Daylight · two dials, one picture
Two things move here and they move on different clocks. Keeping them apart is the whole job, so the picture states which is which everywhere it can — in the colour, in the tracks along the edge, and in what holds still. On the globes the same two things simply swap jobs: the light holds still and the planet turns through it.
Where the sun never rises and where it never sets. Both are facts about the date alone, so both are drawn in gold — solid where the sun never sets, dashed where it never rises.
The edge between lit and unlit ground. On the map it sweeps once a day; on the globes the camera rides the sun, so the same edge holds still and the ground turns through it. Either way it is the hour. Drawn as a cyan line, dawn and dusk alike.
Inside it the sun does not set that day — the midnight sun. Its edge is the latitude where the sun’s lowest point of the day just grazes the horizon. It is a fact about the date alone, which is why it cannot move while the clock runs.
Sunrise on one side, sunset on the other. It moves west at roughly 1,670 km/h at the equator and slows to nothing at the poles — which is why, near the rings, it stops being a line and becomes a place.
Three plates, and each says what it is in its own corners rather than in a caption underneath. The tall one is both poles at once, drawn azimuthal equidistant so that latitude is the radius and the boundary circles come out as true circles. The two round ones are the same planet at 50° north and 50° south, one each side of the equator.
Day length depends on two numbers and nothing else: your latitude, and the sun’s declination — how far north or south of the equator it stands that day. Longitude never enters. That is why the colour of the ground here is made of concentric bands: every place on a circle of latitude gets the same amount of daylight, whatever hour it happens to be there.
The shadow is the other story. It is a single great circle sweeping the planet once a day, and it says nothing at all about how long anyone’s day is — only where in that day they currently are. Two facts, two clocks, one picture. The confusion this page is built to avoid is reading the second as if it were the first.
Nothing else on either plate is allowed to use those two colours, so the question “date, or hour?” is answered by the ink before you have read a word of it.
Watch either plate and the difference is in the motion itself. The shadow glides: it is continuous, and it never pauses. The rings do not glide at all — they hold one position for a whole day and then jump to the next, because the date they answer to is a whole number and nothing here ever reads the fraction. Drop to ½× and you can count the jumps. Anything that steps is the date; anything that glides is the hour.
The strip along the top of the polar plate is the year. Its curve is the real day length at 60°N, month by month, and the gold marker is today. The strip along the bottom is the day. Its curve is the share of the world’s land that is in sunlight, hour by hour, and the cyan marker is now.
They are drawn as different shapes on purpose. The year curve is a smooth annual swell. The day curve is not, because land is not spread evenly round the equator: it peaks when Africa and Eurasia face the sun together and bottoms out eight hours later over the Pacific. On the June solstice that is 74% of all land lit at 12:00 UTC and 38% at 20:00; in December, 65% against 27%. Nearly a three-to-one swing, and it is purely a fact about the hour.
Each dot is lit the way that city is lit right now: gold for sun up, then three shades of blue for civil, nautical and astronomical twilight, then black for full night. The figure beside the name is that city’s day length on the date shown, so the numbers count up and down as the year runs.
Neither camera sits above a place. Each sits at a fixed hour angle from the sun — 55° west of the sub-solar point, on both — so the lit and unlit halves never move on the screen, and everything that crosses the cyan line does so because the planet carried it there. That is the real arrangement, and it is the one thing a spinning-globe animation cannot show you: the terminator is not something that travels, it is a place the ground goes through.
Bolting the camera to the sun fixes the period of the turn as well. One turn is one solar day, and the day here is — — 360° divided by the — of longitude the sun covers every real second. There is no setting that gives a slow turn and a quick year independently, because a year is three hundred and sixty-five turns and the speed control moves both: at 1× the year takes —, and at ¼× the turn stretches to — with the year at —.
What that costs is stated rather than hidden: one turn carries — of date along with it, where a real Earth carries one. The compression is deliberate — it is the only way to watch a season arrive inside a single sitting — and it is why the rings visibly slide while the ground makes a single pass. Everything each of them says about its own clock stays exactly true; only their ratio to each other is squeezed.
