If the Sun vanished this instant, we would carry on in full daylight for about eight minutes and 20 seconds before anything seemed wrong, because that is how long its light takes to reach us

If the Sun could somehow vanish, not merely stop shining but disappear as a source of both light and gravity, the sky would stay bright on Earth’s day side for about eight minutes and twenty seconds, because that is how long its light takes to cross the distance to us. We would carry on in daylight, looking at a Sun that had already gone.

The figure is sound. The thought experiment built on it usually leaves out the more interesting half, and in one respect has the physics backwards.

Where the eight minutes come from

The arithmetic is plain. The average distance from the Sun to Earth, one astronomical unit, is about 149.6 million kilometres. Light travels at 299,792 kilometres per second. Divide the first by the second and you get roughly 499 seconds, or eight minutes and twenty seconds.

The “about” earns its place. Earth’s orbit is an ellipse rather than a circle, so the distance shifts across the year. As BBC Sky at Night Magazine sets out, the trip runs from around eight minutes and ten seconds when Earth is nearest the Sun to about eight minutes and twenty-seven seconds when it is farthest. The commonly quoted 8:20 is the average, not a fixed constant.

Gravity is not instantaneous either

This is the part the popular telling tends to skip. The Sun does not only light the Earth. It holds it in orbit, and the question of how quickly a change in that grip would register is the one that makes the exercise worth doing.

Newton’s gravity was instantaneous. In his model the Sun’s pull acted across the gap with no delay, so remove the Sun and the Earth would fly free at once. Einstein’s general relativity replaced that picture. Gravity is the curvature of spacetime, and a change in whatever is producing that curvature does not appear everywhere at the same moment. It spreads outward at the speed of light.

So if the Sun vanished, Earth would not jump off its path the instant the mass disappeared. It would keep tracing its curved orbit for the same eight minutes and twenty seconds, still answering to the pull of a Sun that was no longer there, until the absence reached us at the very moment the last of the light did. Only then would the planet carry straight on, along a line tangent to the orbit it had been following. In this impossible thought experiment, light and the change in gravity would reach us together.

One caveat keeps this honest. It does not mean the Earth is normally pulled toward where the Sun was eight minutes ago. As the physicist Steve Carlip lays out in the Physics FAQ on the speed of gravity, an orbit built on that kind of delay would be unstable and would not match what we see in the Solar System. For a steadily moving source, general relativity contains velocity-dependent terms that almost exactly cancel the delay, so the pull effectively points toward the Sun’s current position rather than its lagged one. The eight-minute delay applies to a sudden change in the field, such as the source disappearing or sharply accelerating, not to the steady pull of normal orbital motion.

This has been tested across cosmic distance

The equality of the two speeds is not only a prediction; it has been tested directly. On 17 August 2017, the LIGO and Virgo detectors recorded gravitational waves from two neutron stars spiralling into each other about 130 million light-years away. Roughly 1.74 seconds later, the Fermi space telescope detected a short gamma-ray burst from the same event.

Two signals, one carried by gravity and one by light, had travelled for some 130 million years and arrived within about two seconds of one another. The collaboration’s analysis of the event constrains the difference between the speed of gravity and the speed of light to within roughly one part in a thousand trillion. The small lag fits the gamma rays being released a moment after the merger, not gravity moving at a different speed.

The premise does not survive contact with physics

There is a catch sitting underneath the whole scenario. The Sun cannot simply vanish. Mass and energy are conserved, and general relativity is constructed around that conservation. There is no valid solution to its equations in which a star’s mass blinks out of existence and leaves nothing in its place. A real Sun can exhaust its fuel, collapse, or shed its outer layers, but each of those is a process running over time, not an instant deletion.

That is why the disappearing Sun is best treated as a way of building intuition rather than as something that could happen. It is good for one thing in particular. It makes the finite speed of signals concrete, and it shows that the two channels by which the Sun reaches us, its light and its gravity, run on the same clock.

The eight-minute figure survives the scrutiny. What it really demonstrates is not that the Sun could disappear, but that nothing about it travels to us any faster than light itself.