Light Leaves the Sun. Here Is How Long It Takes to Reach Everything

This is Vishala Space. A particle of light has just left the surface of the Sun. It is the fastest thing in the universe. And by the time I finish this sentence, it will not even have reached Mercury. In the time it takes me to say all this, it covers only a few widths of the Sun itself. So this time, we ride along with it, and time the trip to everything. Why does the fastest thing there is still feel slow out here?

On our ruler, Earth is a one-inch marble, the Moon a peppercorn thirty inches away, and the Sun a nine-foot ball about a thousand feet down the road. Shrink the world that much, and light shrinks with it. On the ruler, light moves at twenty-three and a half inches a second. That is slower than a stroll. And at that pace, it has a very long way to go. Every time in this episode comes from NASA and ESA data, and whenever we speed the clock up, the screen will say so. First, the number itself.

Light in empty space moves at one hundred and eighty-six thousand, two hundred and eighty-two miles every second. That is about six hundred and seventy million miles an hour. In one second, it could circle the Earth seven and a half times. And this number is not an estimate. Since nineteen eighty-three, the metre itself has been defined by it, so the speed of light is exact, by definition. Nothing with mass can ever reach it. So the only question left is distance.

Light is fast, but your phone can feel the delay. In one billionth of a second, light travels eleven point eight inches. The computer pioneer Grace Hopper used to hand out wires that long, to show programmers what a nanosecond is. GPS satellites orbit about twelve thousand five hundred miles up, so their signals need about a fifteenth of a second to reach you. Your phone compares those tiny delays from several satellites at once, and turns them into your position. One billionth of a second wrong means about a foot wrong. And the satellite clocks run fast, by thirty-eight millionths of a second every day, because of Einstein's relativity.

Left uncorrected, that drift would add kilometres of error every day. So let us start the clock, and follow one flash of light outward. Mercury: three minutes and thirteen seconds. Venus: six minutes. Then home. Earth, on average, is eight minutes and nineteen seconds from the Sun. The sunlight on your face right now left the Sun before you pressed play.

Mars: twelve minutes and forty seconds. And then the asteroid belt, where the largest body, the dwarf planet Ceres, sits twenty-three minutes out. Four planets and a belt of rubble, all inside the first half hour. Two more numbers from the start line. The closest any spacecraft has come to the Sun is NASA's Parker Solar Probe. On Christmas Eve, twenty twenty-four, it passed three point eight million miles above the surface, so close that sunlight reached it in about twenty seconds. And that sunlight is only the last leg of a much older trip.

Its energy was made in the Sun's core, and it takes a very long time to work its way out. One well-known model puts it at about a hundred and seventy thousand years. The final eight minutes are the easy part. But that eight minutes and nineteen seconds is an average. The figure you usually hear, eight minutes twenty, is the same number, rounded. Earth's orbit is slightly stretched. In early January we are closest to the Sun, and sunlight arrives in eight minutes and eleven seconds.

In early July we are farthest, and it takes eight minutes and twenty-seven. Between the planets, the swing is far bigger. A signal from Earth to Mars can take about three minutes when the two planets pass close, or more than twenty-two minutes when Mars is on the far side of the Sun. Send a rover a question at the worst moment, and you wait about three quarters of an hour for the answer. That is why nobody drives a Mars rover with a joystick. Its moves are planned on Earth and sent ahead, a day at a time. That delay once decided a landing.

On the eighteenth of February, twenty twenty-one, Mars was eleven minutes and twenty-two seconds of light away. The Perseverance rover's descent, from the top of the atmosphere to the ground, took about seven minutes. So when the first signal saying entry has begun reached Earth, the rover had already landed, or crashed, minutes earlier. Nobody on Earth could do anything but wait for the light. Now the clock gets serious. Past the asteroid belt, the gaps open up. Jupiter: forty-three minutes.

On our ruler, that is about a mile from the nine-foot ball, and light has been walking at a stroll the whole way. Saturn: one hour and nineteen minutes. Uranus: two hours and forty minutes. Neptune, the last planet: four hours and ten minutes. And Pluto, out in the cold: about five and a half hours. Now put that on a clock face. If the flash leaves the Sun at noon, it passes Earth before ten past twelve, Jupiter before one o'clock, and Pluto at about half past five in the evening.

Most of a working day, just to cross the planets. Pluto's distance once made for a very nervous evening. On the fourteenth of July, twenty fifteen, New Horizons flew past Pluto, too busy to talk. Pluto was closer than average that year, and the spacecraft's signal saying it had survived still took about four hours and twenty-five minutes to reach Earth. Then came the photos and measurements, more than fifty gigabits in all. At a few thousand bits a second, the last of it arrived fifteen months later. Keep the clock running for a full day.

In twenty-four hours, light covers sixteen billion miles. That distance has a name: one light-day. It is a hundred and seventy-three times the distance from the Earth to the Sun. On our ruler, it is thirty-two miles from the nine-foot ball. For almost fifty years, nothing we built had ever been that far away. Then, in November twenty twenty-six, Voyager 1, launched in nineteen seventy-seven and still talking to us, reaches one light-day from Earth. Send it a command, and the reply comes back two days later.

Fast and far are the same problem. Space is so big that even light needs years to reach the neighbours. So how do we know light takes time at all? The first clue came in sixteen seventy-six, from a moon of Jupiter. Io circles Jupiter every forty-two and a half hours, and slips into Jupiter's shadow like clockwork. But the Danish astronomer Ole Roemer noticed the eclipses ran late when Earth was far from Jupiter, and early when it was close. The difference was the time light needs to cross Earth's orbit.

Roemer's figure was about twenty-two minutes. The true value is sixteen minutes and thirty-eight seconds, but the idea was right. Light takes time. After Roemer, the race was on to pin the number down. In seventeen twenty-nine, James Bradley used the slight tilt of starlight, the way rain seems to slant when you run through it, and found that sunlight takes eight minutes and twelve seconds to reach us. In eighteen forty-nine, Hippolyte Fizeau sent a beam through a spinning wheel with seven hundred and twenty teeth, to a mirror more than five miles away and back. His answer was about five percent too high.

Foucault's spinning mirror, in eighteen sixty-two, came within one percent. Albert Michelson, timing light between two California mountains in the nineteen twenties, got within a few kilometres a second. By nineteen seventy-two, lasers had measured it to about one metre a second. And in nineteen eighty-three, scientists stopped measuring it. They fixed the speed of light, and made it the ruler for everything else. Today the same light-time idea runs NASA's Deep Space Network, timing radio signals to spacecraft across the solar system. And a clock like this tells you one more thing.

But how long is it, really, laid out on the ground? Next time, we take one light-year and lay it along a real road. Next episode: one light-year, laid along a real road. Subscribe, and I will see you there.