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.
However, the earth's orbit takes 365.24 days to complete its solar orbit and an extra day every four years is added to keep calendar seasons synchronised with solar seasons. February was chosen as the month to have the leap day as it is the only month without 30 or 31 days. It has long been suggested that February only has 28 days due to the jealousy of Roman Emperors. According to the theory, the second month of the year had 30 days before the reign of Roman Emperor Augustus.