This week's word is brought to you by the Green Party.

The point diametrically opposed to the zenith, which is the point in the sky directly overhead. An azimuth is an arc from the horizon to the zenith. Nadir can also be taken to mean "lowest point" in the sense that zenith can be taken to mean "highest point".

Of course, the idea that there is a singular point in the sky over one's head is odd. There is an entire line defined by the segment between the center of the Earth and the center of one's head (call it the zenith line). The two segments on either side of the center of the Earth could be called the zenith segment and the nadir segment (equivalently, these can be thought of as vectors). Given a particular distance from the center of the Earth, there is a uniquely defined zenith point and a uniquely defined nadir point.

More information at Dictionary.com.

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