The planetary calendar room is a large machine within Wan Shi Tong's Library. By manipulating various dials, one is able to determine the position of the sun, moon, and stars on any given date.
Inside Wan Shi Tong's Library, Sokka discovered a small burned piece of parchment which read, "The Darkest Day in Fire Nation History" and had a date on top of it. After that, he discovered that the whole wing dedicated to the Fire Nation had been burned down, thus preventing him from finding out what happened on that day. One of the Knowledge Seekers arrived and led Sokka and the others to the planetary room. The fox creature entered through a small opening to the right of the large golden room, and opened it from the inside. When the foursome entered, the fox showed them how the planetary room worked by pushing a lever. Zei exclaimed that the room was a "true marvel; a mechanical wonder". Katara suggested that Sokka enter the date he found on the parchment onto the dials in the middle of the room. When he did so, the heavens moved and showed that the darkest day in Fire Nation history was actually a solar eclipse, thus leading Sokka to discover that firebenders lose their firebending during a solar eclipse.
At the moment Sokka suggested a first draft of the invasion plan, Wan Shi Tong appeared, claiming that the visitors of his library had betrayed his trust by using the knowledge of the library to hurt other people. As a result of this, the Knowledge Spirit started to sink his library in order to trap everyone inside. While Katara and the professor headed for the exit, Sokka and Aang returned to the planetarium in order to find the date of the next solar eclipse. After finding out that it was only a few months away, they too left the room and the library.
This machine was built by Wan Shi Tong and his seekers using phosphorescent crystals, jade, brass, and other fine materials. Though it may seem like advanced technology, the planetary calendar works on gears and cogs like a watch and the ceiling is domed with "stars" projected from a crystal. The room uses its crystal-made stars and rotating planets to act as a kind of almanac showing both recorded history and projected future planetary alignments.
- The machine partly resembles the real-world Antikythera Mechanism, an ancient Greek clockwork device used for calculating the positions of celestial bodies. Though significantly smaller in size, the core mechanisms involved are similar: after the desired date is input by adjusting ring-shaped dials of varying sizes, a series of intricate gears are activated via a crank handle to reveal spacial alignments.