Days and years on each world

On Venus, the planet spins so slowly that one turn takes longer than a trip around the Sun. A day is not the same thing everywhere, and neither is a year. Here is how to work out both, on any world.

A day and a year are two different clocks, and every world sets them differently.

Look

A day is how long a world takes to spin once. A year is how long it takes to go once around the Sun. On Earth these are about 24 hours and 365 days. Other worlds are very different.

World	Length of day	Length of year
Mercury	176 Earth days	88 Earth days
Venus	117 Earth days	225 Earth days
Earth	24 hours	365 days
Mars	24 hours 40 minutes	687 Earth days
Jupiter	9.9 hours	11.9 Earth years
Saturn	10.7 hours	29.4 Earth years
Uranus	17.2 hours	84 Earth years
Neptune	16.1 hours	164.8 Earth years

Some surprises:

• On Mercury, one day (sunrise to sunrise) lasts two of its years.
• Venus spins backwards and very slowly.
• The giant planets spin fastest of all, even though they are the biggest.
• A Mars day is almost the same as ours, which is handy for rover teams on Earth.

Use the comparison panel under the orrery to compare any two worlds.

Understand

There are two kinds of day, and the difference matters.

• The sidereal day is one spin measured against the distant stars. Earth’s is 23 hours 56 minutes 4 seconds.
• The solar day is from one noon to the next. While Earth spins, it also moves about 1/365 of the way around its orbit, so it has to turn a little extra (about 1 degree) before the Sun is back overhead. That takes about 4 more minutes, giving 24 hours.

For most planets the two days are almost the same. For slow spinners they are wildly different. Mercury spins once every 58.6 Earth days, and its year is 88 days. The two motions combine so that one solar day lasts 176 Earth days: exactly two Mercury years. Mercury spins exactly three times for every two orbits, a lock caused by the Sun’s tides.

Venus spins backwards (retrograde), once every 243 Earth days. Because the spin and the orbit go opposite ways, they add up, and the solar day is shorter than the spin: about 117 Earth days.

Years get longer with distance for two reasons: the path is longer, and the planet moves more slowly. Earth moves at about 29.8 km/s; Neptune at about 5.4 km/s along a path 30 times longer.

Master

The solar day follows from the two angular speeds. With the sidereal rotation period P_rot (negative for retrograde) and the orbital period P_orb, both measured in the same direction:

1 / P_solar = 1 / P_rot − 1 / P_orb

Check Earth: 1/0.99727 − 1/365.256 = 1.00274 − 0.00274 = 1.00000 per day, so P_solar = 1 day.

Check Mercury: 1/58.646 − 1/87.969 = 0.017052 − 0.011368 = 0.005684, so P_solar = 175.9 days.

Check Venus (retrograde, P_rot = −243.018 days): 1/(−243.018) − 1/224.701 = −0.004115 − 0.004450 = −0.008565, so |P_solar| = 116.75 days. The minus sign tells you the Sun rises in the west on Venus.

Years follow Kepler’s third law: T² ∝ a³ for everything orbiting the Sun. With T in years and a in au, T = a^1.5. Neptune at a = 30.07 au gives T = 30.07^1.5 = 164.9 years, matching the observed 164.79. The small difference comes from rounding and from the planets’ tiny masses, which Newton’s version of the law includes.