Our modern calendars and clocks are based on the natural cycles of Earth's rotation and revolution around the sun. (NASA)
If you've looked closely at a calendar lately, you may have noticed the bonus day at the end of February.
Yep, 2020 is a leap year, and February 29, the rarest of days, gives us an extra 24 hours of winter—even if they feel more like spring. As an extra bonus, if your birthday falls after February, you get one more day being younger.
What will you do with your calendar dividend?
Leap Year Is a Lull in Time
Broadly speaking, the extra day is added to leap year for the same reason a clock needs occasional adjustment: a small error in the clock's ability to track time gradually builds up to an amount that we notice.
In the case of leap year, we add a day to the end of February (which doesn't have enough days anyway) to correct for a difference in the length of the calendar year and the actual length of an astronomical, or seasonal, year. Julius Caesar started the practice, based on influence from the Egyptian calendar.
We base our calendars and clocks on whole numbers, but the cycles of nature rarely care about that. The actual length of a year—the time it takes Earth to revolve once around the sun—is not precisely 365 days, but 365.2422 days. So, with each passing calendar year, the additional quarter-day compounds, adding up to a full day of mismatch every four years.
So, we add a "leap day" to slow our calendars down and let Earth catch up. Leap years occur on years that are multiples of four, except those divisible by 100 (in case you want to map out leap years of the future).
Other Kinds of Leaps
Much of the modern would uses the common Gregorian solar calendar, in which the first day of the year, January 1, always takes place at the same tick-mark in Earth's orbit around the sun. But many cultures still observe aspects of their own traditional calendar systems, for various purposes. These may include celebrating a cultural New Year, setting important events like weddings and traditional festivals, and other events specific to a culture and history.
The traditional Chinese calendar is a "lunisolar" system, based not only on the solar seasonal cycle, but the moon's phase cycle as well. And as usually happens when humans try to impose a bookkeeping mentality on nature's messy fractional cycles, the Chinese calendar is synchronized to the solar and lunar periods with its own leap year.
Each traditional Chinese month begins at new moon and is 29.5 days long, the period until the next new moon. Do the math. There are 12.37 lunar months in a year. So, to keep the lunar months in close synchronization with the seasons, the Chinese leap year has an extra month added, for a total of 13.
It may sound finicky, but our modern technological world has given birth to the "leap second."
The modern world sets its clocks to Coordinated Universal Time (UTC, also called Greenwich Mean Time). The highly precise UTC atomic clock is kept to within one second of mean solar time.
But while an atomic clock meters time with such precision that it will only deviate by a second in over a million years, the "Earth clock" doesn't follow in perfect lock-step. Mean solar time, based on the sun's average daily motion caused by the Earth's rotation, is gradually slowing, very minutely, requiring timekeepers to add a second every now and then to the UTC atomic clock.
And while the steady decline of Earth's spin, caused by tidal interactions with moon and sun, is predictable, other factors that affect its rotation are more variable. For example, the shifting bulk of ice caps and glaciers as they grow and melt (more melt lately), the rise and fall of magma and ocean currents, and other natural factors.
So, when timekeepers add a leap second depends on how Earth's rotational variations behave.
Strolling With Nature
It doesn't matter what natural cycle humans frame our timekeeping preferences around -- we have to make adjustments to make it fit, like a one-size-fits-none piece of clothing. Nature strolls to the tempo of its own symphony, and we have the glorious luxury of free-falling into Earth's clock, once every four years.
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