Merriam-Webster's Word of the Day for September 08, 2010 is:

zenith • \ZEE-nith\  • noun
1 : the point of the celestial sphere that is directly opposite the and vertically above the observer 2 : the highest point reached in the heavens by a celestial body 3 : culminating point : acme

Example sentence:
The race was either the nadir or the zenith of high jinks politics, depending on one's point of view. (Kathleen Parker, Charlotte Observer, June 14, 2010)

Did you know?
When you reach the zenith, you’re at the top, the pinnacle, the summit, the peak. Zenith” developed from Arabic terms meaning the way over one’s head, and then traveled through Old Spanish, Medieval Latin, and Middle French before arriving in English. As long ago as the 1300s, English speakers used zenith to name the highest point in the celestial heavens, directly overhead. By the 1600s, zenith was being used for other high points as well. As in our example sentence, zenith is often contrasted with nadir. In celestial contexts, the nadir is the point that is vertically downward from the observer (imagine a line going through the earth from the observer's feet and out the other side into the sky). Figuratively, nadir simply means the lowest point.