The Commentariolus (Little Commentary) is Nicolaus Copernicus's brief outline of an early version of his revolutionary heliocentric theory of the universe.

[1] After further long development of his theory, Copernicus published the mature version in 1543 in his landmark work, De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres).

On 1 November 1536, Nikolaus von Schönberg, Archbishop of Capua and since the preceding year a cardinal, wrote to Copernicus from Rome and asked him for a copy of his writings "at the earliest possible moment".

After a brief introduction, the first section states seven postulates from which Copernicus proposes to show that the apparent motion of the planets can be explained systematically.

The second motion is the daily rotation about an axis which passes through the Earth's centre and is inclined at an angle of about 231⁄2° to the perpendicular to the plane of its orbit.

Copernicus specified the rate of this precession with respect to the radial line from the Earth to the centre of its orbit as being slightly less than a year, with an implied direction as being from west to east.

[9] The Moon itself, represented by the point M in the diagram, moves uniformly from west to east around the circumference of the second epicycle so that the period of the angle γ is half a synodic month.

The centre of the second epicycle, represented by the point e2 in the diagram, revolves uniformly from east to west around the circumference of the first, with the same period relative to the radial line joining S to e1.

As a consequence, the direction of the radial line joining e1 to e2 remains fixed relative to the fixed stars, parallel to the planet's line of apses EW, and the point e2 describes an eccentric circle[g] whose radius is equal to that of the deferent, and whose centre, represented by the point O in the diagram, is offset from that of the deferent by the radius of the first epicycle.

In his later work, De revolutionibus orbium coelestium, Copernicus uses this eccentric circle directly, rather than representing it as a combination of a deferent and an epicycle.

For the ratios of the radii of the outer planets' deferents to radius of the Earth, the Commentariolus gives 113⁄25 for Mars, 513⁄60 for Jupiter, and 97⁄30 for Saturn.