Answer:The Length of a Day Measured from noon to noon, a day on any planet is the time it takes for the planet to rotate once on its axis relative to the sun. Earth, of course, rotates once every 24 hours. A martian day, called a sol, is 24 hours, 39 minutes, and 35 seconds long—very close to Earth's rate of spin. That's just a coincidence, though. Planets spin at wildly different rates, from speedy Jupiter, which rotates every 10 earth hours, to sluggish Venus, which only spins once every 243 earth days.
The Length of a Year A year is the time it takes for a planet to make one revolution around the sun. A Martian year is 687 earth days or 669 sols—almost twice our 365-day year. If you're 25 years old on Earth, you'd barely be a teenager on Mars. Mars is, on average, about 50 million miles (80.5 km) farther from the sun than Earth is. And the farther a planet is from the sun, the longer its year. The lengths of planetary years are governed by precise rules. Johannes Kepler (1571–1630) discovered the relationship between the length of the year and the radius of the orbit that applied to all planets, but it took Isaac Newton (1642–1727) to explain this pattern.
As a planet travels in its almost circular orbit, it's continuously accelerating toward the sun. This is due to the gravitational attraction between the sun and the planet, which is stronger the closer they are to each other. But the stronger the pull of gravity, the greater the planet's acceleration, which results in the planet's revolving at a higher speed. This is a good thing—it's what's necessary to keep the planet in its orbit, falling around the sun instead of plunging into it. So the inner planets are compelled to rush around their orbits, while the outer planets revolve around the sun at a leisurely pace.
Going Further
Scientists involved with the robotic exploration of Mars need to know martian time so they can plan a rover's activities to take place during the martian day. So of course they use a Mars clock.
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