Year Lengths and the Earths Orbit

We conclude this subsection by noting that each of the types of years indicated in Table 4.2 will be of slightly different length from year to year; the reason is the Earth's variable speed in its orbit, and the variations that the orbit, and the Earth itself, undergo because of gravitational perturbations. The anomalistic year would be slightly more basic than the others, because in a two-body system, it is the cycle of orbital velocity variation, while the others involve some overlap and, therefore, different integrated values of cycle length. However, the Earth is not in a two-body system, and its orbital elements, including the semimajor axis and period, are continuously changing. The change in the anomalistic year is small, however, and can be ignored for relatively short intervals unless the highest precision is required. For most alignment and calendar questions, a four-place decimal precision in year length when expressed in days, and the use of the average mean solar tropical year length, will be sufficient for most applications.

We now review the general archeological and specifically astronomical methods of determining ages.

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