Discovery

The world through time

In view

Eight planets in their actual place along their orbit, at the year on the time bar. Drag to turn around them.

The distances are compressed. At true scale Mercury sticks to the Sun and Neptune falls off the screen. So the order is right, the proportions are not.

Further back than five thousand years, only the orbits remain. Where a planet stood along its orbit back then can no longer be calculated.

Under Layers you add the comets and asteroids. The belt fills the gap between Mars and Jupiter that otherwise looks empty, and the comets show where they come from: Halley runs out past Neptune and returns every seventy-five years.

A comet's orbit is right, but its place along it drifts. Jupiter pulls at it and that is not in the calculation. Checked against Halley: its 1986 approach comes out two days off, the 1910 one is forty days out and the 2061 one a few months.

The Space probes layer shows what we have sent that way: the two Voyagers, the two Pioneers, New Horizons, Parker Solar Probe, Cassini and Juno. The part already flown is drawn solid, what is still to come dotted, and the dots on the line are the flybys — that is where the path bends, because there a planet's gravity slung the probe on to the next one.

To watch them fly there is the Spaceflight button by the time bar. All of spaceflight fits within a hair's breadth on the normal bar — a hundred and fifty years on an axis of four and a half billion — so sliding moves nothing. That button stretches the stretch from 1955 to 2030 across the whole bar; playing then runs the journey from the start to now, with the probes departing one by one and bending past the planets. One step out shows the rest of the journey, up to 2100.

The Moon in its actual place and with its actual phase, and the path it walks in a month. The lit side points at the Sun.

Distance and size are both right here, unlike on the solar map. Zoomed out the Earth is a dot — which it is, because the Moon stands sixty Earth radii away. Zoom in and the Earth grows with you; at the bottom it says how many kilometres one pixel is.

Here the day counts, not the year: the Moon goes round in a little over twenty-seven days. Hence the day slider at the bottom.

Under Layers you switch on the Earth's surface: the same coastlines as on the globe, and therefore the same drift through time. Set the time bar to two hundred million years ago and you see Pangaea lying there from here. Which side you see is the side you had facing you on the globe — there is no time of day here, so there is no way to work out which meridian points towards us.

The boundary between day and night is worked out per place from the point where the Sun stands directly overhead, and that is a soft band rather than a line: the Sun is half a degree wide in the sky and the atmosphere scatters the light some way further into the night side.

Hence the third slider: the time of day. The Earth turns round in twenty-four hours, so the shadow sweeps across the world once a day — fifteen hundred times faster than the yearly march you see on the day slider. At 12:00 UTC the Sun stands above the prime meridian, at midnight above the date line. Give or take a quarter of an hour: the Earth runs through its oval orbit sometimes ahead of and sometimes behind the clock, and that equation is not in here.

The seasons are in here. Set the day to late June and the North Pole stays in the light for the whole rotation; set it to late December and it does not come out again. So far back that there is no date any more, there is no shadow — which side had daylight then, nobody knows.

Zoom in and you see what people have hung around it: over sixteen thousand objects, with the geostationary ring as the large circle around them. Before 1957 it is empty, and that is history itself.

Two things about that are wrong. Where an object stands along its orbit is the position on one particular day — it goes round every hour and a half. And you only see what is still up there today, filtered by year of launch: what has since fallen back is no longer among them, so the sixties look emptier than they were.

Almost nine thousand stars, everything the naked eye reaches on a dark night, with the lines of the eighty-eight constellations running through them. Drag to look along the sky.

The cross is the celestial pole — the point the Earth's axis points at and which everything turns around. At the bottom it says which star currently stands closest to it.

Slide time back and the pole moves with it: the Earth wobbles round in a little over twenty-five thousand years. Around 2800 BCE Thuban stood there, five thousand years from now Alderamin will. Go tens of thousands of years further and the constellations warp, because every star goes its own way.

The dotted line is the Sun's path through the year — the band the zodiac is named after, and one the Moon and planets never stray far from. At the bottom it says which constellation the Sun actually stands in on this date. That is usually not the sign from the paper: those signs were fixed two thousand years ago and the sky has shifted a whole sign since.

Under Layers you add the rest. The Milky Way as a band. Nebulae and star clusters from the list Charles Messier drew up in 1774 so that he could ignore them — they were the smudges he kept mistaking for comets. The eleven meteor showers, which light up as soon as the day slider reaches their peak. And comets, when they are close enough to really see.

And six supernovae people have watched flare up. Set the time bar to 1054 and there is a star in Taurus that does not belong: visible in daylight for twenty-three days, recorded by Chinese and Japanese court astronomers, 1.1 degrees from the star they called Tianguan. What is left of it is now called the Crab Nebula, and it sits there as a small ring the rest of the time.

land shallow sea moment with a place

1 January

12:00 UTC

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Discovery

The map itself

What you can switch on

Beyond the Earth

What is not in it

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