A year is a measurement before it's a calendar
People have always counted years by the seasons, but a planet's year is a plain geometric fact: the time it takes to return to the same spot in its orbit. For Earth the figure is 365 days and about a quarter, which is why NASA points out that leap years exist. Mars, Venus and the rest have their own years, and between 1609 and 1619 Johannes Kepler worked out how those years relate to one another.
A year on every planet
| World | Earth days | Earth years | Orbital speed | Average distance (million km) |
|---|---|---|---|---|
| Mercury | 88 | 88 days | 47.4 km/s | 57.9 |
| Venus | 225 | 225 days | 35.0 km/s | 108.2 |
| Earth | 365 | 365 days | 29.8 km/s | 149.6 |
| Mars | 687 | 1.9 years | 24.1 km/s | 228.0 |
| Jupiter | 4,331 | 11.9 years | 13.1 km/s | 778.5 |
| Saturn | 10,747 | 29.4 years | 9.7 km/s | 1,432.0 |
| Uranus | 30,589 | 83.7 years | 6.8 km/s | 2,867.0 |
| Neptune | 59,800 | 163.7 years | 5.4 km/s | 4,515.0 |
| Pluto | 90,560 | 247.9 years | 4.7 km/s | 5,906.4 |
Bars show Earth days for one orbit. Mercury's is too short to see next to Pluto's. Values from the NASA Planetary Fact Sheet.
Kepler's rule for how the years line up
Kepler published his first two laws of planetary motion in Astronomia Nova in 1609: planets move on ovals with the Sun off-center, and they speed up when they swing close to it. His third law, in Harmonices Mundi in 1619, links the two things this page is about. Square a planet's year, and you get a number proportional to the cube of its average distance from the Sun.
That explains the shape of the table. Mercury moves at 47.4 km/s and takes 88 days. Neptune crawls at 5.4 km/s over a much longer path, so its year is 680 times longer than Mercury's. Distance does two jobs at once: it lengthens the road and it slows the traveler. If you know a planet's year, you know where it sits, which is how early astronomers mapped the solar system in proportion before anyone could measure it in kilometers. The distance page tells how the scale was finally set.
Neptune's first lap
Neptune is the best example of how long the outer years are. It was found in 1846, after Urbain Le Verrier predicted where it must be and Johann Galle found it at the Berlin Observatory. It then took until July 2011 for the planet to return to the place where it was found, its first full orbit since discovery. Astronomers marked the day, and the Hubble Space Telescope took anniversary pictures of the planet.
Pluto has done even less. It was found in 1930, about 96 Earth years ago, and its year is 247.9 years, so it's still on its first lap since discovery. Uranus, at 83.7 years, takes about a human lifetime to circle once. Try the age calculator to see how old you would be on each world.
The planet where a day outlasts the year
Venus takes 225 Earth days to go around the Sun, but it turns on its axis so slowly that one spin takes 243.0 Earth days, longer than its own year. The same trick makes Mercury's day twice the length of its year. Years are the easy part of planetary timekeeping, and the days page is where it gets strange.
How Mars keeps its own calendar
Scientists who study the weather of Mars needed a way to number its years, so a group led by R. T. Clancy proposed a convention in 2000. Mars Year 1 began on April 11, 1955, at a Martian spring equinox, and Mars years have been counted from there. A Martian year runs 687 Earth days, so a mission that lasts a few Earth years covers only a couple of Mars years. The Mars rovers page lists the machines that have been counting.
Years aren't seasons
A year tells you how long the orbit takes. Seasons depend on the tilt of the axis as well, so a planet with almost no tilt, like Jupiter, has a long year but weak seasons. Planet tilt and seasons sets the two side by side.
Fast on the inside, slow on the outside
Years differ in length because planets also differ in speed. Mercury travels at 47.4 kilometers a second, Earth at 29.8, and Neptune at only 5.4. Neptune moves slowest and has the longest road, so its year is 164 times Earth's while its distance is only 30 times as big. Gravity from the Sun weakens with distance, so the farther a planet is, the more leisurely its orbit can be.
Counting laps since discovery
Long years make a planet's history odd. Neptune hadn't finished a single lap around the Sun since its discovery in 1846 until July 12, 2011, a span of 165 Earth years. Pluto, found in 1930, takes 248 years to go round, so its first lap since discovery will be done in 2178.
How many Martian days in a Martian year?
The Mars calendar used by mission teams counts sols, and a Martian year holds about 668 of them. Because a sol is 24.7 hours long, a year on Mars is 1.88 Earth years, which is why the seasons there come around only about every second Earth year. The age calculator turns these ratios into your own birthday count on each world.
Years and what you see from Earth
The orbit times on this page are measured against the stars. What you see from Earth depends on two orbits at once. Mars takes 1.9 years to go round, but Earth catches up with it only every 780 days, about 2.14 years, and that's the gap between oppositions, when the planet is best placed for viewing. The dates of the next ones are on the Mars opposition page. Jupiter works the same way with a shorter wait, 399 days, because its orbit is slower and Earth overtakes it sooner.
Questions people ask
How long is a year on Mars?
A Martian year is 687 Earth days, about 1.9 years.
Which planet has the shortest year?
Mercury, which goes around the Sun in 88 Earth days.
Which planet has the longest year?
Neptune takes 163.7 years, the longest of the eight planets. Pluto, a dwarf planet, takes 247.9 years.
How long is a year on Jupiter?
About 11.9 years (4,331 Earth days).
