sunrise/sunset variance?

Question

Why do sunrise/sunset times vary from year to year for a particular place and date? Example - the calculated sunrise/sunset for Ray, Minnesota on November 15, 2005 and for Ray Minnesota on November 15, 2006 are slightly different. What factor accounts for this year to year change? I have seen the nasa site for calculating the sunrise/sunset times but I do not understand what the change is from year to year. The variation looking back is only between a few minutes. Thank you for your response.

Answer 1

It is because the year is not 365 days long, it is closer to 365 1/4 days long. That is why we need a leap year every 4 years. That means that after 365 days the Earth is 1/4 day short of completing one complete orbit. There are 360 degrees in a complete orbit which takes 365 1/4 days to complete, so the Earth moves just a little less than 1 degree per day around its orbit. This has a small effect on the exact time of sunset and sunrise.

A regular day, called a mean solar day, is the time from one sunset to the next and is exactly 24 hours long. But a sidereal day, which is the time from one star set until then next star set (for the same star), is 23h 56m 4s long. The 3 minute 56 second difference is the extra time required for the Earth to rotate that extra degree to again bring the Sun exactly to the horizon as seen from a position one degree farther along its orbit. Now if sunset is at some time on some day this year, 365 days later the Earth is 1/4 of a degree short of where it was in its orbit the previous year and sunset is 1/4 of 3m 56s, or 59 seconds, early. The second year of 365 days and it is off by twice 59 seconds (1m 58s) and after the 3rd year it is off by three times 59 seconds (2m 57s). After 4 years it is a full degree, and a full day, short of an orbit so sunset should be the full 3m 56s early, but we add a day to the year every 4 years which cancels out the error. So the sunset time should be 1/4 of 3m 56s earlier each year for 3 years and then back on schedule the 4th year. Approximately. There are other factors too, but I think that should be the major one.

Answer 2

Understand and accept the following - we rotate thru the day in approx 24 hours- we revolve around the sun in approx 365 days.

Therefore - we do not return to the same exact point in space at the same time every year. So after a year, we are close but the earth is not exactly pointing the way it was a year ago. The beginning of the seasons can vary over a three day period.
If you could plot the times I am sure they would repeat, but I do not know of the cycle length.