The ground rises and falls twice a day, cities and all

Your day, drawn by the Moon#

Today, Saturday 31 October, the ground under London travels 29 cm between its lowest and highest points. It is highest at 03:41 (+20 cm) and lowest at 14:18 (−9 cm).

The Moon stretches the whole Earth: one bulge under it and one on the far side. The planet turns through both in a lunar day, 24 hours and 50 minutes, so there are usually two crests a day, each about 50 minutes later than yesterday’s. When the Moon stands far from the equator, one of them passes you much lower and is sometimes barely there.

The rock answers almost at once: it lags the Moon by about 0.2 degrees of the cycle, under half a minute.

A wave of stone crosses Europe#

The crests do not stay put. They slide west with the Moon, across the whole continent at once. On the map, time now runs 1,800 times faster: half an hour a second.

The crest reaches Belfast about 30 minutes after Norwich. From Bucharest to London, 2,091 km, it takes 1 h 49 min on average.

The lines are drawn every 2 centimetres: gold where the ground is above its average, blue where it is below. You can also tap the map to choose your town.

Move the Moon#

Drag across the globe: the Moon moves over the place you touch, and the rock follows. Under the Moon the ground rises; 90 degrees away, on a whole circle, it sinks; on the far side of the planet it rises again.

The real Moon never goes beyond about the 28.7° parallel, north or south, so you cannot take it further here either. The button at the corner of the globe puts it back where it is now.

The Sun stretches too, about half as hard#

The Sun is 27 million times more massive than the Moon, but it is almost 390 times further away, and the force that stretches the Earth falls with the cube of distance. That leaves it about 46 per cent of the Moon’s stretching force.

When the Sun and Moon are in line, at new Moon and full Moon, their crests add up. At right angles, they partly cancel.

The day with the most movement in the next 30: 24 November 2026, 43 cm. The quietest: 18 November 2026, 14 cm.

Why nobody feels a thing#

Because the wave is enormous. From a crest to the trough beside it is a quarter of the Earth’s circumference, about 10,000 kilometres. Over that distance, a few tens of centimetres make a slope no body can feel.

In Bucharest in November 2026, the steepest slope reaches 87 nanoradians: along a kilometre of street, one end rises against the other by at most 87 micrometres, under a tenth of a millimetre. The ground also moves sideways: over the 18.6-year cycle, by up to 8 cm, again unfelt.

As for weight: at CERN, gravity varied with the tide by about a quarter of a millionth. For a 70 kg person that is about 18 milligrams on the scales between the lightest and the heaviest moment.

How far the ground rises depends on how elastic the planet is. Geophysicists capture it in one number, h₂: how far the ground rises compared with an ideal ocean in balance with the Moon on a rigid Earth. For the real Earth, h₂ is about 0.61. A perfectly rigid planet would not move at all.

1914: a tide in a pipe#

At Yerkes Observatory in Wisconsin, Albert Michelson and his colleagues buried two 500-foot pipes (152 m), one north–south and one east–west, 6 inches across, six feet underground, and half filled them with water. The results appeared in March 1914. The water in a pipe has its own tide: when the Moon pulls, it gathers towards one end.

Had the Earth been perfectly rigid, the water would have tilted against the pipe by a certain amount, easy to calculate. It came out less: in the 1916–17 repeat, read with interferometers, 0.690 of the rigid-Earth value. The difference came from the Earth deforming: the ground tilted too, by about 0.6 of the calculated amount, while the deformed Earth’s own extra pull gave back about 0.3. Michelson wrote in 1914 that the Earth’s rigidity is “greater than that of steel”.

The records were cut into 12.42-hour slices, half a lunar day, and the water rose and fell in step with them.

1992: the Moon enters the accelerator#

At CERN, near Geneva, the LEP ring was 26.7 km round. The physicists measuring its beam energy saw variations in 1991 that the magnets’ temperature did not explain. An earlier hunch proved right: the tides of the Moon and Sun.

The stone tide moves the ground there up and down by a few tens of centimetres (the calculations of the time gave up to about 25 cm from the average) and stretches the crust: the ring’s circumference changes by about 4 parts in a hundred million, roughly a millimetre. With that year’s magnet settings, that was enough to swing the particles’ energy by up to about 220 parts per million, end to end. On 11 November 1992 a 24-hour measurement followed it: the points drew the tide’s two humps, as the calculation had predicted.

London: the river and the rock#

At London Bridge, the Thames rises and falls about 6.6 m at spring tides. The ground under London moves too, by about 24 cm on a typical day and up to 45 cm, but no tide gauge sees it: the gauge is fixed to the bank, and the bank rises with it.

Near the Atlantic there is a third movement. The weight of the ocean’s own tide presses the rock down and lets it spring back. In Cornwall this ocean loading reaches 4.5 cm in amplitude for the main twice-daily tide, and GPS stations measure it. The numbers on this page leave it out: they are the body tide alone.

The day you were born#

Enter the date, and the time if you know it. The page works out how the ground under London moved that day. Nothing leaves your device.

Pick a date to see the day.

When you are highest#