Eye of Harmony

The Eye of Harmony (also known as Rassilon's Star) was a power source utilised by the Time Lords.

Legend
According to The Book of the Old Time Rassilon "with a great fleet" found the Eye of Harmony in a "black void" and returned it to Gallifrey at which "the people rejoiced". (DW: The Deadly Assassin)


 * The passage, in full, implies that the Eye of Harmony may refer not to the black hole but the singularity contained within it.

In the Panopticon
In the time of the Doctor, the Eye of Harmony lay secretly under the floor in the center of the Panopticon. The Time Lords had forgotten its location, some even believing it to be mythical or no longer in existence. (DW: The Deadly Assassin)

The Rod and the Sash of Rassilon
The ancient artifacts ceremonially given to the Lord Presidents had a practical function with relation to the Eye. (DW: The Deadly Assassin)

The Rod of Rassilon opened the floor of the Panopticon like a key, allowing a black hexagonal obelisk to rise from the floor which contained the Eye of Harmony itself. (DW: The Deadly Assassin)

The Sash of Rassilon, also given to the President, supposedly served as means of protecting the wearer against the forces of a black hole. The practical functions of the Rod and the Sash and the true nature of the Eye of Harmony remained unknown to most on Gallifrey. (DW: The Deadly Assassin)

Attempt to open the Eye
The Doctor deduced the true use of the artifacts by interpreting enigmatic passages from The Book of the Old Time. He also realized that the name Eye of Harmony symbolically described a black hole contained and balanced against the mass of the planet Gallifrey by Rassilon's engineering. The Master had also separately come to the same conclusions. He deduced that the Sash of Rassilon would give him immunity against the forces unleashed by the Eye if opened (it didn't, due to being damaged by the time he used it). If it had opened, the Eye of Harmony would have functioned as a normal black hole and destroyed Gallifrey and also the Master, who had hoped it would trigger a new cycle of regenerations for him. (DW: The Deadly Assassin)


 * Omega had also utilised a black hole in order to enable the Gallifreyans to acquire the power of time travel.

Inside the Doctor's TARDIS
It also seems that the Eye of Harmony existed within the Doctor's TARDIS during the later part of his seventh and eighth incarnation.The Eye of Harmony was stated to be the TARDIS's primary power source. The Eye responded to a physical linking device, opening if a Human put their eye in the link. It was also shown that leaving the TARDIS's Eye open for too long would result in space-time distortion. If opened partially, the Eye gave visions of the past. The particular structure of a Human eye had the effect of opening it. (DW: Doctor Who)


 * A popular theory has speculated that the Eye can exist in more than one place at the same time by having a link from Gallifrey to the TARDISes powered by it, much like the inside of the TARDIS was actually located in a different dimension than the outside. It would make sense that the Eye of Harmony could be linked to the TARDIS while still on Gallifrey. With the later destruction of Gallifrey, the Eye of Harmony presumably no longer served as a source of power.

All TARDISes built after a certain point; including the Type 40 the Doctor uses, have a mathematically modelled duplicate of the Eye with all its attendant features. (PDA: The Quantum Archangel)


 * The Doctor has also discussed artron energy as a power source for his TARDIS.

Dalek Eye of Harmony
During their advancement, the Dalek Empire developed the use of harmonic energy that was derived from their own Eye of Harmony that was based on that of the Time Lords on Gallifrey. Several adjustments were made through the use of temporal barrier technology used on Kar-Charrat. (BFA: The Time of the Daleks)

Behind the scenes

 * No explanation of how the Eye of Harmony could exist in two places at once exists onscreen. A popular fan explanation states that each TARDIS had a link to the actual Eye of Harmony (possibly in the same way the Time Lords transmitted energy from Gallifrey to the TARDIS in DW: The Three Doctors). The BBC official guide to Doctor Who (including the TV Movie) confirms this, and states that following the destruction of Gallifrey, the Doctor's TARDIS now powers itself with the energy from time rifts.