Astronomy in the World of Avatar

People in the World of Avatar have limited knowledge about the universe.

Eclipses
People know the cause of eclipses and it can be predicted through a Planetary Calendar Room. An eclipse is an astronomical event that occurs when one celestial object moves into the shadow of another.

Lunar Eclipse
A Waterbending weakness is its dependence on the moon. Although Waterbenders gain power from the moon (with Waterbending at its zenith during full moons), the moon's absence during an eclipse results in a complete loss of Waterbending ability (as seen in The Siege of the North, Part 2).

Solar Eclipse
Day of Black Sun: The Day of Black Sun, refers to a day in history when a solar eclipse occurs when the sun is entirely blocked out by the moon and rendering Firebenders powerless. There have been two known days of black sun in history.

Solstices
A solstice is an astronomical event that occurs twice a year, when the tilt of the Earth's axis is most oriented toward or away from the Sun, causing the Sun to reach its northernmost or southernmost extreme. Solstices can be predicted too.

Winter Solstice
The Winter Solstice is a natural phenomenon that occurs when the sun is at its greatest angular distance from the equator. It is also the shortest day of the year. In the Avatar World, as the solstice approaches, the natural world and the spirit world grow closer together until the line between the two is blurred. During the solstices and equinoxes, the spirits can exert incredible power in the natural world. This is also the ideal time for people (with the ability) to enter the Spirit World. Spirits can appear in the real world for a short time on the solstice, like the Avatar Roku in the episode Winter Solstice Part 2: Avatar Roku.

Summer Solstice
The Summer Solstice is a natural phenomenon, occurring when the sun is at the closest angular distance to the equator. It is also the longest day of the year. In the Avatar World, both the Winter Solstice and the summer solstice, and the equinoxes are the only times that the spirit world and the mortal world come close together enough to interact freely with each other for enough time. This event is experienced throughout The Avatar and the Fire Lord, when Aang meditates in order to communicate with Avatar Roku about the latter's past events experienced when alive, such as his friendship, friendly rivalry as well as the eventual conflict with his friend and eventual Fire Lord Sozin. In The Firebending Masters, Zuko and Aang go to the ancient Sun Warrior civilization and towards a chamber that has a celestial calendar in front of it. Zuko interprets this is having to wait for the solstice in order for the door to open.

Comets
A comet is a small Solar System body that orbits the Sun and, when close enough to the Sun, exhibits a visible coma (atmosphere) or a tail — both primarily from the effects of solar radiation upon the comet's nucleus. The coming of a comet can be predicted.

Sozin's Comet
Sozin's Comet is a comet that seems to pass close to the world of the Avatar every hundred or so years. The comet passes close enough that Firebenders are able to draw power from it as they normally do the sun, greatly increasing their powers. Fire Lord Ozai claimed it enhanced Firebending by a hundred times.

Meteorites
A Meteorite is a natural object originating in outer space that survives an impact with the Earth's surface. A meteorite is seen in Sokka's Master. Sokka chose the meteorite as the material for make his sword.

Astronomical Objects
People in the World of Avatar have a number of important astronomical objects.

Star Chart
The Star Chart is a map of the stars and constellations.

Eclipse Glasses
Eclipse Glasses are a gadget invented by the Mechanist. When the invasion force invaded the Fire Nation on the Day of Black Sun, they could observe the eclipse safely as a means of keeping track of their time.

Sunstone
A Sun Warrior treasure, the Sunstone is kept hidden inside a room in the Sun Warrior's Ancient City which is supposed to be unlockable only during a solstice.

Planetary Calendar Room
The Planetary Calendar Room is in the Wan Shi Tong's Library, in the Si Wong Desert. Aang and Sokka used the room to search information about the eclipse.

Celestial Calendar
Located in center of the courtyard at the Sun Warrior city is a large orange gem that refracts slivers of light onto a calendar on the ground. This gem is part of a celestial locking system that protects a sacred room the room can only be opened on specific days, like the summer solstice.