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.
How long has the earth been here? ...a very long time, at least according to the way humans measure time. Based on relative and absolute age dating techniques, geologists now think that the earth was formed about 4,600,000,000 years ago. One of the challenges of any study of the earth is being able to comprehend just how long this is, and for what an extremely small part of it humans have been here.