Starts With A Bang

# Why today really isn’t 24 hours long

As the Earth spins and wobbles on its axis and revolves elliptically around the Sun, each day changes from the last. “24 hours” isn’t right.
Key Takeaways
• On average, over the course of a full calendar year, the amount of time it takes Earth to spin completely around is marked by our 24 hour day.
• But the true definition of a “day” isn’t a full 360 degree revolution of our world, but rather the amount of time it takes the Sun to return to the same position in Earth’s sky, day after day.
• While 24 hours is about right on average, it’s not true for nearly every day, including today. Here are the scientific reasons why.

In marking the passage of time, we’ve assigned 24 hours to each and every day.

While that’s a day’s length on average, most days aren’t actually 24 hours.

Counterintuitively, a day isn’t the time required for a planet-wide 360° rotation.

We rotate 360° each 23 hours, 56 minutes and 4.09 seconds, leaving us 00:03:55.91 short.

A full rotation, astronomically, is a sidereal day: different from a solar (calendar) day.

Conventional days are defined by the Sun returning to its prior position the day before.

This requires accounting for Earth’s motion through space.

Earth requires ~1° of additional rotation to account for its daily motion around the Sun.

That “extra” 0.9856° of rotation equates to an additional 235.91 seconds, lengthening the solar day to 24 hours.

But Earth’s orbital speed also varies, moving faster near January’s perihelion and slower around July’s aphelion.

Nearest the Sun, Earth orbits at 30.3 km/s, while at its farthest, it moves at 29.3 km/s.

Factoring in our varying speed and our non-circular, oblique trajectory, each day’s length varies by several seconds throughout the year.

Those variations explain our analemma’s “figure 8” shape.

Only four times each year will your day actually be precisely 24 hours long.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.

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