Draconitic Year

Draconitic Year[1]

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.

Because the paths of the Sun and Moon are at an angle to each other, the moon crosses the path of the Sun twice, each time the Moon goes once around Earth, once every month.

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.

Draconitic 346.620075883
Lunar 354.370000000
Tropical 365.242189000
Calendar 365.250000000
Sidereal 365.256363004
Anomalistic 365.259636000

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?”

[1] Sun, Moon and Stars (February 26, 2013) and Astronomy (January 2016, p. 12)

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