Avatar: The Last Airbender is an incredible world right on the line between science and magic. Bending, a genetic trait that allows people to interact with and manipulate the four elements is the basis of the expanded ATLA universe. While there is something magical about the art of bending, the mechanics behind it is also treated with a very scientific rationale in the show. People cannot learn to possess bending abilities, only those who are natural-born benders can learn to master their respective elements. There are also several ways in which a bender’s power can be disrupted or even taken away permanently. A solar eclipse, however, might not be a consistent cause for this to happen.
In, Book 2, Episode 10, Team Avatar finds the lost library of the all-knowing spirit, Wan Shi Tong, who reluctantly allows them to peruse his vast collection. Sokka uncovers the remaining evidence of an event that the Fire Nation tried to destroy. Looking into the meaning of the darkest day in Fire Nation History, they learn that during a solar eclipse called The Day of Black Sun, firebenders lose their abilities temporarily.
The Northern Water Tribe and Earth Kingdom plans a large scale invasion of the Fire Nation around the exact time of the eclipse so that they can infiltrate its island and so that Aang can take out the Firelord. The sun is eclipsed by the moon, causing firebenders to be powerless for about eight minutes because they draw their power from the sun. This is reminiscent of Book 1, Episode 20, wherein waterbenders lose their bending after Zhao kills the moon spirit. This is not the same as an eclipse, though, since the moon itself was stripped of the source of waterbenders' power.
The Day of Black Sun, however, doesn’t seem consistent with the physics of bending. If the firebenders lose their bending during a solar eclipse because the moon stands between them and the sun, by that logic, firebenders shouldn’t be able to bend at nighttime either. If the moon can disrupt a bender's power, then nighttime would arguably have the same effect since the earth rotates away from the sun each day, meaning that the entire earth would be standing between the firebenders and the sun.
The waterbenders losing their bending was much more plausible since it was the spirit of the moon being permanently destroyed, rather than a mere physical object being placed between them and the moon. While waterbenders draw power from the moon, they still bend during the day because the moon still exists; they’re just stronger at night when they’re closer. So, if firebenders still have their abilities at night, their power should not be disrupted by a solar eclipse. Unless...
There is one reasonable cause that might explain the firebending phenomenon during an eclipse. It may be that it is not the sun being blocked from the earth that deprives the Fire Nation of their bending, but more specifically, what stands in the way. The moon, being the source of waterbenders’ power and also the natural opposite of firebending, could have the ability to disrupt firebenders' abilities for that unique reason. Disrupting a bender's ability can be both physical and spiritual. The physical disruption of bending is seen when Ty Lee demonstrates her ability to temporarily disable a bender by hitting them in specific pressure points and blocking their chi. The ATLA expanded universe went further to show the ability a waterbender might have to permanently take a person’s bending away by blocking their power through bloodbending, as Amon does to the benders of Republic City in The Legend of Korra.
Bending can also be disrupted or taken in a spiritual capacity, as Aang does to Firelord Ozai in the series finale. Korra also uses her spiritual connection in the Avatar state to restore the bending that was taken by Amon’s bloodbending. While the physics of the solar eclipse would not reasonably disrupt the Fire Nation’s bending, the spiritual nature of the event could suffice to explain The Day of Black Sun.
