And it’s not because of leap seconds; it’s a fundamental property of most days.
Human beings, in marking the passage of time, account for each day equally: with 24 hours.
However, 24 hours is only the length of one Earth day on average; in reality, most days are either longer or shorter.
A day isn’t the time it takes Earth to rotate 360°, which leaves us 3 minutes and 55.91 seconds short.
That’s what astronomers call a sidereal day, quite different from a common, solar day.
We need the Sun to return to its previous day’s position, and that requires accounting for Earth’s motion through space.
Owing to its revolution around the Sun, the Earth must rotate approximately 361° to mark a solar day.
That extra rotation takes 235.91 seconds, which is why our solar day is 24 hours on average.
But Earth’s orbital speed isn’t uniform: it’s faster near perihelion (early January) and slower near aphelion (early July).
Earth’s actual motion around the Sun varies from a low of 29.3 km/s to a high of 30.3 km/s.
Factoring this in, our day’s length varies by about ±4 seconds throughout the year.
This is why our analemma doesn’t create a symmetric shape.
Only four times annually, latitude-dependent, are days actually exactly 24 hours.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.