Every Planet Fits Between Earth and the Moon (Only on Some Days)

This is Vishala Space. Line up every other planet in the solar system, side by side, and they fit in the gap between Earth and the Moon. That is the famous version. The true version is stranger. They fit only on some days of the month. And on most days, they do not. So which days?

And why would the answer change at all? Last time we built a ruler: Earth a one-inch marble, the Moon a peppercorn thirty inches away. This time we put Jupiter, Saturn and the rest into that gap, one by one, and check the famous claim the way it should be checked. By the end, you will know how to tell, on any day, whether it is true. First, the claim the way it usually goes around. Add up the widths of the seven other planets: Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. You get about two hundred and thirty-six thousand miles.

The average distance to the Moon is about two hundred and thirty-nine thousand miles. So they fit, with roughly two thousand seven hundred miles to spare. That is the version on the posters. Every width in this episode comes from NASA's planetary tables, and every one is drawn at the same scale as last time: one inch for seven thousand nine hundred and eighteen miles. So let us build it. Jupiter goes in first: about eighty-seven thousand miles across. On the ruler, eleven inches.

A basketball. Saturn next: seventy-two thousand miles, nine inches, with its rings left at home for now. Then Uranus and Neptune, two softballs, about four inches each. Then the small ones. Venus, almost a twin of the marble, just under an inch. Mars, half an inch. And Mercury, a pea, three-eighths of an inch.

Seven worlds, touching, in one line. Twenty-nine point eight inches of planets. The peppercorn sits at thirty point two. Watch them go in. A third of an inch to spare. It works. Almost too neatly.

So now we check it. Look at where that thirty point two inches is measured from. It runs from the centre of the marble to the centre of the peppercorn. But the planets cannot sit inside the Earth. The line starts at the Earth's surface, and it has to stop at the Moon's surface. Take away half the marble and half the peppercorn, and the real gap is twenty-nine and a half inches. In miles, two hundred and thirty-three thousand eight hundred.

The planets need two hundred and thirty-six thousand. On an average day, they do not fit. They are about two thousand three hundred miles too long, and Mercury is left hanging off the end. And there is a second catch, hiding inside the giants. Jupiter spins once in less than ten hours. Saturn in less than eleven. Spinning that fast, they bulge at the middle.

The widths on the posters are averages. But stand the planets upright in a row, the way every picture shows them, and what touches the next planet is the equator, the widest part. Jupiter is nearly two thousand miles wider across its equator than its average. Saturn, two and a half thousand. Measured side by side, the line grows to two hundred and forty-one thousand miles. Thirty point four inches on our ruler. Now it is more than seven thousand miles too long for the average gap.

Almost a whole Earth of planets with nowhere to go. And that is with Saturn's rings left out. The bright rings alone are about a hundred and seventy thousand miles across. Put them back, and Saturn fills nearly three-quarters of the gap on its own. So is the famous fact just wrong? Not quite. Because the gap itself does not stand still.

The Moon's orbit is not a circle. It is a slightly stretched loop, and the Moon runs around it once every twenty-seven and a half days. At its closest point, called perigee, it swings in. At its farthest, called apogee, it swings out. Take one example month, November twenty twenty-six: the Moon reaches its farthest point on the thirteenth, two hundred and fifty-two thousand miles away. Twelve days later, on the twenty-fifth, it is at its closest: two hundred and twenty-three thousand. A swing of nearly twenty-nine thousand miles.

On our ruler, the peppercorn slides from almost thirty-two inches in to twenty-eight, and back out again, every month. And our line of planets, equators and all, needs the Moon's centre at least two hundred and forty-six thousand miles away. Just over thirty-one inches. Here is that month, hour by hour, sped up. So the answer is yes, but only sometimes. When the Moon is out near apogee, the gap opens wide enough for every planet. When it swings in, the gap closes and the line jams.

Run the numbers hour by hour, and the planets fit about one third of the time: a window of eight to eleven days each month, around apogee. The famous fact is not really a fact. It is a calendar. So how do you know about any given day? NASA's Jet Propulsion Laboratory publishes the Moon's position for every hour, from an orbit model tuned to more than fifty years of laser measurements. Look up the Moon's distance for your day, measured centre to centre. If it is more than two hundred and forty-six thousand miles, the line-up fits.

If it is less, the line jams. In our example month, the window opens on the eighth of November and stays open until the seventeenth. The same thing happens every month, around each apogee. And the windows for the months after this video comes out are in the description. Those laser measurements are how we know any of this so precisely. For decades, observatories in the United States, France, Italy and Germany have fired laser pulses at mirrors that Apollo eleven, fourteen and fifteen left on the surface, along with two more on Soviet Lunokhod rovers. Only a tiny trickle of the light comes back, two and a half seconds later, and the timing pins the distance down to a few millimetres.

They also show that the stretched orbit is not the same every month. The Sun keeps tugging on it. In twenty twenty-six, the deepest dive is on Christmas Eve: two hundred and twenty-one thousand six hundred miles, the closest of the year, right as the Moon is full. That full Moon will look about fourteen percent wider than the Moon at its farthest. And on that night, the planets miss by more than twenty-four thousand miles. One more thing the lasers show: the Moon is drifting away from us, about an inch and a half every year. So the gap is slowly opening.

If that drift held steady, the planets would fit on an average day in about three hundred million years. And Pluto? Since two thousand and six it is officially a dwarf planet. Squeeze it in anyway, and the window shrinks to under three days in ten. So that is the line-up. Seven planets, thirty point four inches. A Moon that swings between twenty-eight and thirty-two inches, every month.

It fits about a third of the time. The next time someone tells you every planet fits between the Earth and the Moon, you can ask them one question. Which day? Which brings us to light. A flash from the marble crosses that thirty-inch gap in one point three seconds. Sunlight, leaving the nine-foot ball a thousand feet away, takes eight minutes and twenty seconds to reach the marble. Fast and slow at the same time.

So how fast is light, really, out here? Next time, a photon leaves the Sun, and we follow it to everything. Next episode: light leaves the Sun, and how long it takes to reach everything. Subscribe, and I will see you there.