Recently we presented an article on the six definitions of a year. A year is not necessarily a year. “Annual” means yearly, but what is a year? There are actually 6 different years: Anomalistic year, Draconitic year, Julian year (this one you are familiar with — 365.25 days), Luna year, Sidereal year, and Tropical year.
The draconitic year, sometimes called the eclipse year, is the time it takes for the Sun to move from one lunar node (where the Moon’s path intersects the Sun’s) to the same lunar node from our perspective.
These points of crossing are called, lunar nodes, and they precess gradually westward, performing a complete circle in approximately 18.6 years: a draconitic or nodical period.
The Draconitic year is actually 346.620075883 days. (Yes, we can measure it that precisely, out to 9 places, or to 0.55629 seconds!)
The chart below shows the lengths of each of the years in days.
It surprises people that our common conception of how to measure time is not precisely fixed, and there is the question: “A year is a year is a year?”