Ask what a day is and the honest answer is a question back: which day do you mean? The number on your wall clock, 24 hours, is an average that has been rounded off and frozen for convenience. The Earth itself keeps at least two different days at once, neither of them exactly 24 hours, and the length of the one we live by is very slowly changing. None of this is speculation. It falls straight out of two facts you already know, that Earth spins and that Earth orbits the Sun, and it is worth walking through because it explains why timekeepers occasionally have to nudge the world's clocks to keep them honest.
Two things happen every day, not one
A day, in the everyday sense, is one turn of the Earth. But a turn relative to what? There are two natural reference points, and they give two different answers.
The first is the Sun. A solar day is the time from one local noon to the next, one appearance of the Sun on your meridian to the following one. This is the day human life is built around, because it tracks daylight. On average it is 24 hours, and that average is exactly where the 24-hour figure comes from.
The second is the stars. A sidereal day is one rotation of Earth measured against the distant stars, which are so far away that they make an effectively fixed backdrop. This day is about 23 hours, 56 minutes, and 4 seconds, close to four minutes shorter than the solar day. Astronomers care about it because the coordinates of stars are tied to the sky, not to the Sun, so a sidereal clock tells you which part of the sky is overhead right now.
The gap between the two is not a rounding error or a measurement quirk. It is a direct consequence of Earth doing two motions at the same time, and once you see why, the whole subject clicks into place.
Why the star day is nearly four minutes shorter
Picture Earth at noon, with the Sun directly over your meridian and a particular distant star also crossing overhead. Now let Earth spin exactly once, a full 360 degrees measured against that star. The star is back on your meridian, so one sidereal day has passed. But during that same spin, Earth has also travelled a small distance along its orbit around the Sun, roughly one degree of the way around. From your new position the Sun is no longer quite on your meridian; it has slipped back by about that same degree.
To bring the Sun back overhead and complete a solar day, Earth has to keep turning for a little longer, about one extra degree of rotation. One degree of spin takes roughly four minutes, which is exactly the amount by which the solar day exceeds the sidereal day. So the solar day is longer precisely because Earth has to rotate a bit extra each day to catch up with its own orbital motion and face the Sun again.
That daily four-minute head start the stars gain on the Sun compounds. A given star rises about four minutes earlier each night, so over a month it shifts by roughly two hours, and across a full year the accumulated slippage comes to one whole extra rotation. A year holds about one more sidereal day than it holds solar days, and that single bonus rotation is the orbit itself, quietly folded into the count. If you want to watch the two day lengths run side by side, the sidereal day clock shows a solar clock next to Greenwich sidereal time so the drift is visible in real time.
Even the solar day is not a constant
Here is the part that surprises people: the 24-hour solar day is only an average. The interval between one real noon and the next is not the same on every date. Two features of Earth's motion make it wander through the year.
The first is that Earth's orbit is an ellipse, not a circle. When Earth is closer to the Sun it moves faster along its orbit, and when it is farther away it moves slower. Since the length of a solar day depends on how far Earth has swung around the Sun during that day, the faster and slower stretches of the orbit lengthen and shorten the true solar day.
The second is that Earth's axis is tilted relative to its orbit. The Sun's apparent path across the sky is angled against the equator, and projecting that tilted motion onto the daily east-to-west track changes how far the Sun appears to move along the meridian direction each day. This too speeds up and slows down apparent noon depending on the season.
Add those two effects and you get the equation of time: the running difference between what a sundial reads (apparent solar time) and what a clock reads (mean solar time). Across the year the Sun can run ahead of the clock or behind it by up to about a quarter of an hour, which is why a well-made sundial carries a correction table. It also explains a familiar puzzle, that the earliest sunset and the latest sunrise do not fall exactly on the shortest day. The seasonal swing in daylight itself, separate from this noon-timing effect, is easy to see on the daylight chart, which plots how day length changes through the year for any location.
The day is slowly getting longer
So far everything has been about variation within a single year, effects that average out. But there is a genuine long-term trend on top of them: the mean solar day is slowly getting longer.
The main cause is the Moon. The Moon raises tides in Earth's oceans, and the friction of those tides against the rotating Earth acts as a gentle brake, bleeding rotational energy out of the planet. That energy is not lost; it is handed to the Moon, which drifts very slowly outward. The result on Earth is that the spin gradually slows and each day grows a tiny bit longer. The rate is measured in milliseconds of added day length per century, far too small to notice in a human lifetime but unmistakable over geological spans.
