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A year on Earth is 365 days – unless it is a leap year – but the actual definition of a year is the time it takes a planet to orbit the Sun. Mercury orbits the Sun in 88 Earth days, which is the shortest orbit of any of the planets. This makes sense considering it is the closest object to the Sun. In comparison, a year on Mercury is about a quarter of a year on Earth. Mercury moves at different speeds depending on where it is in its orbit. When it is near its perihelion – the closest point to the Sun in an object’s orbit – it moves faster. It moves slower when it is near its aphelion – the farthest point from the Sun in an object’s orbit.
A year on Venus lasts 224.7 Earth days, which is 0.62 the length of a year on Earth. The odd thing about Venus is that a day on Venus is 243 days, so Venus’ day is longer than its year.
Earth’s year is actually 365.25 days long, not the 365 days that we round it to. Because an Earth year is not exactly 365 days, we have a leap year – a year with 366 days – every four years. That extra day is added to the month of February so in a leap year it has 29 days instead of 28. The next leap year is in 2012.
A year on Mars is 687 Earth days. You may have noticed by now that the length of the planet’s year increases the further the planet is from the Sun, which is only common sense because the further out planets have to make a bigger orbit around the Sun. Because two years on Earth is only a little more than a year on Mars, the two planets are close together every two years, which is when astronomers sends probes to Mars.
While it only takes about 10 hours for Jupiter to rotate, it takes 11.9 Earth years to orbit the Sun. Although this seems like a long time, it is actually much less than some of the other planets.
Saturn’s year is 29.7 Earth years long. Our planet rotates 30 times in the time it takes Saturn to rotate once.
A year on Uranus lasts 84.3 Earth years. One of the odd things about Uranus is that its axial tilt is so radical (97.8°) that it actually rotates on its side. This results in extreme seasonal changes.
Since Pluto lost its status as a planet, Neptune has the longest year. It takes the planet 164.8 Earth years to orbit the Sun. Our planet orbits the Sun almost 165 times before Neptune orbits it once, and we have not even seen an entire orbit since the planet was discovered.
Astronomy Cast has episodes on all the planets including Venus.