Inquiry
What is the origin of the Sun Stone and its history?
Keep in mind while reading these theories that the Sun Stone was apparently powered up long before 65000000 B.C., as it was found and used extensively by Zeal in 12000 B.C., while the time travelers cannot use it until exactly 65002300 years of charging have passed. That it is absent from the Sun Keep also suggests that it was powered up long before prehistory, and simply wasn't found until the founding fathers of Zeal located it.
Theories
Planetary Connection
See [[Sun Keep (Origin of)]], Planetary / Magical Connection. The Sun Stone, along with the Sun Keep, could have been a natural formation akin to [[Power Spot]]s. By extension, the Sun Stone could also be a natural formation; it would have been charged up before 65000000 B.C. and then removed from the Sun Keep somehow, where it would have lay dormant for millions of years until found by Zeal and utilized there. Once again, this does not account for the torches in the Sun Keep, but those may have been placed there by the Iokans or Reptites prior to Crono's visiting the area in prehistory. Two other problems plague this theory. Mainly, the formation of a unique "battery" and its lucky placement in a unique "charger" is almost ludicrously coincidental. Secondly, the material the Sun Stone is made out of is not observed anywhere else in the game, suggesting it is completely unique (perhaps engineered by a sentient species).
Complete Mystery / Plot Oversight
Some civilization of group of beings were using the Sun Stone prior to 65000000 B.C. Since nothing corroborates such a claim except a quote by Lucca stating that solar energy was used before Lavos, the Sun Stone was perhaps an oversight by the developers, who didn't realize that by having Crono and his team leave it in the Sun Keep, they precluded the ability for it to be left in the Sun Keep by anyone else to be found by Zeal before 12000 B.C.
Temporal Displacement Mystery
The Sun Stone was created as a way of harnessing the planet's elemental energy by either the Reptites or others, and somehow was displaced in time. This theory is pure, unbridled speculation, sadly.
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Inquiry
How does the Sun Keep maintain a fixed position in the crust? What is its source of light, which shines eternally? The Sun Keep also does not erode with geological change. How is this accomplished?
Keep in mind while reading these theories that the Sun Stone was apparently powered up long before 65000000 B.C., as it was found and used extensively by Zeal in 12000 B.C., while the time travelers cannot use it until exactly 65002300 years of charging have passed. That it is absent from the Sun Keep also suggests that it was powered up long before prehistory, and simply wasn't found until the founding fathers of Zeal located it.
Theories
Light Source Only
Pole Alignment
The world of the Chrono series, as depicted in the ending, is shown not to have a very tilted axis. Conceivably, a structure at the North Pole could receive constant exposure of sunlight. This theory only attempts to explain the presence of light; the theories below attempt to describe origin, and either use this theory or attribute the light to magic design.
Origin
Reptite Construction
GrayLensman
The Reptites possibly created the Sun Keep. They were fairly advanced, although we don't know much about their technology or understanding of magic, if any. They were able to predict Lavos' fall, however, and Azala could lift boulders with telekinesis. The real question is whether the Reptites used the Sun Stone for power. Some evidence may be inferred from Chrono Cross, in which the Reptite Dimension appears. In that dimension's history, the Reptites lived in close harmony with the planet, utilizing its elemental powers. Back in Zeal, the Sun Stone was identified as a source of elemental, planetary power; it was retired after the Mammon Machine was discovered and the scholars of Zeal decided that the society did not need the energy of a "tired, old planet." The Sun Keep thusly may have been a Reptite experiment, with the Sun Stone being placed in the keep before the Tyrano Lair was destroyed. However, this fails to account for why no Sun Stone (except the one Crono places in the Keep) exists in the Sun Keep for the millions of years after 65000000 B.C. The absence of the Sun Stone in the keep is a fundamental problem with attributing its creation to a known civilization. This may be a developer oversight in the plot.
