Only four days each year are actually 24 hours long

To mark the passage of time, we assign 24 hours to each day, universally.

While that’s accurate for a day’s average length, most days aren’t 24 hours long.

Counterintuitively, a day isn’t the time required for Earth to rotate 360°.

We rotate 360° in 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 complete when the Sun returns to its earlier position from the previous day.

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 adding 235.91 seconds, extending the average 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 travels 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 per year will Earth’s day endure for precisely 24 hours.

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

This article was first published in June of 2023. It was updated in September of 2026.

This article is featured on Big Think.