We all know that one day on the Earth lasts for 24 hours and that a year has 365 days (or 366 in a leap year), but we have to keep in mind that it is a very relative measure. The duration of the days is the result of the time that takes a planet in making a complete turn onto its axis, and the duration of the year corresponds to the time that takes a planet to make a turn around its star.
Even if we can think that it has a shorter day because of its small distance to the Sun, this isn’t the case. Mercury takes approximately 58,5 terrestrial days in completing a turn onto its axis. What it does have to do with the distance to the Sun is the duration of the year. Mercury only needs 88 days to do a complete orbit around the Sun.
Venus has an even longer duration of days. This one is the planet in the solar system that has the longest period of rotation. Additionally, unlike the other planets that turn around the Sun with an orbit more or less elliptic, the path of Venus around the Sun is an almost perfect circle. Venus is also the unique planet that turns onto its axis counter-clockwise.
To complete a lap around the Sun, Venus needs 225 days. However, to make a complete turn onto its axis he takes 243 terrestrial days. This means that a year on Venus (225 days) lasts less than one of its days (243 days). This is because Venus spins onto its axis at a very low speed of 6,52 kn/h at the equator.
Our planet, the third more distant from the Sun, has a day that lasts 23 hours, 56 minutes, and 4 seconds. Due to this, Each year lasts approximately 365,25 years. It is for this reason that every four years there is a leap year, as every 4 years, the 0,25 days end up forming a day.
Moreover, due to the distancing of the Moon, the days are getting longer at a rhythm of about 2 milliseconds each century. Even if it seems a very short time, we have to keep in mind that, after the formation of the Moon, when this one was much nearer to the Earth, the days on the Earth only lasted for 5 hours.
The thing that is really peculiar on Mars, in relation to the duration of the days, is its similitude with the Earth. In fact, a day on Mars lasts 24 hours and 37 minutes. Even so, regarding the rotation time around the Sun, it is almost double the Earth’s. A year on Mars lasts about 667 terrestrial days. A Martian day is named sol.
Even if it has a very similar duration of the day to the Earth’s, Mars has only a third of the gravity that we suffer on the Earth. In this way, on Mars, we would weight three times less than we do here on Earth.