy. There are obvious contextual differences in the use of the two words in Kepler’s books. By distinguishing the two words helps modern researchers to understand the conceptual change process in Kepler‘s working on new astronomy.