The question remains how the Reptites were able to anchor the Sun Keep to resist changes in the planet's crust. This all hinges on their level of technology; since they later came to specialize in a symbiotic relationship with the planet in the Reptite Dimension, it is not impossible for them to have had the technology, but the chances are very slim. After all, if they were in possession of these advanced skills, they could have dispatched the Early Humans with ease.
This theory is supported by a note by Lucca, who states that
Lucca: Solar energy was used aeons ago, well before Lavos Energy existed... It might be just the thing we need to defeat Lavos...
Since Lucca uses the words "before Lavos Energy existed", and not "before it was made available", it suggests that the Sun Stone was used before Lavos' came into the picture.
Planetary / Magic Connection
Aitrus, GrayLensman, ZeaLitY
In some form or fashion, the Sun Keep has a very strong connection with the planet, and naturally occurred. This bond is physically strong enough to prevent movement by the keep from continential drift; it also supplies the constant presence of light. It was noted in Chrono Cross that certain "Power Spots" exist on the planet's crust, said to be in close connection with the planet's inherent elemental power. Perhaps the Sun Keep is an extreme case of this type. The sunlight would be a case of elemental power (and keep in mind that elemental in this term refers to real magic, not Dragonian Elements). The actual structure is resistant to geological change in this manner; it may be connected to the planet's core. If someone were able to tap into the core of the earth, they would have access to a tremendous source of energy. The Sun Keep could be a geothermal power plant, with some sort of shaft leading down to the core of the earth. Resting on the shaft through the mantle, the Sun Keep would be unaffected by the pressure of shifting tectonic plates. Whatever mechanisms maintained the power plant could also preserve the appearance of the mountain. Another possibility is that magic is used to extract the power of the sun akin to the way the Mammon Machine takes Lavos's energy. Compare this theory to the one at [[Sun Stone (Origin of)]].
This theory fails to take into account the torches that exist there, suggesting the Sun Keep was built by a sentient race. It is not impossible that the Iokans or Reptites placed them there out of wonderment and worship, however.
Misconceptions
Zeal Creation
A few stipulations discount Zeal's possible role in constructing it. Firstly, the Sun Stone would have been found by Zeal fully powered up, since Zeal obviously did not exist for 65 million years. Secondly, the Sun Keep would have had to have been built with extraordinary foresight; it would be a significant long-term investment unless the scientists of Zeal found a way to place the Sun Stone in history millions of years before 12000 B.C., a tenet with no evidence supporting it. Lastly, this theory is also troublesome when considering that the Sun Stone (and possibly other elemental, planetary artifacts) was responsible for the magic and power of Zeal from its creation until the phasing in of the Mammon Machine. Additionally, instead of placing it in the Sun Keep after its use was precluded by the Mammon machine, the Stone was instead placed in another palace created by Zeal, the Sun Palace, complete with a guardian to prevent its theft. Lastly, the Sun Keep exists in 65000000 B.C., well out of Zeal's reach. Even the Epoch was not complete by the time of Zeal's fall, precluding time travel ability.
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Inquiry
What is the power of the Sun Stone?
Answer
At the equator, at noon, the sun delivers 950 Watts per meter. Assume the Sun Stone is 0.1 meters in diameter and has 100% efficiency. Then,
Power (absorbed) = (950 Watts/meter) (3.14) (0.05 meters)^2 = 7.46 Watts
Energy = Power * Time
65,013,300 years = 2.05*10^15 seconds
E = (7.46 Watts) (2.05*10^15 seconds) = 1.53*10^16 Joules
One Ton of TNT = 4.184*10^9 Joules
Sun Stone = 3.66 megatons of TNT
A vertically orientated solar collector, situated at the pole, would receive the exact same energy.
If the Sun Keep operates on geothermal energy or absorbs the sun's power directly, the Sun Stone could contain even more energy. And the Zealians gave that up in favor of Lavos. Lucca receives it by the end of the game.