The toggle above the globes swaps which half of that is allowed to stand still. Earth turns is the arrangement just described. Shadow moves bolts each camera to a longitude instead — 28°W and 62°W — and then it is the light that crosses a fixed view, the way it does on the map. Neither is more correct than the other; both read the same real longitudes, and the only difference is which motion you are being asked to watch. A crossing takes one solar day either way.
Earth wobbles nails the light down completely. Where the terminator meets the edge of the globe is decided by one thing — which way the sun lies on the screen — because a point on the edge is at a right angle to the camera, a point on the terminator is at a right angle to the sun, and the two conditions leave only the direction perpendicular to both. So holding the sun at a single screen position holds those contact points still, and in fact holds the whole line still, ends and middle alike.
Sun is a point answers the obvious next question: if the sun is not allowed its figure-of-eight, where does the eight go? Onto the ground. Pinning the sun means something has to absorb the wobble, and the only thing left to absorb it is the planet. So this mode marks one fixed spot on Earth and draws its path through the year, in the frame where the sun does not move — and it is the same curve, the same width against the same height, moved from the sky to the surface. Which of the two is “really” wobbling is not a fact about the solar system; it is a choice of what to hold still.
Seeing it costs one honest departure, and the plate admits to it: the globes are strobed. They are drawn at the same mean time every day rather than on the running clock, because the daily spin is a full turn and the wobble is a few degrees, and the first would drown the second entirely. That is exactly how an analemma photograph is taken — one frame a day, always at the same hour — and it is the only way the residual motion is visible at all. The map keeps the real clock throughout.
The price of pinning is the camera’s fixed latitude: to keep the sun there as the declination swings, the camera has to nod with it, from 25° to 77° and back over the year. That is the wobble. Underneath it the planet still turns once a solar day, so the ground both spins and rolls while the light does nothing at all. The corner read-out counts the latitude off as it goes.
One thing does still move the line under Earth turns, and it is worth waiting for: the date. As the declination swings through the year the terminator tilts and slides while the ground keeps turning at its own rate. So on the globes the two facts have exchanged roles — the ground carries the hour, and what is left of the line’s motion is the season — and they are still drawn in the same two colours they are on the map.
South rather than Africa because of how far the land actually goes: Africa stops at Cape Agulhas, 34.8°S, while South America runs on to Cape Horn at 56°S — twenty-one degrees further, and the difference between a continent that sees a long summer evening and one that sees a proper white night.
Relief is real, on all three plates. The land is shaded by a terrain model lit from the sun’s actual direction at each point — not from a fixed studio light — so mountains catch the light as they turn into the dawn line, and the Andes, the Rockies and the Atlas stand up strongest there. By the time a range reaches noon, with the sun overhead, its relief has flattened out; which is exactly what happens on the ground. On the night side there is none, because there is no light to cast it.
Both cameras hold the same hour angle, so both read the same local time. Every difference you can see between them is the season, and the first figure is the one that matters: how long the day lasts at that camera’s latitude. Watch them swap as the date crosses an equinox.
A single turn carries every longitude past the fixed dawn line once: Asia, then Europe and Africa, then the Americas, then the Pacific — the emptiest water on Earth, and the reason the land-in-sunlight curve under the map falls as far as it does. Follow one city across and you are watching a sunrise, at the right speed relative to everything else on the page.
The sun. Declination and the equation of time from the standard low-precision solar position — mean anomaly, equation of centre, obliquity 23.4397°. Sunrise and sunset are taken at a solar centre of −0.833°, which is the usual allowance for the sun’s upper limb plus average refraction. Day lengths agree with published almanac tables to about a minute; they are geometric, and take no account of terrain or local horizon.
The ground. Coastlines are a quarter-degree land mask built here from NASA’s Blue Marble imagery combined with its terrain model — the terrain alone below 60°S, so the Antarctic edge is the continent rather than the sea ice in the photograph. Its area-weighted land share comes out at 30.1% against a true 29.2%, the difference being coastal pixels rounded in. Relief shading uses the same terrain model, which is a relative bump map, not elevation in metres — the mountains are in the right places and the right order, but the shading is not a measurement.
The cities. Coordinates are city-centre positions; day lengths are computed from them, not looked up. Times throughout are UTC, so a city’s local clock is not shown — the point here is a single shared instant, which is the one thing local time destroys.