Rotation also varies on much shorter timescales for reasons that are not tidal: the atmosphere shifting mass around, water and ice redistributing on the surface, and motion in Earth's liquid outer core all tug the spin rate up and down by small amounts. Because of all this, the true length of a day is not something you can pin to a fixed formula. It has to be measured, and it is, continuously, by the international services that watch Earth's rotation.
Where leap seconds come in
This is the practical crux. Modern civil time is kept by atomic clocks, which tick at an utterly steady rate defined by physics and do not care what the Earth is doing. Coordinated Universal Time, UTC, is built on that atomic rate. But Earth's rotation, as we have just seen, is neither perfectly steady nor perfectly 24 hours, so an atomic timescale left alone would slowly pull away from the actual position of the Sun in the sky.
To stop that drift from accumulating, timekeepers occasionally insert a leap second into UTC, a single extra second that shows on the clock as the unusual reading 23:59:60 before the day rolls over. Each inserted leap second nudges UTC back into close agreement with Earth's measured rotation, keeping the two within a fraction of a second. Twenty-seven leap seconds have been added since the modern definition of UTC began in 1972. They are not on a fixed schedule, because Earth's rotation is not on a fixed schedule; each one is announced in advance only once the accumulated difference calls for it.
Leap seconds are also on their way out. In 2022 the international body that defines the second voted to stop inserting them by 2035, letting a larger tolerance build up before any future correction. The full record of insertions and the running offset between atomic time and UTC is laid out on the leap second history page. The reason the whole leap-second mechanism has to exist at all is the theme of this article: the day the Earth actually keeps is not the tidy 24 hours our clocks assume.
So why do we still call it 24 hours?
Because a usable civil clock has to be simple, and the honest astronomical picture is not. If every day were literally its own slightly different length, no ordinary clock or calendar could work, timetables would be impossible, and coordinating anything across distance would be a nightmare. So civil time takes the mean solar day, rounds it to exactly 24 hours of exactly 60 minutes of exactly 60 seconds, and holds that fixed by convention. The small, real differences between that convention and the sky are absorbed elsewhere: by the equation of time for sundials, by measured rotation data for astronomers and satellite systems, and by the occasional leap second for UTC.
That is the reconciliation. The 24-hour day is not wrong, but it is a deliberate simplification. The Earth keeps a solar day that breathes through the year, a sidereal day almost four minutes shorter, and a spin that is imperceptibly winding down over the ages. Civil time smooths all of that into a single round number so the rest of life can get on. Knowing what the number hides is what turns a clock reading into an actual understanding of time.
Frequently asked questions
If a day is not 24 hours, how long is it really?
It depends on which day you mean. The mean solar day, the average interval from one local noon to the next, is 24 hours by definition, but any single real solar day runs a little longer or shorter than that average through the year. The sidereal day, one rotation of Earth measured against the stars, is about 23 hours 56 minutes 4 seconds, close to four minutes shorter. Civil clocks use a fixed 24-hour day, which is a rounded convention rather than a measurement.
Why is the sidereal day shorter than the solar day?
Because Earth does two things at once: it spins on its axis and it moves along its orbit around the Sun. After one full spin relative to the stars, the same stars are back where they were, but the Sun has shifted slightly because Earth has moved along its orbit that day. Earth has to turn roughly one extra degree, taking a little under four minutes, for the Sun to return to the same place in the sky. That extra turn is the whole difference between the star day and the Sun day.
What is the equation of time?
It is the running gap between apparent solar time, what a sundial shows, and mean solar time, what a clock shows. Two effects drive it: Earth's orbit is an ellipse, so the planet moves faster near the Sun and slower far from it, and Earth's axis is tilted relative to its orbit. Together they make the true length of a solar day drift across the year, so the Sun can be ahead of or behind the clock by up to about a quarter of an hour depending on the date.
Is the length of a day actually changing over time?
Yes, very slowly. Tidal friction between the oceans and the Moon transfers rotational energy away from Earth, so the planet's spin gradually slows and the mean solar day lengthens over geological time, on the order of milliseconds per century. Earth's rotation also wobbles on shorter timescales because of tides, the atmosphere, and motion in the fluid core, which is why precise timekeeping relies on measured rotation data rather than a fixed formula.
Why were leap seconds added to UTC?
Atomic clocks tick at a perfectly steady rate, but Earth's rotation does not, so an atomic timescale slowly drifts away from the actual position of the Sun. To keep Coordinated Universal Time within a fraction of a second of Earth rotation, timekeepers occasionally insert a leap second, shown on the clock as 23:59:60 UTC. Twenty-seven leap seconds have been added since the modern definition of UTC began in 1972. In 2022 the international body responsible for the second voted to stop inserting them by 2035.