Geocentrism

Geocentrism

the motionless centre

The geocentric cosmos: the Earth held motionless at the centre, the seven planetary spheres turning about it. A 1660 plate of Ptolemy's system — its author seated, lower left — wrong, and for fourteen centuries the best predictor of the sky there was.

Type	Cosmological doctrine
Claim	The Earth is fixed at the centre of the universe
Status	Displaced c. 1543–1700; held today in small numbers
Rests upon	The sensation of stillness
Answered by	Aberration, parallax, Doppler shift, the pendulum, the background light
The Earth, in fact
Turns	Once a day
Travels	≈ 30 km/s about the Sun; ≈ 370 km/s through the cosmos
Central to	Nothing

Geocentrism is the doctrine that the Earth stands fixed and unmoving at the centre of the universe, with the Sun, Moon, planets, and stars revolving about it.^[1] It is the oldest cosmology and the most intuitive, and — for a system wrong in both its claims — among the most reasonable ever held: the Earth gives no sensation of motion, and everything dropped falls toward it, as if toward a centre. It was displaced, across the four centuries from Copernicus to Newton, by the discovery that the Earth both turns and travels, and is central to nothing.

That the planet moves is not felt but measured, and the measurements are by now many and independent.^[2] Geocentrism survives, in small numbers, as the position that all of them are somehow mistaken and the senses correct — that the stillness one feels is the stillness there is.

The motionless centre

In the system perfected by Ptolemy in the second century, the Earth sat at the centre of a set of nested transparent spheres bearing the heavenly bodies, the whole turning about it once a day.^[3] To save the wandering of the planets — which periodically slow, halt, and reverse against the stars — the scheme rode small circles, epicycles, upon the larger ones, in numbers that grew with the precision demanded of it. It was geometry of real power: it predicted the sky well enough to keep calendars and cast horoscopes for fourteen centuries, and its central claim agreed, at every point, with what a standing observer could feel.

It also held a strong argument in reserve. If the Earth circled the Sun, the nearer stars should shift against the farther ones across the year — a parallax — and no such shift could be seen. The observation was sound and the logic valid; only the silent assumption, that the stars stood near enough for the shift to show, was wrong. The parallax was there. Catching it would take until 1838, and instruments Ptolemy could not have dreamt of.

The displacement

Nicolaus Copernicus, in De revolutionibus (1543), put the Sun at the centre and set the Earth in motion about it — a planet among planets, turning daily on its axis. Tycho Brahe, measuring the heavens more finely than anyone before him and unconvinced, offered a compromise in which the planets circle the Sun while the Sun still circles a fixed Earth. Johannes Kepler, working from Tycho's figures, found that the planets move not in circles but in ellipses, and the last of the epicycles fell away. Galileo turned the new telescope upward and saw the phases of Venus — which only a Sun-circling Venus can show — and four moons in attendance on Jupiter, proof that not everything turns about the Earth. Newton supplied the reason the Sun should hold the centre at all: it is by far the most massive body present, and the small things fall around the large.^[4] Among astronomers, the matter was closed by 1700.

The evidence of motion

The classical demand — show the Earth move — has since been met many times over, and from independent directions.^[5]

The first direct proof was aberration: in 1729 James Bradley found that every star traces a tiny yearly ellipse, the light arriving at a slight tilt because the Earth, carrying the observer, is itself crossing the incoming beam — as rain appears to slant when one runs through it. A century on, in 1838, Friedrich Bessel measured the long-demanded parallax of the star 61 Cygni, catching at last the annual shift the geocentrists had rightly required and wrongly believed absent. The Doppler shift adds a third: the lines in a star's spectrum slide toward the blue and back toward the red across the year, as the orbiting Earth bears down upon the star and then withdraws.

Rotation can be shown without leaving the building. In 1851 Léon Foucault hung a long pendulum and set it swinging; over the hours its plane of swing turned — not because the pendulum changed direction, but because the floor, the Earth, turned beneath it.^[6] And the cosmos, questioned directly, returns an answer the geocentrist might almost have wished for: the background radiation that fills the sky is faintly warmer in one direction than its opposite, the signature of the Solar System's drift through it at some 370 kilometres a second. There is, it emerges, a frame against which absolute motion can be measured. The Earth is merely not at rest in it.

The persistence

Geocentrism did not quite end. It survives in small religious and contrarian communities as the claim that the Earth is, after all, the motionless centre, and that the measurements above are mistaken, misread, or arranged.^[7] The doctrine rests, as it always has, on a single sensation — I feel no motion — which is exactly the sensation the pendulum was built to address: the floor turns beneath the swinging weight whether or not anyone standing on it can feel the turn.

The position has a wider cousin in the keepers of absolute rest. The Aetheric Concord, holding still to a luminiferous aether that the Michelson–Morley experiment declined to find, wants for the universe a single still frame against which all motion is real; the geocentrist wants that same frame, and wants the Earth to be sitting in it. Both are met by the same evidence, and refused by it in the same way: there is no privileged rest, save the cosmic one the background light defines — and nothing anyone stands on is at rest there.

See also

• Earth — the body in question, which moves
• The figure of the Earth — its shape, as this is its motion
• Ptolemy — the centred system, perfected
• Nicolaus Copernicus and Johannes Kepler — the displacers
• The Aetheric Concord — the modern keepers of an absolute rest
• Flat Earth — the shape denied, as this is the motion

References

1. ^ Strictly, the doctrine makes two claims — that the Earth does not move, and that it lies at the centre — which are separable but have almost always been held together, motion and centrality being equally unwelcome to the senses.
2. ^ The independence is the point. A single measurement may be doubted; a dozen unrelated ones, drawn from starlight, spectra, pendulums, and the background sky, that all agree the Earth moves, can be doubted only by supposing a conspiracy among instruments that have never met.
3. ^ Ptolemy, Almagest, c. 150 AD. The work was authoritative for some fourteen hundred years, a tenure no physical theory has since approached, and a useful reminder that longevity and correctness are different virtues.
4. ^ More exactly, both bodies fall about their common centre of mass; but the Sun outweighs the Earth some three hundred thousand times over, so that the shared centre lies deep inside the Sun, and the description "the Earth goes round the Sun" is wrong only by the width of a remark.
5. ^ Aberration, parallax, and Doppler shift each independently require the Earth to orbit; Foucault's pendulum and the trade winds each require it to rotate; the microwave dipole requires the whole Solar System to be moving. No one of these was designed to answer geocentrism, which is why their agreement is difficult to dismiss.
6. ^ The rate of the turn depends on latitude: a full turn a day at the poles, none at all at the Equator, and the in-between everywhere else — itself a fact that no motionless Earth has ever been able to explain.
7. ^ The modern movement is small, articulate, and well supplied with literature. As with its flat-earth neighbours, its arguments are best read as a long, careful demonstration that the senses, consulted alone and trusted absolutely, will confirm very nearly anything.

External links

• Gaia: mapping the Galaxy by stellar parallax — European Space Agency
• The cosmic microwave background — NASA / WMAP
• The Celestial Atlas of Andreas Cellarius (1660) — source of the Ptolemaic-system plate reproduced above; public domain