Tropical Year

Tropical 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.

A tropical year is the time it takes for the Sun to travel 360° along the ecliptic, its apparent path through the sky. Since the Sun doesn’t actually go around Earth, this should be restated as the time it takes Earth to complete a full orbit around the Sun. It varies from year to year, but to 8 significant figures it is 365.24219 days. The exact length of a tropical year can vary by up to around half an hour. For instance, the tropical year 2032 will last longer than 365 days and 6 hours. 2027, however, will only last 365 days, 5 hours, and 39 minutes.


The chart below shows the lengths of each of the years in days. Note how much shorter than the Calendar Year is the Sidereal Year—just a

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?”



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