Why the night sky looks the way it does

Go somewhere properly dark. Not the end of the garden — somewhere half an hour from the nearest town. Then stand still for twenty minutes and don't look at your phone.

The sky that turns up is not the one you're used to. There are far too many stars. They're different colours. And a pale band runs right across the top of everything, as if someone spilled milk up there, which is roughly what the Greeks concluded had happened.

You don't need a telescope for any of what follows. You need about six ideas.

Everything up there is old news

Light is fast. Space is worse.

Sunlight takes eight minutes and twenty seconds to reach us, so the Sun you squint at is the Sun from eight minutes ago. The Moon is a second and a half away, near enough to feel present. After that it stops being a rounding error.

Alpha Centauri, the nearest star system to ours, is four and a bit light-years out. Some of the stars in Orion are hundreds or thousands of light-years away, so you're catching light that set off while Rome was still a going concern. And if you can find the faint smudge of the Andromeda galaxy — you can, from a dark site, with nothing but your eyes — that light left about two and a half million years ago. There were no people yet. There were things that would eventually become us.

So the sky isn't a photograph. It's a stack of different centuries all arriving at once, sorted by distance. Two stars sitting side by side in the same constellation might be showing you the Bronze Age and last Tuesday.

The fastest trick in stargazing

Stars twinkle. Planets don't. That's the whole trick, and it works in about ten seconds.

The reason is apparent size. A star is so far away that it stays a single point of light no matter what you look at it through — even a large telescope can't turn it into a disc. That one thin beam has to cross our atmosphere, which is turbulent and shifting about, and it gets nudged off course on the way down. The point shimmers.

A planet is close enough to be a tiny disc instead. A disc is made of many beams arriving at once, and they get nudged in different directions simultaneously, so the wobble averages itself out. The light sits still.

The bright thing that refuses to flicker, then, is a planet — nearly always Venus, Jupiter, Mars or Saturn. It's also why everything twinkles harder down near the horizon, where you're looking through a much fatter wedge of air, and why observatories keep ending up on top of mountains.

Brightness numbers run backwards

Every guide throws magnitudes at you and none of them stop to mention that the numbers go the wrong way round. Smaller is brighter. The good stuff is negative.

Blame Hipparchus. In the second century BC he sorted the visible stars into six buckets: first magnitude for the show-offs, sixth for the ones you're not entirely sure are there. When astronomers could finally measure brightness properly, they kept his buckets rather than start again, and in 1856 Norman Pogson pinned the thing down — five magnitudes means a hundredfold difference in brightness. One step is therefore about two and a half times.

Bolting a real scale onto an old eyeball ranking shoved the brightest objects off the end into negative numbers. Sirius sits at about −1.5. Venus reaches −4. The full Moon is around −13, which is why it wrecks everything else in the sky for a week.

The number to keep is 6. That's roughly as faint as the naked eye goes under a genuinely dark sky. If a guide lists something at magnitude 5, there is no point at all hunting for it from a lit street.

The band is us, seen edge-on

From somewhere dark, a soft band arches over your head. That's the Milky Way, and for once the name is a fair description of the thing.

We live inside a flattened disc of a few hundred billion stars, roughly halfway out from the middle. Look along the plane of that disc and you're looking through the greatest possible depth of stars — so many, so far off, that they smear together into a glow instead of resolving into points. Look at right angles to the disc and you're staring out through the thin part, which is why the rest of the sky seems comparatively empty.

The dark lanes running down the middle of the band aren't gaps. They're dust: cold clouds blocking the light of everything behind them. That dust is the raw material the next round of stars gets built from.

Constellations aren't things

Worth being blunt about this one. The stars in a constellation are almost never neighbours. They look close together from where we happen to be standing, and that is the entire relationship. Along the line of sight they're often hundreds of light-years apart.

Orion is the usual example. From Earth it's a figure with a belt and a sword hanging off it. Move a few dozen light-years sideways and the hunter falls apart into unrelated stars with nothing to do with each other. The pattern lives in your visual cortex, not in space.

That isn't a complaint. Constellations are one of the oldest pieces of human technology still in daily use, and they do a job nothing has replaced: they let you tell someone else where to look.

Your hand is a protractor

Star charts tend to assume you already know where you're pointing, which is the one thing a beginner doesn't.

Fix that first. Hold your arm straight out. Your closed fist covers about ten degrees of sky. Your three middle fingers, about five. Your little finger, about one. This works for a faintly ridiculous reason — people with big hands tend to have long arms, so the two errors cancel. Horizon to straight overhead is ninety degrees, which is nine fists. Once "about fifteen degrees left of that bright one" means something physical, written directions start to work.

Then two signposts. The two stars on the outer edge of the Plough's bowl, the end away from the handle, point almost exactly at Polaris, roughly five fists off.

And here's what nobody warns you about: Polaris is unimpressive. Everyone expects the North Star to be the brightest thing up there. It's somewhere around fiftieth. It matters because of where it sits rather than how it looks — parked near the point the whole sky pivots around, so it barely moves all night while everything else wheels past it. Its height above your horizon is your latitude, which is how people crossed oceans without instruments.

In winter, Orion's belt handles the other half. Follow it down and left to reach Sirius, the brightest star in the night sky. Run it up and right and you land on orange Aldebaran, and past that the Pleiades, which look like a thumbprint until you glance slightly to one side of them and they quietly resolve into a cluster. In summer it's Vega, Deneb and Altair — the Summer Triangle, wide and bright enough to survive a fairly poor sky.

The sky runs two clocks

Watch for an hour and the whole thing slides west at fifteen degrees an hour. That's just Earth turning, and it's why a telescope drifts off its target within a minute or two unless something is driving it.

