A pocket watch given to Negi Springfield by Chao Linshen, allows him to time travel. Only three have been shown to exist so far, with Chao owning two and Negi with one. The amount of magic used is the determining factor of how far one can travel. According to Chao, 24 hours is the limit for a single mage, and the device is only demonstrated to allow travel in time, not space. Negi used the device during the Mahora Festival, in order to keep his promises to visit each of his students' festival activities.
Unfortunately, the watch is also vulnerable to sabotage - when it was brought into Evangeline's Resort, a trap that Chao Linshen had built into it caused the resort to keep them for a week in the real world when it should have only been a few hours. Afterwards, it became inactive since the main source of energy was the World Tree during the Mahora Festival. Negi and the gang were able to reactivate the watch by going deep into the core of the World Tree, which still had some residual magic left for a time jump. Apparently, it was enough to make it back to the third day of the Mahora Festival, but the watch was damaged at the end of the jump when Negi collapsed and dropped it, implying that its further usefulness would be limited.
During the Mages vs. Mars mock battle, Negi managed to use the Cassiopeia to counter the time-displacement effects of Mana Tatsumiya's bullets. Later, during the climatic battle against Chao Linshen, Negi used a pair of specially-empowered spirits in order to make minute adjustments to his own Cassiopeia and keep pace against Chao's time-jumping attacks through her Battlesuit Cassiopeia. In the end, the damaged Cassiopeia self-destructed from over-use, but not before Negi destroyed Chao's.
Even if the watch designs are the same, Cassiopeia No.2 is carried by Chao and allows her to travel through time. She used it to escape from the teachers after the Mahora Fighting Tournament half-a-day. It can be inferred that this is the same device that her birth parents used to send her into the future, and that she later used on the Orbital Defense Platform Athens to save her life from the Covenant boarders. Because of this, it can be assumed that this version of the Cassiopeia has much looser restrictions on the time an individual can travel and can also allow travel through space.
For the battles in the second and final day of the festival, Chao's Battlesuit has a third Cassiopeia unit built into the back of the suit, which is controlled by her AI, Pheidippides. The Cassiopeia unit enables her to circumvent attacks by moving to a different time period or simulate time stopping by continuously jumping to the same time period in a different dimension, thereby giving her the illusion of teleportation. She uses these techniques to take out Negi and escape from holds by both Setsuna Sakurazaki and Kaede Nagase on the second day. On the third day, she takes out Asuna Kagurazaka and Setsuna using the suit and sends Takamichi Takahata into the future with her time-displacement bullets. However, during her fight with Negi he manages to destroy the unit with a well-placed punch, thereby preventing her from using it to skip through time. It is not clear exactly how the unit is operated since unlike the other units the suit has no clear setting or triggering mechanism. Given the assumption that Chao comes from a future where the Halo games are a reality, it is likely that the device is controlled through a neural interface and her uplink to Pheidippides.
A new device built in 2558, intended for inclusion in the mechanical frame and inner workings of the combat android Myu. This version is not a single unit, rather, its mechanisms are dispersed throughout Myu's body and have multiple redundancies, preventing disabling the device through a single lucky hit. Intended for combat use, it is functionally identical to the third unit, but can be toggled to allow greater flexibility in time and space travel like the second unit.
- The name of the device is derived from the beautiful but vain queen Cassiopeia of Greek mythology. Legend goes that the boast of Cassiopeia was that both she and her daughter Andromeda were more beautiful than all the Nereids, the nymph-daughters of the sea god Nereus. This brought the wrath of Poseidon, ruling god of the sea, upon the kingdom of Ethiopia.
- The same queen supplies the name for the constellation that the device shares its name with.