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Leap Year 2024: Why Do We Get an Extra Day?

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The white sun shines in the distance behind blue Earth.
Earth and Sun — image captured from the International Space Station as it orbited above Canada.  (NASA)

How many of you were born on Feb. 29? Well, happy upcoming birthday, 2024! You get to celebrate in your actual birth month, not your surrogate party day, March 1.

An extra day to play

Leap year is upon us, giving us a chance once again to get some extra stuff done and ponder one of nature’s cycles — in this case, the ongoing role of Earth spinning through the days and revolving around the years.

Every four years, we add an extra day to the calendar, tacked onto the end of February. Why bother? Why not just celebrate the textbook 365 days every year?

Earth’s clock ticks to its own rhythm

The answer is that Earth doesn’t keep perfect time like your smartphone does (neither does your smartphone, really, but it periodically syncs to a highly accurate atomic clock time to give you that impression). And, like your smartphone aligning itself to atomic time, Earth and the solar calendar must be occasionally synched to make up the difference.

The common (calendar) year is 365 days, compared to the tropical year, which is 365.24 days. Every four years, an extra day is added to the calendar on Leap Year to make up the difference.

The solar calendar is based on the seasonal cycle of the sun and Earth, where natural events like the winter and summer solstices, or the vernal and autumn equinoxes, happen on the same calendar days every year. And the calendar would remain in step with those events if Earth actually took exactly 365 days to go around the sun. Noon on the winter solstice would take place every 365 days without fail, and that would be that.

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In reality, Earth takes an extra quarter day (5.8 hours to be exact) to complete one orbit around the sun, so the exact time of winter solstice, or midnight on Jan. 1, or the moment of your birth gradually slides on the calendar.

Celebrating New Year, your birthday or whatever 5.8 hours later than last year would be no big deal, and you wouldn’t notice the difference from one year to the next anyway. But over time, the deficit builds up and eventually becomes noticeable without slowing down the calendar with that extra leap year day. Left unadjusted, events like the solstice or equinox would migrate several weeks on the calendar over your lifetime.

An illustration of a blocky satellite and large square clock.
In 2019, NASA launched the Deep Space Atomic Clock experiment on the General Atomics’ Orbital Test Bed spacecraft. DSAC is a technology concept for equipping future deep space missions with onboard atomic clocks for navigation instead of relying on Earth-based clocks to supply accurate time.

Nature’s beat

Our calendars and clocks are designed to track time in a rigid, metronome-like cadence, but the actual natural cycles they are based on are more fluid, like a symphony of string and wind instruments. The drum-beat tempo of the clock must follow the lead of nature’s flowing composition.

Even the speed at which Earth moves around the sun does not remain steady but grows and ebbs cyclically as our planet falls along its elliptical orbit, speeding up and slowing down like a rollercoaster car on a looping track and making the Northern Hemisphere’s summer season about five days longer than winter.

So, enjoy the ride! This year, you have an extra day to do it.

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