Why Is a Day Not Actually Twenty Four Hours? Two Different Questions

Measuring a day means measuring the time until something returns to the same position, and the answer depends on what that something is. Against the background stars, which are so distant that the Earth's orbital motion does not change their direction appreciably, one full rotation takes about twenty three hours fifty six minutes and four seconds. Against the sun, one rotation is not enough, because during that rotation the Earth has moved roughly one degree around its orbit, so the planet must turn about one extra degree to bring the sun back overhead. That extra degree takes approximately four minutes, which accounts for the whole difference.

The four minutes accumulate and have visible consequences:

• •A star rises about four minutes earlier each night than the night before
• •Over a month that amounts to two hours, and over a year a full twenty four
• •The constellations visible in the evening therefore change through the year
• •A telescope must be driven at the star rate, not the sun rate, to track an object
• •Observatories keep clocks on the star rate for planning
• •The two rates differ by about one day per year in total count

Clocks are set to the sun rather than to the stars for the obvious reason that human life follows daylight, and a timekeeping system in which noon drifted steadily through the night over a year would be useless. The practical day is defined as the average interval between successive noons, averaged because the actual interval varies through the year by up to about half a minute either way, owing to the tilt of the axis and to the fact that the orbit is elliptical so the Earth moves faster when nearer the sun. That variation accumulates to a difference of as much as sixteen minutes between clock time and sundial time, which is the reason a sundial and a watch disagree by an amount that changes with the season.

Determining how fast the planet turns to any useful precision is harder than it sounds, because every reference eventually turns out to move. Observing stars was the standard method for centuries and is limited by the fact that stars themselves drift slightly. Modern measurement uses radio telescopes on different continents observing the same distant quasars simultaneously and comparing arrival times, which fixes the orientation of the Earth against objects so remote that their motion is undetectable. Those measurements show the rotation is not constant, slowing over long periods from tidal friction, wobbling on shorter ones as mass moves in the atmosphere, the oceans and the core, and occasionally speeding up slightly. Leap seconds exist to reconcile clocks with a planet that will not keep time.

Applying the same distinction elsewhere produces results that are frequently startling. Mars turns in a period close to Earth's, giving a day about thirty nine minutes longer, which mission teams operating rovers have had to live on, shifting their working hours later every day and finding it genuinely difficult. Venus turns so slowly, and backwards, that its solar day is about a hundred and seventeen Earth days while its rotation takes two hundred and forty three. Mercury turns three times for every two orbits, producing a solar day about a hundred and seventy six Earth days long, which is two of its years. The distinction is not a technicality but the difference between a planet's spin and its relationship to its star.

One rotation relative to the stars takes about twenty three hours fifty six minutes, and the planet must turn a further degree to bring the sun back overhead, which takes the extra four minutes. Those minutes accumulate, which is why stars rise four minutes earlier each night and why the evening constellations change through the year. Clocks follow the sun because daylight governs human life, and the two rates differ by one day per year.