Zenith
The point directly overhead is called an observer's zenith. Opposite the zenith is the nadir, directly beneath one's feet.
Another way to say it is that the maximum altitude, 90 degrees, is the zenith.
Zenith is a great name for a TV: Since dust and horizon haze obscure the sky at lower altitudes, when you look toward the zenith you get a "clearer picture."
Check your understanding:
- Are zenith and nadir points horizon-dependent? That is, do they differ for observers at different locations?
- Are the stars at the zenith and nadir points time-dependent? That is, do they differ for an observer at the same location but at different times?
- Is it meaningful to speak of the azimuth of a star at the observer's zenith?