Day and Night Map
Where it is light on Earth right now, where it is dark, and the twilight in between. Drag the slider to move through the next seven days, or press play and watch the line sweep round. The same picture as the world clock's map, with time attached.
Play runs at about six hours a second: one full turn of the Earth takes four seconds, and the whole week about half a minute. Nothing on the map slides sideways while it runs — the shaded shape is re-solved for each moment, meridian by meridian — so the picture never runs off the edge and jumps back. The one thing that does cross the edge is the sun marker, and that is honest: this map is a cylinder cut open at the date line, so the point the sun stands over leaves at one side and arrives at the other.
How to read this map
Half of the Earth is in sunlight at every moment, and half is not. This map draws that division for one instant: the bright half is where the sun is above the horizon, the dark half is where it is below, and the soft band between them is twilight — the places where the sun has set but the sky has not finished going dark.
The yellow dot is the one place on Earth where the sun is directly overhead at that moment — straight up, no shadow at noon. It is the middle of the lit side, and everything else follows from it: the further you are from that dot, the lower the sun sits in your sky, until at ninety degrees away it is exactly on your horizon and you are standing on the line.
The line is called the terminator, and it moves west at about 1,670 kilometres an hour at the equator — the speed of the Earth's own turn. Watch it with Play and you are watching sunrise sweeping across one edge of the world while sunset sweeps across the other. Every place on the map crosses it twice a day, which is the definition of a day.
Where the sun is standing, seen from the side
The map above is drawn looking down at the ground. This is the same instant seen from beside the Earth's orbit — level with the sun, out in the plane the Earth travels in. It is the picture that explains the other one.
The sun is so far away that its light arrives in parallel lines, which is why exactly half the globe is lit whatever the date. One of those lines is special: the one that runs from the centre of the sun to the centre of the Earth. It is drawn solid yellow, and it meets the surface at a single point — the place where the sun is straight up, where a flagpole casts no shadow at noon. That point is the yellow dot on the map above. This is the same dot, from the side.
That line lands at 10.8° N — 12.6° short of the Tropic of Cancer, which is as far north as it can ever get. The northern half of the world is leaning into the light, so more of it falls inside the lit half than outside, and its days are longer than its nights.
So what are the Tropic of Cancer and the Tropic of Capricorn?
They are the two limits of that yellow line. The Earth's axis is tilted 23.4° and it keeps pointing the same way all year — it does not wobble round to follow the sun. So as the Earth goes round its orbit, the amount of that tilt facing the sun changes: fully toward it in June, fully away in December, and sideways-on at the two equinoxes in between. That is the lean you see in the picture, and it is the whole of the seasons.
Because the lean can never be more than the tilt itself, the centre-to-centre line can never land more than 23.4° from the equator. The two circles round the Earth at exactly that distance — 23.4° north and 23.4° south — are the Tropic of Cancer and the Tropic of Capricorn. They are drawn dashed in the picture above and on the map at the top of this page.
So the tropics are not a climate, a rainforest, or a stripe on a globe somebody chose. They are a measurement of the Earth's tilt, written onto its surface. If the tilt were 10°, the tropics would sit at 10°. If it were zero, they would both collapse onto the equator and there would be no seasons at all.
The dashed line through the middle of the globe is the axis, and the faint upright line down the centre is the day/night line — the terminator — seen edge-on. On the map at the top of the page that same line is the curve you can watch sweeping west. Here it is a straight cut through the middle of the world, which is what it always was.
How the shadow changes through the year
If the Earth spun upright, the day/night line would be a straight vertical line — every place on the planet would get twelve hours of daylight, every day, forever, and this map would be very dull. The Earth does not spin upright. Its axis is tilted by 23.4°, and that single fact is what puts a lean on the line, gives us seasons, and makes the length of your day depend on where you live and what month it is.
Here is the part that is easy to get backwards. The amount of shadow never changes. Half the Earth is lit and half is dark at every moment of every day of the year, because the sun lights half a ball — that is what a ball in a beam of light does, in June, in December, always. Winter is not more shadow.
What changes is where the line falls, and therefore who is inside which half. When the northern end of the axis leans toward the sun, the lit half is tipped north: it swallows the whole Arctic and leaves a wide slice of the south in the dark. The north is not getting a bigger world, it is getting a bigger share of the same lit half — so northern days run long and northern nights run short, and the southern hemisphere gets the opposite deal at the same instant. Six months later the Earth is on the far side of its orbit, the axis still points the same way, and now it leans away from the sun instead. The shading flips top to bottom.
Press June solstice and look at the top of the map: the whole Arctic is inside the lit half, all the way round, so the sun there does not set at all. Press December solstice and the same region is inside the dark half instead — that is the polar night, and in a place like Utqiaġvik in northern Alaska it lasts for weeks. Fairbanks, further south, does get a sunrise in December, but only about four hours of daylight before the line catches up with it again.
Two motions are on show at once and they run at wildly different speeds. The turn — the line sweeping west — takes a day. The lean — the whole pattern tipping north and south — takes a year, so it barely changes across the seven days the slider covers. That is why the jump buttons exist: they are the only way to see the slow one.
Why this map is flat — and what that costs
The Earth is a sphere, and no flat picture of a sphere can be honest about everything at once. Every world map ever drawn has chosen what to get right and what to sacrifice. This one is an equirectangular map, and its choice is simple: longitude is drawn evenly across, latitude evenly down. Every vertical line is a meridian, every horizontal line a parallel, and the grid is a plain rectangle.
That is exactly the right choice for this picture and the wrong one for most others. Because longitude runs evenly across it, an hour of the Earth's turn is the same distance everywhere on the map — 15° of longitude, one twenty-fourth of the width, at the top of the map and at the bottom alike. The terminator can therefore sweep across at a constant rate, and time zones are straight vertical strips. On a Mercator map, or a globe drawn in perspective, none of that holds.
