How many stars can you see?

A star you can see with the naked eye is one bright enough to reach you without binoculars or a telescope. Astronomers measure brightness in magnitudes, and the scale runs backwards. The smaller the number, the brighter the star, and the very brightest have negative numbers.

On a perfectly dark night, a healthy eye can pick out stars down to a magnitude between 6 and 7. Astronomers have listed every star that bright in one catalogue, the Yale Bright Star Catalogue. If you count the stars in it until it reaches your eye's limit you get between 5,100 and 8,400 for the whole sky (north and south together). The range is honest: it depends on where exactly the limit sits.

That limit matters more than you might expect. There are far more faint stars than bright ones. Of all the stars you could see on the best night, most sit near the very edge of vision. So a sky that is just a little bit brighter loses those stars first, and a lot of them at once.

You never see the whole sky at once. The ground hides half of it. And near the horizon, starlight passes through far more air than it does overhead. This means that the faintest stars fade out before they set.

Put those together and, on a clear, moonless night far from lights, you can see between 1,100 and 3,200 stars at once. The count shifts with your latitude and the hour, because the crowded band of the Milky Way rises and sets like everything else. These numbers come from our own calculation with the catalogue, so treat them as good estimates, not a census.

As Earth turns the sky changes, and new stars rise in the east while others set in the west. Since you opened this page, the ground at the equator has carried people 0 kilometres eastward, under a sky that stays put. Over a night, and over a year, the turning Earth shows you far more than one moment can. Where you live decides which part of the sky you never see. From Europe, some of the southern sky never rises. From Australia, the stars around the North Star never do.

Three things define how many stars you see: your eyes, the air and the light around you.

Your eyes need time. In the dark your pupils open within minutes, but the chemistry of night vision takes much longer to build up, and a few seconds of bright light, a phone screen or a torch, undoes it. Look slightly to the side of a faint star and it often appears, because the cells that work best in dim light sit away from the centre of the eye.

The air matters too. Haze, dust and humidity dim the stars, most of all near the horizon.

Then there is the glow of our own lights. Street lamps, windows and signs send light up into the sky, where the air scatters it back down as a dull glow. Faint stars drown in it. Just outside a small town, a clear night can still reach the low end of the dark-sky count. In a suburb, you may see between 100 and 210 stars at once. In a city centre only the brightest survive: between 3 and 27, plus the Moon and any bright planets.

The Milky Way is the soft band of light that crosses a dark sky. It is our own galaxy, seen edge-on from inside: the glow of countless stars too far away to see one by one. Before artificial light spread, it was part of every clear, moonless night.

In 2016, scientists published a world atlas of how bright the night sky is, built from satellite data and tens of thousands of measurements on the ground. On their figures for 2014, about 36 percent of the world's people lived where artificial light hides the Milky Way. That atlas is still the best global count we have.

Since then the glow has grown. Volunteers around the world compare the stars they see with star charts and report the faintest they can make out. From those reports, the night sky grew brighter by about 9.6 percent a year, give or take a little, between 2011 and 2022. That is faster than satellites show, because they miss much of the blue light of modern white LEDs. So today the share who can't see the Milky Way from home is very likely higher.

Even the best dark night shows you only our planetary neighbourhood. Nearly every star you can see belongs to the Milky Way, most of them relatively close. Our own galaxy holds between 100 billion and 400 billion stars, although nobody has counted them. The number comes from the galaxy's mass and from models of how many small, faint stars there are, so careful estimates differ widely.

Take the low end. For every star you can see on the darkest night, the Milky Way holds about 12 million times as many. Almost all of them are too faint, too far away or hidden behind dust. Their light still reaches us, blended into that pale band.

There are more stars beyond. One faint smudge you can see with your eyes alone from the north, on autumn evenings, is not a star at all. It is the Andromeda Galaxy, a whole galaxy of its own! From the southern half of the world, the Large Magellanic Cloud, a small neighbouring galaxy, hangs in the sky as a bright smudge.

The stars you can see on a dark night are the ones people have always looked up at. Scientists think even the earliest humans knew red Betelgeuse in Orion, and many cultures have woven it into their stories.

They are still there over every city. The glow hides them; it doesn't remove them. And unlike most losses, this one can be undone. Good lighting points down, uses only as much light as it needs and switches off when no one needs it. Those ways of lighting are well understood, and many of them save electricity too.

So the count of stars you can see is partly a measure of the sky and partly a measure of us. Find a dark place on a clear night, give your eyes time, and the number climbs, star by star, towards the sky your ancestors knew.