What if the Sun disappeared?

First, some good news. The Sun is not going anywhere! It will shine for a very, very long time.

But here is a puzzle. Why does Earth go round the Sun?

Let's start with something important: the Sun cannot just disappear. NASA scientists expect it to keep shining for about another 5 billion years.

So why ask? Because imagining it gone is a clever way to answer a real question. Earth has been circling the Sun for billions of years. What keeps it going round, instead of flying off into space? In this lab you can switch off the Sun's pull in a model, and the answer becomes clear.

The Sun is a stable star roughly halfway through its life; NASA estimates it has about 5 billion years left before it changes dramatically. It cannot simply vanish.

The question is a thought experiment: if the central force of an orbit were removed, what would the orbiting body do? The answer separates two ideas that are easy to confuse, the motion Earth already has and the force that bends it.

Earth is always zooming sideways through space. It goes very, very fast.

The Sun pulls on Earth all the time. The pull bends Earth's path into a big loop.

Earth is always moving sideways through space, at about 30 kilometers every second. That is more than 100,000 kilometers every hour.

At the same time, the Sun's gravity pulls Earth toward it. The pull does not slow Earth down; it bends Earth's path, so instead of going straight, Earth curves around the Sun. One full loop takes a year.

Earth's mean orbital speed is 29.78 km/s at a mean distance of about 149.6 million km. In the model, the Sun's gravity supplies exactly the centripetal acceleration needed for a circle, GM/r² = v²/r, and the orbit takes 365.26 days.

The force is always at right angles to the motion on a circular orbit, so it changes Earth's direction without changing its speed. That is why the speed readout stays at 29.8 km/s all year.

In our pretend lab, Olly switches off the Sun's pull.

Now Earth stops going round. It rolls on in a straight line, like a ball let go from a spinning game.

Run the model, then switch the Sun's pull off in the middle of an orbit. Earth does not fall, stop or spin. It simply carries on in a straight line at the same speed it already had, about 30 kilometers per second, heading off along the direction it was moving at that moment.

In the model, a month later Earth is about 168 million kilometers from where the Sun was, and half a year later more than three times as far as today.

With the central body's gravity removed, the model's Earth keeps its velocity: x = x₀ + v·t. The speed readout stays at 29.8 km/s while the distance grows, reaching about 168 million km after 30 days and about 487 million km after 180 days. The 'stays in orbit' readout switches to no.

This is a thought experiment carried out with a model. The Sun cannot vanish, and the model removes its gravity instantly, which real physics does not allow.

Moving things keep going until something pushes or pulls them.

The Sun's pull was bending Earth's path. No pull, no bending. So Earth goes straight.

Isaac Newton worked out that a moving object keeps moving in a straight line at the same speed unless a force pushes or pulls it. Earth's sideways motion is not caused by the Sun; Earth already has it.

What the Sun does is bend that motion into a curve. Take away the bending pull and nothing is left to change Earth's direction, so it goes straight, like a ball released from a spinning ride.

An orbit is continuous free fall: the velocity is tangential, the acceleration points to the Sun, and the balance between them gives a closed curve. Remove the acceleration and Newton's first law leaves uniform straight-line motion along the last tangent.

The model's limits: point masses in one plane, Newtonian gravity, and an instant switch. In reality, changes in gravity travel at the speed of light, so Earth would keep curving for about 8 minutes, the same time sunlight takes to reach us.

The Sun gives us light and warmth. Living things on Earth need its energy.

The Sun will keep shining for a very long time. We are lucky to have it!

Sunlight takes about 8 minutes and 20 seconds to travel the 150 million kilometers to Earth, so the sunshine you feel left the Sun a few minutes ago. That light warms our planet, and NASA says that without the Sun's energy, life as we know it could not exist.

Scientists explain that a world with no Sun would also lose that light and warmth. That is one more reason this can only ever be a pretend experiment, and one more reason to be glad the Sun is steady and reliable.

Light covers the roughly 150 million km from the Sun in about 8 minutes 20 seconds (about 500 seconds). Sunlight is the energy that drives Earth's climate, and NASA notes that without the Sun's energy life as we know it could not exist.

The model only tracks motion, not light or heat. What would happen to Earth's temperature and life without sunlight is an idea well beyond it, and in any case the Sun is expected to remain stable for billions of years.