One of my favorite words.
Go out on a clear night an look straight up. That’s the Zenith.
The terms Zenith and Nadir are also used when referring to the two of the six Common Berthing Mechanisms (CBMs) of the Unity and Harmony modules of the International Space Station. CBMs are the docking ports where other modules of the space station are connected.
The image below depicts Node 2 now renamed Harmony. The image shows the Zenith CBM (on top), the Forward CBM (to the left) and the Port CBM (to the right) are shown. Not shown are the Nadir CBM (on the bottom opposite the Zenith CBM), the Starboard CBM (opposite the Port CBM) and the Aft CBM (on the opposite end to the Forward CBM).
ESAs laboratary Columbus is now permanently berthed to the Starboard CBM of Harmony.
The Experiment Logistics Module Presurized Section (ELM PS) of the Japanese Experiment Module (Kibo) will be temporarily berthed to the Zenith CBM during the current STS-123 mission.