How Long Is a Day on Each Planet?

What Is the Difference Between a Solar Day and a Sidereal Day?

A sidereal day measures a planet’s rotation against a nearly fixed background of distant stars. Imagine drawing a meridian through a marked spot: one sidereal turn returns that meridian to its original direction in space. A solar day instead asks when the Sun next crosses that same local meridian in the same cycle.

On Earth, a sidereal turn takes about 23 hours, 56 minutes and 4 seconds. During that turn Earth also moves along its orbit, so it must rotate a little farther for the Sun to return to the meridian. The average solar day is about 24 hours. This is why “one rotation” and the familiar clock day are close, but not identical.

Neither definition means the number of hours of daylight. Sunrise, sunset and whether the Sun rises at all depend on latitude, seasons, axial tilt and atmospheric effects. The simulator compares whole rotations and mean noon cycles, rather than local hours of sunshine.

Which Planet Has the Shortest Day?

Jupiter has the shortest sidereal rotation period among the eight planets: approximately 9 hours, 55 minutes and 30 seconds using the JPL/IAU reference adopted here. Saturn comes next at roughly 10 hours, 39 minutes in the legacy radio reference used by the same table. Both rotate more than twice during one Earth sidereal turn.

These are reference periods for giant planets, not the time taken by every visible cloud. Jupiter’s atmosphere rotates at different rates at different latitudes. Its System III convention follows a magnetic-field rotation reference, providing a reproducible way to compare the planet with other worlds.

Which Planet Has the Longest Day?

The answer depends on the definition. Venus has the longest sidereal day, at about 243 Earth days. Mercury has the longest mean solar day among the rocky planets, at about 176 Earth days. Their slow rotations make orbital motion a major part of the noon-to-noon calculation.

Mars is much closer to Earth’s daily rhythm. Its sidereal turn lasts about 24 hours, 37 minutes and 23 seconds. Its mean solar day, known as a sol, lasts about 24 hours, 39 minutes and 35 seconds. That small difference matters when coordinating surface missions over many sols.

Why Is a Day on Venus Longer Than Its Year?

Venus completes an orbit in about 224.7 Earth days, but one sidereal rotation takes about 243.0 Earth days. In that specific sense, its day is longer than its year. Its mean solar day is approximately 116.75 Earth days, so a solar day is shorter than its year.

Venus rotates retrograde: its spin has the opposite sense to its orbital motion in the convention used here. The apparent Sun therefore travels around the reference meridian more quickly than it would if the planet’s slow spin and orbital motion acted in the same direction. The diagram uses a negative spin angle for Venus and a positive orbital angle; it never treats the negative source period as negative elapsed time.

Why Does Venus Spin Backward?

As discussed in NASA’s conversation with planetary scientist David Grinspoon, its rotational history is not settled. Early impacts and long-term tidal torques, including effects involving its massive atmosphere, have been investigated. The observed retrograde direction is well established; a single certain explanation for how it developed is not. The animation demonstrates the present reference motion without pretending to reconstruct that history.

Why Does a Solar Day on Mercury Last So Long?

Mercury turns once relative to the stars in about 58.65 Earth days and orbits in about 87.97 Earth days. Its 3:2 spin–orbit resonance means it makes three sidereal turns in two orbits. After one orbit, a marked meridian does not face the Sun in the same way as at the beginning; the configuration repeats after about two Mercury years.

That gives a mean solar day near 176 Earth days. In the circular demonstration, choose Mercury and stop at a solar cycle: the spin counter approaches 1,080° while the orbital counter approaches 720°. The tiny differences from exact whole multiples come from rounded reference periods.

NASA’s Mercury overview describes its unusual apparent solar motion. Actual Mercury has an eccentric orbit and does not move at constant orbital angular speed. Around perihelion, the apparent Sun can reverse direction briefly for some observers. The simple frequency model does not predict those local events or exact sunrise times.

Why Do Jupiter and Saturn Spin So Quickly?

Planets inherited angular momentum as they assembled from the young solar system’s material. Accretion, contraction, impacts and later torques shaped their rotation. Large radius alone does not determine the spin period: the two largest planets rotate much faster than the much smaller Earth.

Saturn also illustrates the difficulty of assigning one number to a giant planet. The JPL table retains a roughly 10-hour-39-minute reference period. A 2019 Cassini ring-seismology analysis reported by NASA/JPL inferred an interior rotation period of 10 hours, 33 minutes and 38 seconds. These values come from different tracers and methods; they should not be silently mixed inside one comparison.

The simulator consistently uses the labeled JPL reference, including in the race and calculator. It does not claim that this legacy radio period is the uniquely correct rotation of Saturn’s interior.

Do Uranus and Neptune Have Days?

Yes, they rotate, even though neither ice giant has a solid surface like Earth’s. The adopted Voyager magnetic/radio references are about 17 hours, 14 minutes for Uranus and 16 hours, 7 minutes for Neptune. Their atmospheric winds can carry features at different speeds, so a cloud is not a universal clock.

Uranus is especially unusual: its spin-axis obliquity is about 97.8°, placing it almost on its side. Under the signed-period convention used by the fact sheets it is classified as retrograde. The side-view glyph shows that tilt; the rotation dial is deliberately schematic and does not combine a tilted sphere with a second, conflicting sign reversal.

Solar days on a selected giant-planet reference meridian can be modeled, but there is no fixed solid-ground clock shared by every atmospheric layer. To avoid implying one universal observable noon cycle, this page leaves giant-planet solar-day fields reference-dependent. High obliquity also makes a simple noon-cycle number an inadequate description of local sunlight and seasons.

Reference Data, Method and Limitations

Sidereal periods follow JPL’s Planetary Physical Parameters, checked October 28, 2026: Mercury 58.6462 d; Venus −243.018 d; Earth 0.99726968 d; Mars 1.02595676 d; Jupiter 0.41354 d; Saturn 0.44401 d; Uranus −0.71833 d; Neptune 0.67125 d. Signs identify direction. Displayed durations are rounded, approximate reference values, even when shown in hours, minutes and seconds.

Axial tilts and the terrestrial mean solar references use NASA’s metric fact sheet and the detailed Mars fact sheet. The fact-sheet notes explain rotation conventions. Orbital periods are reused from our planetary-age dataset, in conventional 24-hour Earth days. They are sidereal reference years, not tropical years, seasons or date-specific ephemerides. Small differences between rounded published tables do not imply changing clocks.

The rotation angle is 360° × elapsed time / signed sidereal period. The schematic solar-cycle period is calculated as 1 / Psolar = |1 / Prot − 1 / Porbit|, with negative Prot for retrograde rotation and positive Porbit. Published terrestrial solar references are used in the cards, table and calculator; the orbital demonstration uses the frequency formula. Both are mean approximations. Circular, coplanar orbits, constant rotation and no precession are assumed in the demonstration.

Each comparison uses a shared elapsed time for its planets. Independently controlled sections run through one animation-frame driver. Relative-speed playback is limited to six simulated hours per real second; One Rotation mode gives only the selected planet a normalized eight-second turn at 1×. Its Earth counter still reports the same modeled elapsed time, but the Earth dial is held still to avoid severe aliasing. Nothing autoplays. Hidden tabs pause, reduced-motion users can use sliders and steps, and long frame stalls do not jump the clock forward.

All calculations run locally. The illustrations are stylized reference dials, not photographs, physical size comparisons, measured cloud tracking or predictions for an observer’s latitude. The tools do not require a third-party astronomical library or an external data request.