2 Measuring vitamin D levels is also expensive, costing between $50 and $220 per test in commercial laboratories for patients without insurance. Another challenge is that the level of 25-hydroxy-vitamin D that should be considered normal is unclear.
The closer a planet is to the sun, the shorter the time to complete one revolution is. Mercury, for example, has an orbital period of 88 days while it takes Jupiter about 11.86 years. The period of the Earth's orbit is usually thought to be 365 days as calendars show. However when averaged it is actually about 365.25 days, which requires an extra day on the calendar every four years (causing a leap year). Contradictions within orbit times are probably caused by the ages of the sources.
There are 4 Eons on the geological time scale in total. Era - 12 total, spanning several hundred million years each. Period - a timespan longer than an Epoch, but shorter than an Era. Epoch - spans tens of millions of years each. All of these time spans relate events recorded in the stratigraphy to a certain point (or span) in the history of the Earth.