In The Republic by Plato, The Spindle of Necessity is mentioned in Book 10, Myth of Er. The cosmos is represented by the Spindle attended by sirens and the three daughters of the Goddess Necessity known collectively as The Fates. Their duty is to keep the rims of the spindle revolving. The Fates, Sirens and Spindle are used in The Republic, partly, to help explain how known celestial bodies revolved around the Earth according to Plato's understanding of cosmology and astronomy, which was a geocentric model.

The "Spindle of Necessity", according to Plato, is "shaped is like the ones we know". In other words, it was the same as the standard Greek spindle. It consisted of three main parts; a hook, shaft and whorl. The hook was fixed near the top of the shaft on its long side. On the other end resided the whorl, which acted as sort of a base. The hook was used to spin the shaft, which in turn spun the whorl on the other end.

Placed on the whorl of his celestial spindle were 8 "orbits". Each of these orbits created a perfect circle. Each "orbit" is given different descriptions by Plato, which no doubt represent known bodies within our solar system.

There has been much debate about this passage about the "Spindle of Necessity" has been subject of much discussion over time. However, there are three main points can be made:

First - Even though it is factually inaccurate, the main purpose of "the Spindle of Necessity" is to give a working, visual model to the solar system.

Second - Based on Plato's descriptions within the passage, the orbits can be identified as actual heavenly bodies:

• Orbit 1 - Stars
  
• Orbit 2 - Saturn
  
• Orbit 3 - Jupiter
  
• Orbit 4 - Mars
  
• Orbit 5 - Mercury
  
• Orbit 6 - Venus
  
• Orbit 7 - Sol
  
• Orbit 8 - Moon
  

Third - The descriptions of the rims accurately fit the relative distance and revolution speed of the respected bodies as would appear to an observer from Earth.