Earth, planets and orbits

1543

The Sun in the centre

Nicolaus Copernicus

What if the Earth is the thing that’s moving?

Copernicus put the Sun at the centre and let the Earth move around it. Planets drifting backwards then became an illusion: we are overtaking them, like passing a slower car. He still used perfect circles, so he still needed small extra circles to match the sky.

First, make a guess

Plotted night by night from Earth, what does Mars’s path look like?

How they figured it out

Copernicus was a church administrator in Frombork, Poland, who did astronomy on the side. Around 1510 he passed round a short handwritten sketch putting the Sun near the centre, then spent decades refining it before publishing. On the Revolutions appeared in 1543; the story goes that he saw the first printed copy on his deathbed.

A preface, added without his knowledge by the theologian Andreas Osiander, called the whole thing just a convenient way to calculate. But the Sun-centred picture explained the backward loops of the planets naturally, and for the first time gave every planet a definite order and relative distance from the Sun.

The equation

Earth→Sun-centred orbit\text{Earth} \to \text{Sun-centred orbit}

Change where you stand, and a complicated motion becomes a simple one.

Everyday echoOvertake a car on the motorway and it seems to slide backwards.

See it in real life

Overtaking a train

When your train overtakes a slower one, the other train seems to slide backwards, even though both are going forwards. Every 26 months or so Earth overtakes Mars on the inside track, and Mars appears to go backwards across the stars.

Where engineers use it

  • Choosing a frame

    Engineers pick the reference frame that makes a problem simple: a rotating frame for turbines, a moving one for vehicles.

  • Mission planning

    Spacecraft trajectories are planned around the Sun, then converted into what ground stations will see.

Where it came from

What it led to