## Who used the concept of epicycles deferents and equants?

Ptolemy (c. 90 – c. 168 A.D.)–The Great Compiler He defined the modern magnitude system. He refined the geometric model of the Solar system using epicycles, deferents, and equants to explain the motion of the planets.

What were deferents and epicycles in Ptolemy’s model?

In order to explain the motion of the planets, Ptolemy combined eccentricity with an epicyclic model. In the Ptolemaic system each planet revolves uniformly along a circular path (epicycle), the centre of which revolves around Earth along a larger circular path (deferent).

Who introduced epicycles and equants?

Ptolemy
By carefully coordinating these two cycles, the epicyclic model explained the observed phenomenon of planets retrograding when at perigee. Ptolemy enhanced the effect of eccentricity by making the epicycle’s centre sweep out equal angles along the deferent in equal times as seen from a point that he called the equant.

### What is the difference between the deferent and the epicycles?

In both Hipparchian and Ptolemaic systems, the planets are assumed to move in a small circle called an epicycle, which in turn moves along a larger circle called a deferent. Both circles rotate clockwise and are roughly parallel to the plane of the Sun’s apparent orbit under those systems (ecliptic).

What are epicycles?

Definition of epicycle 1 in Ptolemaic astronomy : a circle in which a planet moves and which has a center that is itself carried around at the same time on the circumference of a larger circle. 2 : a process going on within a larger one.

Who invented epicycles?

The primitive epicycle theory invented by the Pythagoreans was quite good for the Sun, Mercury and Venus, but not for the outer planets, so the search for a better theory was fully justified. In Section 5 I shall present my interpretation of Plato’s text anew.

## Why did Ptolemy believe in epicycles?

In order to preserve the geocentric cosmology of the time and to account for retrograde motion of Mars, Ptolemy had to make a model of planetary motion that invoked the use of epicycles. An epicycle is basically a little “wheel” that orbits on a bigger wheel.

Why did Copernicus use epicycles?

Natural consequence of observing moving planets from a moving Earth. By contrast, Ptolemy’s system required epicycles to get retrograde motion. Copernicus still needed epicycles to reproduce the non-uniform speeds of the planets correctly.

What do epicycles explain?

epicycle. / (ˈɛpɪˌsaɪkəl) / noun. astronomy (in the Ptolemaic system) a small circle, around which a planet was thought to revolve, whose centre describes a larger circle (the deferent) centred on the earth.

### How do epicycles explain retrograde motion?

Epicycles Explain Retrograde Motion. As a planet moves around on its epicycle, the center of the epicycle (called the “deferent”) moves around the Earth. When its motion brings it inside the deferent circle, the planet undergoes retrograde motion.

Did Copernicus use epicycles?

While Copernicus’ system does not need epicycles to produce retrograde motion, because of his insistance on uniform circular motion he still had to use them in order to get his model to make accurate predictions (i.e., to “preserve appearances”), particularly to reproduce the non-uniform speeds of the planets.

Who introduced epicycle?

Ptolemy explained the apparent “looping motion” of the planets by placing the center of one rotating circle, called the epicycle, which carried the planet, on another rotating circle, called the deferent, so that together the motions of the two circles produced the observed looping motion of the planet.

## Why are eccentrics called epicycles?

Because one half of an epicycle runs counter to the general motion of the deferent path, the… …revolving, off-centre circles called eccentrics; epicycles, little circles whose centres moved uniformly on the circumference of circles of larger radius (deferents); and equants.

What is the equant of the epicycle?

The new point was called the equant, which was located on the opposite side of the eccentric, at the same distance as the earth, as shown in the figure below. The center of the epicycle moves at a constant distance from the eccentric, but at a constant angular speed about the equant.

What do you mean by eccentrics?

…revolving, off-centre circles called eccentrics; epicycles, little circles whose centres moved uniformly on the circumference of circles of larger radius (deferents); and equants.

### Who were the critics of the equant?

Noted critics of the equant include the Persian astronomer Nasir al-Din Tusi who developed the Tusi couple as an alternative explanation, and Nicolaus Copernicus, whose alternative was a new pair of epicycles for each deferent. Dislike of the equant was a major motivation for Copernicus to construct his heliocentric system.