Above the line the rattleback turns anticlockwise seen from above; below it, clockwise. Yellow dots mark the moments it crossed zero.

Spin it

Or start it the other documented ways

Launch spin Skew of the mass axes Rolling resistance Time Design Show the two axes

The lab

Everything in this panel is computed live, in this page, by the same engine that is drawing the stone — not read from a table.

Right now

What the linear theory says

The body

Where the rolling-resistance slider sits

The verdict on the claim

“Spin it either way and it just spins the way you spun it.”

What the harness measured

Generated by tools-harness.js and written into js/findings.js. The page cannot print a number the harness did not produce.

Help, credits and sources

What this is

A rattleback — also called a celt, an anagyre, a Celtic stone, a wobblestone or a rattlerock — is a semi-ellipsoidal top that prefers one direction of spin. Spin it the other way and it rattles to a stop and turns round. Archaeologists digging Celtic and Egyptian sites in the nineteenth century found stone adze-heads that did it.

This page is an independent reimplementation: a rigid body whose lower surface is an ellipsoid, rolling without slipping on a horizontal plane, integrated from the Newton–Euler equations with the contact force eliminated. Nothing about the reversal is scripted. The stone is not told which way to turn.

How to play with it

Spin it one way, then the other, with the same launch rate. Watch the trace under the table.
Skew is the angle between the stone's mass axes and its shape. At zero it is an ordinary top and nothing happens. The two coloured lines on the deck are the shape's long axis (blue) and the mass axis (pink).
Rolling resistance is the only dissipation. Slide it up and the weak direction stops reversing before the strong one does — that band is where a real rattleback lives.
Rock it end to end, or side to side, without spinning it at all. It spins up by itself, and the two rocking axes give opposite senses.

Trade marks — what the register actually says

A register was reachable and the check was run, not assumed. TMview answered a POST to www.tmdn.org/tmview/api/search/results on 2026-09-20. The query rattleback returned 7 marks. Verbatim, the live ones:

RATTLEBACK   GB  UK00003039841  Registered  app 2014-01-29  reg 2014-07-11  class 28
             proprietor: Kiddy Wholesale Ltd
RATTLEBACK   US  85791834       Registered  app 2012-11-30  reg 2013-07-16  class 42
             proprietor: Rattleback, Inc.
RATTLEBACK   US  85791822       Registered  app 2012-11-30  reg 2014-02-11  class 35
             proprietor: Rattleback, Inc.
RATTLEBACK   EM  015064157      Registered  app 2016-02-01  reg 2016-06-30  classes 33, 25, 43
             proprietor: RATTLEBACK, LLC
-- and, no longer live --
RATTLEBACK   US  86645744       Ended       app 2015-05-29  reg 2017-01-10  classes 25, 33
RATTLEBACK   GB  UK00915064157  Expired     app 2016-02-01  reg 2016-06-30  classes 25, 33, 43
RATTLEBACK   US  73253844       Ended       app 1980-03-13  reg 1981-11-17  class 28
             proprietor: Moore; A. D.

Class 28 is games and toys and class 42 is software services, so two of those live marks sit squarely in the classes a browser toy falls under. For that reason this page is named Wobblestone — another traditional name for the same object — and not after the registered word. "Rattleback" still appears in the text as the ordinary descriptive name of the physical phenomenon, which is how the scientific literature uses it. No clearance is claimed and none was sought; this is a statement of intent, not a legal opinion. A TMview search on 2026-09-25 for wobblestone and wobble stone (exact and contains, all classes) returned zero marks; earlier searches for anagyre and rattlerock also returned zero.

What is faithful, and what is this project's own

Faithful. The phenomenon, the names, the two documented design types (a skewed base, or a symmetric base with offset end weights), the documented fact that some rattlebacks reverse from either direction and take several reversals per spin, and the documented explanation that friction is what makes most of them look one-way.
This project's own. The equations, the closed form for the growth-rate difference between the two senses, the regime boundaries, the numbers, the renderer, and every figure in the panels above. The three primary papers are paywalled from the machine that built this and were not read — see below.
Different from a real stone. A real rattleback sits on a table with real friction and a real contact patch. This one rolls on a mathematical point with an optional rolling-resistance couple and no slipping at all. The model demands a static friction coefficient of up to -- at the violent peak of a reversal, which a plastic stone on a desk may well not supply — so a real one probably slips there, and this one never does.

Sources

Rattleback — Wikipedia. en.wikipedia.org/wiki/Rattleback. The phenomenon, the names, the history, the sizes, the two design types, and the friction explanation.
G. T. Walker, On a curious dynamical property of celts, Proc. Camb. Phil. Soc. (1890s), and On a dynamical top, Quart. J. Pure Appl. Math. — named by the encyclopaedia as the first modern descriptions. Not retrieved.
H. Bondi, The rigid body dynamics of unidirectional spin, Proc. R. Soc. Lond. A 405, 265–274 (1986). DOI 10.1098/rspa.1986.0052.
A. Garcia & M. Hubbard, Spin reversal of the rattleback: theory and experiment, Proc. R. Soc. Lond. A 418, 165–197 (1988). DOI 10.1098/rspa.1988.0078.
H. K. Moffatt & T. Tokieda, Celt reversals: a prototype of chiral dynamics, Proc. Roy. Soc. Edinburgh A 138, 361–368 (2008). DOI 10.1017/s0308210506000679.
V. Ph. Zhuravlev & D. M. Klimov, Global motion of the celt, Mech. Solids 43, 320–327 (2008). DOI 10.3103/s0025654408030023.
J. Awrejcewicz & G. Kudra, Rolling resistance modelling in the Celtic stone dynamics, Multibody Syst. Dyn. 45, 155–167 (2018). DOI 10.1007/s11044-018-9624-9.
TMview, www.tmdn.org/tmview/, for the trade-mark records quoted above.

What could not be obtained. royalsocietypublishing.org returned HTTP 403 for Bondi 1986 and for Garcia & Hubbard 1988; the Cambridge endpoint for Moffatt & Tokieda 2008 returned a 746,406-byte HTML paywall page, not a PDF. Every author, journal, volume, page and year above was verified against the Crossref metadata API, which carries no article text. Nothing here is quoted from or attributed to the contents of those papers. The closed form in the verdict panel is this project's own derivation, checked three independent ways rather than copied from anyone.

Credits

The rattleback is a natural object and a folk toy; it has no author. The nineteenth-century archaeological finds and Gilbert Walker's 1890s papers are the first records named by the encyclopaedia. This page is an independent reimplementation written for the SkillSafe platform. It is not affiliated with, endorsed by, or connected to any holder of the trade-mark records quoted above.

How to reproduce every number

node tools-harness.js          # engine, all assertions, regenerates js/findings.js
node tools-harness-page.js     # page: assets, wiring, CSP, every printed figure
node tools-static.js <port>    # local server
node tools-smoke.js http://127.0.0.1:<port>/ out.png

The table