How long are the years and days on the other planets in the solar system?

To answer this seemingly idle question, we must first agree on what kind of day we are talking about.

Yes, because there are two kinds of days: The sidereal day, which corresponds to the time it takes for a planet to rotate on its own axis, and the solar day, which instead corresponds to the time between one sunrise and another, that is, the time it takes for the sun to reappear in the same place, and it depends both on the rotation of the planet on its own axis and on its rotation around the sun.

In the case of the Earth, the two days are practically equal, the sidereal day lasting 23 hours 56 minutes and 4 seconds, the solar day 24 hours.

The length of days

We begin our brief journey through the solar system with Uranus, the seventh planet in the system in order of distance from the Sun. Uranus rotates around its axis at a speed of 17 hours, 14 minutes, and 24 seconds (sidereal day). However, its solar day is very different because of the strange tilt angle of its axis (about 90 degrees): it is basically a bit like a spinning top tilted to one side. Because of this peculiarity, it takes 84 years for the Sun to appear in the same place again!

Venus, the second closest planet to the Sun, has a peculiarity in its rotation around its own axis, it is the only one that rotates clockwise (retrograde rotation). In fact, its rotational speed is only 5.5 km per hour (Earth rotates at 1,670 km per hour). In practice, Venus takes about 243 Earth days to rotate around its axis. In terms of solar days, we have to wait 116 Earth days to see a new sunrise at the same location.

Mars is the fourth closest planet to the Sun. Its rotation speed around its own axis is very similar to that of our planet, as is its solar day: 24 hours and 39 minutes.

The planet farthest from the Sun, Neptune, has a sidereal day and a solar day of 16 hours, 6 minutes, and 36 seconds, according to reliable estimates.

Jupiter, the gas giant, is also the fastest rotating planet (45,300 km/h). Its solar day, which is not easy to determine because it is a gaseous planet, is very short: only 9 hours, 55 minutes and 30 seconds.

Finally, Saturn, also gaseous, has an estimated solar day of 10 hours and 32 minutes.

The days are thus very "variable" - a hypothetical human being on Uranus could be born and die without ever having seen the light of the Sun, or vice versa, while on Venus we would have to wait 115 Earth days before we would see the Sun's rays shining on our space base again.

The variability of the years

On Saturn and Jupiter, leaving aside the small problem of how to survive on a gaseous planet with extreme conditions, we would have a markedly accelerated day/night rhythm of about 11 and 10 hours, respectively.

If we move to the length of years, the "oddities" are not necessarily less. Let us start with Mercury, the closest planet to our star. Well, the very hot planet completes its orbit around the Sun in only 88 Earth days, so a Mercury year lasts about a quarter of ours, while the solar day, as we have seen, lasts a full 59 days.

Venus, which is nearly the size of the Earth, orbits the Sun at a distance equal to 75 percent of the Earth-Sun distance, in a time of about 225 Earth days (7 1/2 months).

The most Earth-like planet, Mars, has a year of about 686 Earth days. The farther we get from the Sun, the longer it takes to orbit it.

The gas giant that dominates our system, Jupiter, has an orbital period of 398.88 days, at the end of which the celestial body begins a phase of apparent retrograde motion, in which it appears to move backward in the night sky relative to the background of "fixed" stars, following a sigmoid trajectory. Jupiter, which passes through all the constellations of the zodiac in the 12 or so years of its revolution, has a year that lasts 12 Earth years!

Saturn, which has a day of about 10 hours and 40 minutes, has a year that lasts 29.5 Earth years. The year of Uranus is equal to 84 Earth years. In its 4 seasons (of 21 years each) it presents to the Sun first the North Pole, then the equator, then the South Pole and again the equator.

Finally, Neptune, the planet farthest from the Sun, about 30 astronomical units away, has a very long year, 164 Earth years. Only the dwarf planet Pluto is worse, taking 248 years to complete its orbit around the Sun.

A hypothetical and unlikely Earth colonist born on Uranus would thus have a life expectancy of about 1 year, while someone who managed to survive the hellish temperatures of Mercury would have a life expectancy of about 400 dwarf planet years.