The second clock is slower and catches everybody out. Stars rise about four minutes earlier every day, because Earth has to turn slightly past a full rotation to bring the Sun back round to the same place, while the stars only need the rotation itself. Four minutes a day compounds to two hours a month, and two hours a month is why the early-evening sky in January and the early-evening sky in July have almost nothing in common.

It's also why Orion is a winter constellation in the north. It hasn't gone anywhere in summer. It's just up during the day.

Colour is a thermometer

Stars aren't all white, though it takes a moment of deliberate looking. Betelgeuse, on Orion's shoulder, is orange. Rigel, diagonally opposite, is blue-white. Same constellation, no equipment needed — it's the easiest colour comparison the sky offers, and once you've seen it you can't unsee it.

The difference is temperature, and nothing else. Hot things glow at shorter wavelengths, which is why a poker in a fire goes red, then orange, then white as it heats. Stars do exactly the same. Red is the cool end, around 3,000°C at the surface. Yellow, like our Sun, about 5,500. Blue-white runs to 10,000 and beyond.

The part that feels wrong: hot stars die young. Blue giants tear through their fuel in a few million years and then go out spectacularly. The little red dwarfs are stingy, and can keep burning for longer than the universe has so far existed. Not one red dwarf has ever died of old age. There hasn't been time.

Telling apart the things that move

Most of what crosses the sky sorts itself out within seconds, once you know the tells.

A plane blinks. Red and green, on a regular beat, and if it's close enough you'll hear it a moment later.

A satellite doesn't blink at all — a steady point travelling in a straight line, taking several minutes to cross. You can see it because it's high enough to still be in sunlight while you're standing in the dark. The Space Station is the obvious one, often brighter than anything else up there, and it has a lovely exit: it doesn't set, it fades out halfway across the sky as it slides into Earth's shadow.

A meteor is finished before you've got your arm up. A fraction of a second, two at the outside. Nearly all of them are made by something about the size of a grain of sand burning up eighty to a hundred kilometres overhead, which is a strange thing to know while watching one. On an ordinary dark night you'll catch a few an hour. During the Perseids in August, or the Geminids in December, considerably more, all appearing to fan out from the same patch of sky.

And since 2019 there's a new category: a line of satellites travelling in convoy, which is a recent Starlink launch before the batch spreads out to its working orbits. Astronomers are not delighted about it.

Nobody has properly explained the Moon

A Moon sitting on the horizon looks enormous. The same Moon overhead looks ordinary. Photograph both with the same lens and they measure identically, which means the whole effect is happening inside your head rather than in the sky.

The usual explanation is about distance cues. Near the horizon you see the Moon beyond buildings and hills and trees, all of which tell your brain "that is a long way off", and something judged to be far away while taking up the same amount of sky gets interpreted as bigger. Overhead there's nothing to measure it against.

Whether that's actually the answer is still argued about, and has been for more than a thousand years, which is faintly embarrassing for a species that has walked on the thing.

Two things it definitely isn't. It isn't the atmosphere magnifying anything — if anything the air squashes the low Moon into a slight oval. And it isn't distance. The Moon's orbit does vary, but on a monthly cycle with no connection to where it happens to be sitting tonight.

What the streetlights have taken

From a properly dark site you can pick out a few thousand stars, and the Milky Way isn't subtle — it's the most obvious feature up there. From the middle of a city you'll manage a few dozen stars and no galaxy at all.

Astronomers grade this on the Bortle scale, one to nine, pristine to hopeless. Most people in Europe and North America have never stood under the better half of it. A good number have never seen the Milky Way in their lives, which is a peculiar thing to be true of a species that spent almost all of its history sleeping outdoors.

The cause is light thrown upwards, scattering off the air. That's also the one cheerful thing about it: unlike more or less every other kind of pollution, it stops the moment you aim the lamp downwards. Shielded fittings fix it, and they aren't expensive.

One last thing, and it's the reason most people give up early. Your eyes need a solid twenty to thirty minutes to adapt to darkness, and a single glance at a phone throws most of that away. Red light does the least damage, which is why observers use red torches. If you've decided you can't see many stars, there's a decent chance you just didn't wait.

Binoculars beat a cheap telescope

If you want more than your eyes give you, don't buy a telescope. Buy binoculars, or dig out the pair already in the cupboard.

They're easier to point, easier to hold, and an ordinary 10×50 pair does more than people expect. Jupiter becomes a small disc with up to four dots strung out beside it, and the dots shuffle position from one night to the next — that's the observation Galileo made in 1610, and the one that started taking apart the idea that everything went round the Earth. The Pleiades stop being a smudge and become several dozen stars. Andromeda turns into a definite oval. And along the Moon's terminator, the line dividing lit from unlit, craters throw long shadows, because sunlight there is arriving almost horizontally.

Rest your elbows on a wall or the roof of a car. Most complaints about binoculars turn out to be complaints about a shaking image.

Everything is moving, just slowly

The sky looks permanent, and on our timescale it very nearly is.

Every star up there is moving, some of them quickly, but at those distances it takes thousands of years for anything to become obvious. The constellations are quietly deforming as we watch. Give it a hundred thousand years and the Plough will not be a plough.

Earth's axis wanders as well, tracing a slow circle over roughly 26,000 years. Polaris hasn't always been the north star and won't remain one. When the pyramids were going up, the pole pointed near Thuban in Draco. In about twelve thousand years the job passes to Vega. Sailors in different centuries were steering by genuinely different skies, using the same method.

None of which you need in order to enjoy any of it. Find somewhere dark, give your eyes twenty minutes, and look up. You're looking at a slice of a galaxy, sorted by distance, arriving out of the past, colour-coded by temperature. Nobody charges for it.