Science labs › Earth and space › Solar System
A desk orrery run by real gravity: why does Mars take longer than Earth to go round?
Solar System lab · Last updated: 16 October 2026
You run a desk orrery, a working model of the solar system, where the planets move under real gravity. They watch Earth lap Mars again and again and find out why planets further from the Sun take longer to go round. It turns a list of planet names into a system that makes sense.
What you'll discover
- The Sun's gravity holds all the planets in their orbits.
- Planets further from the Sun move more slowly and have a much longer path to travel.
- Mars takes about 687 Earth days to orbit the Sun, so a year on Mars is almost two of ours.
The science in plain words
The solar system is the Sun and everything held by its gravity: eight planets, their moons, dwarf planets such as Pluto, asteroids and comets. The Sun holds more than 99 per cent of all the mass, so its gravity steers everything else. Each planet is moving sideways very fast, and the Sun's pull keeps bending its path into a nearly circular orbit.
Gravity gets weaker with distance. A planet close to the Sun feels a strong pull, so it must move fast to stay in orbit without falling inwards. A planet further out feels a weaker pull and orbits more slowly. Earth travels at about 30 kilometres per second, while Mars, about one and a half times as far from the Sun, moves at about 24. Mars also has a longer path to cover, so both effects add up: a Martian year lasts about 687 Earth days.
In the early 1600s Johannes Kepler found a rule linking distance and time: the square of a planet's year is proportional to the cube of its average distance from the Sun. Isaac Newton later showed that this rule comes from his law of gravity. It holds for every planet, from Mercury's 88-day year to Neptune's year of about 165 Earth years.
Where it fits
Common questions
How long is a year on Mars?
Mars takes about 687 Earth days to go once round the Sun, so a Martian year is nearly twice as long as ours. A Martian day is only a little longer than an Earth day, at about 24 hours and 40 minutes.
Why do planets further from the Sun take longer to orbit?
The Sun's gravity is weaker further away, so outer planets move more slowly in their orbits. They also have a much longer path to travel round the Sun. Together these make their years far longer: Neptune takes about 165 Earth years to go round once.
What keeps the planets going round the Sun?
The Sun's gravity pulls each planet towards it, while the planet's own sideways motion carries it forward. The balance between falling inwards and moving sideways bends the planet's path into an orbit. Without gravity, each planet would fly off in a straight line.
Related labs
- Orbital Mechanics: Newton's cannonball: how fast must you fire it sideways so it never lands?
- Earth's Seasons: Carry the globe round the lamp. Why is summer long and winter short?
- Comet Orbits: Why does a comet grow a tail near the Sun, and why does it rush past so fast?
See all earth and space labs or every Taz science lab.
Try it yourself
The Solar System lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.