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.
The orbit of a planet around the Sun measured with respect to the âfixed starsâ is used to determine the sidereal period. The sidereal period of the Earth is 365.25636 mean solar days, which is slightly longer than the tropical year (measured between successive vernal equinoxes) of 365.24219. In most calendar systems, the length of the year is taken to be 365 days, which is close to the Earthâs sidereal period.