How a Seasonal Snarl-Up in the Mid-1500s Gave Us Our Strange Rules for Leap Years

aristarchus writes:

Over at The Conversation, a good explanation of the relation between solar system celestial mechanics and calendars.

Happy February 29! It doesn't come round very often, so make sure you enjoy it.

But why do we have these extra days? Well, if we didn't, the seasons would gradually move around the calendar. Rather than midsummer in the southern hemisphere falling around December 21, it would arrive in January, then February, and so on. After a few centuries, the Australian summer would end up be in July!

And Australians are confused enough as it is. Also, nice to talk about something not scaring the scaredy among us.

The sidereal year
The "sidereal year" is the classic classroom definition. It's the time it takes Earth to complete one lap of the Sun and return to exactly the same place in its orbit, judged by the position of the Sun relative to the background stars.

But one sidereal year doesn't take 365 days. Rather, it takes 365.256 days.

Then there's another problem. In addition to spinning on its axis (which gives us day and night), our planet also wobbles. More accurately, Earth's axis "precesses", spinning around once every 26,000 years or so, like a wobbling spinning top.

Then, there is this:

The tropical year
Fortunately, we have another way to define a year that can fix this problem. Instead of measuring the exact time it takes to orbit the Sun, we can instead measure the time between the vernal equinox of one year and the next.

The vernal equinox is the point in Earth's orbit where the Sun moves from the southern hemisphere of our sky to the northern one. Each year it falls on or around March 21.

The time between one equinox and the next is called the "tropical year", and is slightly shorter than the sidereal year. It comes in at 365.24219 days.

This difference is pretty small (about 20 minutes), but it equates to the amount that Earth's axis has precessed in that time - just under 1/26,000 of a full lap.

Article goes on to compare the Julian calendar, and the (modified) current calendar:

The Gregorian calendar
To fix this very slow drift, a new calendar was devised in the second half of the 16th century. Named after Pope Gregory XIII, the Gregorian calendar was released in 1582.

Of course, given relativity, a flat earth, and the administration's attitude toward science, oh, and Daylight Saving Time, we are lucky just to find lunchtime in any given day. Happy 29th!

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