|Earth Science. Englewood Cliffs, NJ: Prentice Hall, 1987: 80.||"365 days in a period of revolution in earth time"||365 days|
|Random House Encyclopedia. New York: Random House: 174.||"Time to orbit sun - 365 days 6 hours 9 minutes 9 seconds"||365.25635 days|
|Duncan, David Ewing. Calendar. 1st edition. New York: Avon, 1998: 202.||"365 days 5 hours 48 minutes 46 seconds (historical length of the tropical year)"||365.24220 days|
|Davis, Don and Amy Regourd. Our Sun and the Inner Planets. BLA, 1978.||"365.25 days Orbital Period"||365.25 days|
|Earth Planet Profile. Welcome to the Planets. NASA/JPL, 1995.||"Revolution period (length of year in Earth days) 365.26"||365.26 days|
The Earth is one of the nine planets that orbit around the sun in elliptical orbits. An orbit is the time it takes for a planet or other satellite to go around the sun and end up in the same place it started. 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, I believe that the time given by the NASA website is more right because of NASA specialized experience of space and because its web pages are constantly being updated with accurate information. I also found that the year decreases due to a gradual slowing of the earth's rotation by a rate of half a second per century. So far through 20 centuries, the year has slowed down by 10 seconds.
The atomic clock is another accurate source, however it recorded 365.24 days, which is lower than NASA's results by about 8 minutes. Between these two results, I go with the atomic clock's results since atomic clocks examine atoms and their age and are accurate to the quadrillionth of a second. Atomic clocks use oscillations of atoms instead of pendulums or other mechanical devices and are thus extremely accurate. Their oscillations are compared to the atomic system and then a time is calculated.
Mitchell Krasnerman -- 1999