What is the relationship between the distance from the Sun and the time taken to orbit the Sun?

A year on Earth is approximately 365 days. Why is that considered a year? Well, 365 days is about how long it takes for Earth to orbit all the way around the Sun one time.

A year is measured by how long it takes a planet to orbit around its star. Earth orbits around the Sun in approximately 365 days. Credit: NASA/Terry Virts

It’s not exactly this simple though. An Earth year is actually about 365 days, plus approximately 6 hours. Read more about that here.

All of the other planets in our solar system also orbit the Sun. So, how long is a year on those planets? Well, it depends on where they are orbiting!

Planets that orbit closer to the Sun than Earth have shorter years than Earth. Planets that orbit farther from the Sun than Earth have longer years than Earth.

A planet orbiting close to its star has a shorter year than a planet orbiting farther from its star. Credit: NASA/JPL-Caltech

This happens for two main reasons.

If a planet is close to the Sun, the distance it orbits around the Sun is fairly short. This distance is called an orbital path.

The closer a planet travels to the Sun, the more the Sun’s gravity can pull on the planet. The stronger the pull of the Sun’s gravity, the faster the planet orbits.

Check out how long a year is on each planet below!

Year: 88 Earth DaysDistance from Sun: ~35 million miles

(58 million km)

Year: 225 Earth DaysDistance from Sun: ~67 million miles

(108 million km)

Year: 365 Earth DaysDistance from Sun: ~93 million miles

(150 million km)

Year: 687 Earth DaysDistance from Sun: ~142 million miles

(228 million km)

Year: 4,333 Earth DaysDistance from Sun: ~484 million miles

(778 million km)

Year: 10,759 Earth DaysDistance from Sun: ~887 million miles

(1.43 billion km)

Year: 30,687 Earth DaysDistance from Sun: ~1.78 billion miles

(2.87 billion km)

Year: 60,190 Earth DaysDistance from Sun: ~2.80 billion miles

(4.5 billion km)

Why does NASA care about years on other planets?

NASA needs to know how other planets orbit the Sun because it helps us travel to those planets! For example, if we want a spacecraft to safely travel to another planet, we have to make sure we know where that planet is in its orbit. And we also have to make sure we don’t run into any other orbiting objects — like planets or asteroids — along the way.

Scientists who study Mars also need to keep a Martian calendar to schedule what rovers and landers will be doing and when.

Mars and Earth are always moving. So, if we want to land a robotic explorer on Mars, we have to understand how Earth and Mars orbit the Sun. Watch this video to learn more about the Martian year. Credit: NASA/JPL-Caltech

*Length of year on other planets calculated from data on the NASA Solar System Dynamics website.

article last updated July 13, 2020

QUESTION #477

Asked by: Navid Dianaty There is no connection between a planet's distance from the Sun and its rate of rotation (spin on its own axis or Planetary Rotation Period). While it is true that, generally speaking, the gas giants have higher rotation rates than the earth like planets closer to the Sun, any correlation is only coincidental. Mars, for example, is further from the Sun than Earth but has a slightly slower rate of rotation - its planetary rotation period is 1.03 Earth days. Pluto. the furthest planet from the Sun, has a planetary rotation period of 6.39 Earth days. You can see in the plot below that there is no correlation between the planetary rotation period and the distance from the Sun.

There is, however, a relationship between a planet's distance from the Sun and its period of revolution. Kepler's third law of planetary motion says that the square of the planet's orbital period is proportional to the cube of its semimajor axis. For now, just read that as saying that a planet's 'year' is determined ONLY by its average distance from the Sun. The further away from the Sun it is, the slower the planet's orbital speed and the longer its path. Both of those factors result in taking longer to make one complete orbit and a planet having a longer year.

Answered by: Paul Walorski, B.A., Part-time Physics Instructor