What does the night sky look like tonight?

A star map is a picture of the whole sky above one place at one moment. Imagine lying on your back in an open field, with your head to the north. The centre of the star map circle is the point straight above you. The rim is the horizon all the way round, marked N, E, S and W. East is on the left, because that is where it falls when you look up from there.

The brighter a star, the bigger its dot. Thin lines join the stars of each constellation, and the constellations' names are in capitals. The pale band is the Milky Way: the glow of countless stars in the disc of our own galaxy, too far away to see one by one. Small open symbols, each with its name, mark star clusters, nebulae and galaxies, such as the Andromeda Galaxy. Each planet is a dot inside a small ring, and the Moon is drawn with its phase.

Under the circle, the map prints its date, its local time with the offset from UTC, and its place. On a dark night the circle is black. In twilight it turns deep blue, and by day it is paler still, with a note that the Sun outshines the stars. The stars are drawn anyway, because they are still there.

The stars are not racing across the sky. We are turning beneath them. Earth spins once on its axis, measured against the stars, in 23 h 56 m 04 s. As it turns, stars rise in the east, climb, and set in the west, just as the Sun does. At the equator the spin carries the ground along at 0.4651 kilometres per second.

Earth's axis points at one spot in the northern sky, very close to the star Polaris. So Polaris hardly moves, and the whole northern sky wheels around it.

That turn against the stars is a little shorter than the day on our clocks, which follows the Sun. Earth is also moving along its orbit, so each day it has to turn a little extra to face the Sun again. The result is that every star rises slightly earlier each night, and over a year the whole sky slides past. The stars of a winter evening are the stars of a summer morning. Set the map to tonight, then to the same hour half a year from now, and watch the constellations change places.

Light is the fastest thing there is, but space is wide. A light-year is the distance light travels in a year: 9.5 trillion kilometres, enough to go around Earth's equator about 240 million times.

So the starlight in your eyes tonight is old. Sirius, the brightest star of the night, is 8.6 light-years away, so the light you see left it that many years ago. Rigel, the bright blue-white star in Orion, is about 860 light-years away; careful measurements still place it between 790 and 1,030 light-years from us. For Deneb, in the Swan, two careful ways of measuring give two different answers, somewhere between 1,240 and 2,830 light-years. The question is still open.

Each star on the map sends you a different date. Some of that light set out before you were born, some before your great-grandparents were. The sky you see is not one moment but many, arriving together. That is the quiet wonder of looking up: we all stand under the same old light, and it is still on its way.

The map can show the sky over any place at any minute from 1900 to 2100. Type the town where you were born, the date and, if you know it, the time. The sky that appears is the one that stood over you then, whether or not anyone looked up. Time zones before 1970 were not always written down, so the map prints the UTC offset it used, and you can check it.

Since then, Earth has carried you once around the Sun for every year of your life, at 29.78 kilometres per second. At that speed, London to New York would take about 3.1 minutes. Since you opened this page, Earth has carried you 0 kilometres around the Sun.

There is a quieter link too. Altair is 16.7 light-years away, Vega 25.0, Arcturus 36.7, Capella 43 and Aldebaran 67. If one of those numbers is close to your age, the light reaching you from that star tonight left it around the year you were born.

Or try 20:17 UTC on 20 July 1969, the minute Apollo 11 landed. From Aarhus, the Moon hangs low in the west with Jupiter just above it, and Mars glows low in the south beside Antares.

Where you stand on Earth decides which part of the sky you can see. The map is drawn for your latitude, so the sky over Aarhus and the sky over Sydney share some stars and miss others.

In the north, the sky turns around Polaris. South of the equator, Polaris stays below the horizon, and no bright star marks the southern pole of the sky. Instead the Southern Cross points the way towards it, and two soft patches of light, the Large and Small Magellanic Clouds, hang nearby. They are neighbouring galaxies, visible to the eye from dark places.

The south also has the nearest stars the eye can see. Rigil Kentaurus and Toliman, the bright pair of Alpha Centauri, are 4.34 light-years away. Proxima Centauri, a small red star close to them in the sky, is nearer still, but too faint to see without a telescope.

Familiar shapes look different too. Orion, a winter sight from Europe, stands on his head in a southern summer sky. And from near the equator, over a year, you can see almost every star there is to see. Wherever you live, half of the sky belongs to someone on the other side of the world, and they are looking up at it too.