How long does sunlight take to reach Earth?

The Sun is a star, a huge ball of hot gas held together by its own gravity. Deep in its core, hydrogen fuses into helium, and that is where the energy in sunlight begins. What leaves the Sun's surface is light, and light is the fastest thing there is.

It moves at 299,792.458 kilometres every second. At that speed it could circle Earth's equator in 0.13 seconds, and reach the Moon in 1.3 seconds.

Even so, the gap to the Sun takes minutes to cross. The light warming your face left the Sun's surface 8 m 15 s ago. It was already on its way when you started reading this page.

Earth's path round the Sun is not a perfect circle. It is a slightly stretched loop, so our distance changes through the year. We pass closest in early January, at about 147 million kilometres, and farthest in early July, at about 152 million. The next close pass is on 3 January 2027, and the next far one on 5 July 2027.

So sunlight arrives a little sooner in January than in July. At the closest point it takes about 8.2 minutes, at the farthest about 8.5 minutes.

That is not why we have seasons. In early January, when Earth is nearest the Sun, it is winter in the north. The seasons come from Earth's tilted axis, which turns each half of the planet towards the Sun for part of the year. Compared with how far away the Sun is, the change in distance makes little difference to our weather.

Because light takes time, every look at the sky is a look back. The Sun in your sky is the Sun of a few minutes ago. If something changed on its face, we would not see it for 8 m 15 s.

What if the Sun vanished altogether? We would keep seeing it for those same minutes. Earth would also keep circling it for those minutes, because a change in gravity travels at the speed of light too. In 2017, gravitational waves and light from the same collision of two neutron stars reached Earth less than two seconds apart, after a trip of about 130 million years. Then the sky would go dark and Earth would carry straight on into space.

The same is true for everything else up there, only more so. The light of Proxima Centauri, the nearest star after the Sun, left it about 4.2 years ago.

The eight minutes are only the end of the trip. The energy in sunlight is made in the Sun's core, where hydrogen fuses into helium. Getting out of the Sun is the slow part. In the dense radiative zone the energy is absorbed and given off again countless times, a step at a time in a random direction. Higher up, in the convection zone, rising bubbles of hot gas carry it to the surface.

How long it all takes is not known well. Careful calculations with models of the Sun's interior give answers between 10,000 and 170,000 years. Older textbooks print much bigger numbers, from rough sums that treat the Sun as the same density all the way through.

Even the low end is longer than all of recorded history, about 5,200 years. When the energy in today's sunlight set out from the core, people like us were already alive: our species is about 320,000 years old.

Sunlight reaches Mercury in about 3.2 minutes, Mars in about 13 minutes and Jupiter in about 43 minutes. Out at Neptune it arrives after about 4.2 hours.

Out there it is also faint. Light spreads out as it travels. Above Earth's air, each square metre facing the Sun gets 1,361 watts of sunlight. At Neptune the same square metre gets 1.508 watts.