The average distance between Earth and Mars is about 225 million km. The distance from Earth to Mars varies a lot because Earth’s orbit around the sun is shorter than Mars’ orbit. The minimum distance from the Earth to Mars is about 54.6 million kilometers and the farthest is about 401 million km.
Both Earth and Mars are following elliptical orbits around the Sun, like two cars travelling at different speeds on two different shaped racetracks. If you are a fan of NASCAR racing you know on a racetrack the car on the inside lane has a shorter distance to travel then a car on the outside lane. Earth being the third planet from the Sun is on the inside lane having a shorter distance to travel when compared to farther fourth planet, Mars.
Sometimes the planets are closer, other times they’re on opposite sides of the Sun, their distance is always changing.
How do we calculate the distance?
The first person to ever calculate the distance to Mars was the astronomer Giovanni Cassini in 1672 from Paris. Cassini used the parallax method to calculate the distance to Mars with surprising accuracy.
The parallax method is the apparent displacement of an object because of a change in the observer’s point of view. As the Earth orbits the sun, Mars will appear to move against the more distant background stars. Astronomers can measure Mar’s position once, and then again 6 months later and calculate the apparent change in position. The star’s apparent motion is called stellar parallax.
Today, astronomers calculate the distance to Mars (or any other object in the Solar System) using the speed of light. They measure the time it takes for signals to reach spacecraft orbiting other planets and measure the time it takes for signals to return. This allows them to measure the distance to planets, like Mars, with incredible accuracy.