What it sacrifices is size. On the real Earth the meridians converge — they meet at the poles — so a degree of longitude is a shorter distance the further you get from the equator, by exactly the cosine of the latitude. This map draws them all the same width anyway. So everything is stretched sideways, by 1 ÷ cos(latitude), and the further from the equator you look, the worse it gets:
Greenland is the famous victim. It sits between about 60° and 83° north, so this map stretches it by a factor of two to eight, and it ends up looking like a continent. It is not. Greenland covers about 2.17 million square kilometres; Africa covers about 30.4 million — roughly 14 Greenlands. On the map above they look comparable. Africa straddles the equator, where this projection is honest, so Africa is the one drawn nearly right and Greenland is the one blown up.
The same distortion is why this map is cropped at 82.0°N and 62.0°S rather than drawn to the poles. At the poles the stretch is infinite: a single point becomes the entire width of the map. Those two bands are where the projection stops meaning anything, so they are not drawn.
The honest way to see all of this is a globe. The Sun, Earth and Moon simulator draws the Earth as a ball with the same day/night line on it, from the same maths — the terminator you see leaning here is the one you see curving round the globe there. Compare the two and the projection stops being an abstraction.
Things to try
- Find your own bedtime. The map marks where you are as soon as your browser shares it — if it did not, there is a My location button on the map. Then drag the slider to tonight and watch the shading arrive over your dot: that is your sunset, to the minute the sun goes down where you are standing.
- Who is asleep right now? Press Now and look at the four city rows under the map. Then pick a country the class has a connection to and check whether anyone there would answer the phone.
- Race the line. Press Play and follow the terminator west. It crosses the whole map in 24 hours, which at the equator is about 1,670 km/h — faster than an airliner. Ask which way you would have to fly to keep the sun from setting.
- Break the tilt. Jump to the June solstice, then to December, and watch the top of the map swap from all-light to all-dark. Ask what would happen to seasons if the tilt were zero — the answer is on the map, because the line would simply stand up straight.
- Catch the equinox. Jump to an equinox and check the line: nearly vertical, and every place on Earth getting about twelve hours of each. It is the only date the map is symmetric.
- Argue with the map. Compare Greenland with Africa, then look up their real areas. This is the cheapest possible lesson in why the projection matters, and it lands harder when the map is one they have just been using and trusting.
Common questions
What is the line between day and night called? The terminator — or the day/night line, or the grey line. It is the circle around the Earth where the sun is exactly on the horizon, so everywhere along it is either rising or setting at that moment. It sweeps west at the speed of the Earth's rotation, about 1,670 km/h at the equator and slower the further from the equator you go.
Why isn't the day/night line straight up and down? Because the Earth's axis is tilted by 23.4° relative to its orbit. The line is only vertical at the two equinoxes, when the sun stands over the equator. At every other time of year the sun is north or south of the equator, one pole leans into the light and the other away from it, and the line leans by the same amount — up to 23.4° at the solstices.
What is the yellow dot? The subsolar point: the single place on Earth where the sun is directly overhead at that instant. It circles the planet once a day and drifts slowly between the two tropics over the year. It is always at the centre of the lit half, so the sun's height in your sky is set by how far you are from it.
What are the Tropic of Cancer and the Tropic of Capricorn? They are the two limits of the sun's travel north and south, and they are set by the Earth's tilt. Draw the line from the centre of the sun to the centre of the Earth: it meets the surface at the one place the sun is directly overhead. Because the axis is tilted 23.4° and keeps pointing the same way all year, that point can never land further than 23.4° from the equator. The circle at 23.4°N is the Tropic of Cancer, reached at the June solstice; the circle at 23.4°S is the Tropic of Capricorn, reached in December. They are the Earth's tilt written onto its own surface — not a climate zone, and not a line anybody chose.
Does the Earth get more shadow in winter? No — and this is the thing most often got backwards. Exactly half the Earth is lit and half is dark at every moment of every day of the year, because that is what sunlight does to a ball. What the tilt changes is where the dividing line falls, and so which places end up inside which half. In winter your hemisphere is leaning away from the sun, so more of it sits in the dark half as the Earth turns: your nights are longer, not because there is more darkness in total, but because you are getting a bigger share of it.
Why is there a soft band instead of a sharp edge? Because sunset is not the end of daylight. The band is twilight — the sun is below the horizon but still lighting the sky from underneath. The band's outer edge is 18° below the horizon, the point where the sky is fully dark. It is wider near the poles, which is why a summer night in Scotland or Scandinavia never gets truly dark.
Why does the map look stretched near the top and bottom? Because it is an equirectangular projection: it draws every degree of longitude the same width, when on a sphere they get narrower toward the poles. Everything is stretched sideways by 1 ÷ cos(latitude) — 1.4× at 45°, 2× at 60°, 4.8× at 78°. It is why Greenland looks like Africa here when Africa is really about 14 times bigger.
Is the map showing real time? Yes. It opens at the current moment and repaints every minute while it is left alone. The slider moves it forward through the next seven days, and Now brings it back. Everything is computed in your browser from the sun's position — nothing is fetched from a server, and there is nothing to sign up for.
Where can I see the exact sunrise and sunset time for my town? On the sun pages: they give sunrise, sunset, day length, twilight and a seven-day outlook for more than a thousand cities, or for any location you name. This map is the shape of the thing; those pages are the numbers.
Keep going
This map answers "where", to the nearest few hundred kilometres. For "when", to the minute, in your own town:
Why do we have seasons? Sunrise & sunset for your city Tonight's moon phase What time is it there? The same line, on a globe How sunrise is worked out Using these in a lesson