Why the Earth–Mars Distance Changes So Much

Quick answer: Earth and Mars are never a fixed distance apart because they orbit the Sun at different speeds and different distances from it. Their separation ranges from roughly 56 million kilometres at the closest oppositions to more than 400 million kilometres when the two planets sit on opposite sides of the Sun. Check the live calculator for today's exact figure.

Earth circles the Sun once every 365.25 days at an average distance of about 149.6 million kilometres — one astronomical unit (AU). Mars takes much longer to complete a lap: 687 Earth days, at an average distance of roughly 228 million kilometres, or 1.52 AU. Because the two planets are moving at different angular speeds, Earth periodically "laps" Mars, the way a runner on the inside track overtakes one on the outside.

This difference in orbital period is what makes the Earth–Mars distance change constantly, rather than staying fixed the way it would if both planets moved in lockstep.

Roughly every 26 months, Earth catches up to and passes Mars on the inside track. At that moment — called opposition, because Mars appears opposite the Sun in Earth's sky — the two planets are about as close as they get. But even oppositions aren't identical to one another, because Mars's orbit is noticeably elliptical (its eccentricity is about 0.093, compared with Earth's nearly circular 0.017).

A "perihelic opposition," when Mars is simultaneously near the closest point in its own orbit, can bring the planets to within about 56 million kilometres of each other — the closest realistic approach. An "aphelic opposition," when Mars is near the farthest point in its orbit, only brings the two planets to around 101 million kilometres apart. Both are opposition events; the difference is entirely down to where Mars happens to be in its elliptical path.

At the other extreme, when Earth and Mars sit on opposite sides of the Sun — a configuration called solar conjunction — their separation stretches to somewhere between about 378 million and 406 million kilometres, again depending on where Mars sits in its elliptical orbit at the time. That's more than seven times farther than the closest possible approach.

Configuration	Approx. distance
Closest possible opposition	~56 million km
Typical opposition	~75–100 million km
Farthest opposition	~101 million km
Solar conjunction (farthest overall)	~378–406 million km

You'll sometimes see a single figure quoted for "the" Earth–Mars distance — often around 225 million kilometres. That number depends heavily on how you average: averaging over time gives a different answer than averaging over all possible orbital positions of both planets. Neither figure tells you today's actual distance, which is why a live, position-based calculator is more useful than any single quoted average. See today's real distance for the number that actually applies right now.