The zenith sector..can be used only on the meridian; and its measures are referred to the zenith point of the place of observation. Its principal uses are to determine the latitude of the place of observation by a star of known zenith distance; to measure the zenith distance of a star..; and to ascertain the zenith point.
W. Pearson, Practical Astronomyvol. II. 531
1970
A 5-foot portable zenith sector was found to be difficult both to manipulate and to rectify.
Notes & Rec. Royal Societyvol. 24 266
2006
The expedition was equipped with a 9-foot-tall zenith sector constructed by the London instrument maker George Graham.
zenith sector typically occurs fewer than 0.01 times per million words in modern written English.
zenith sector is in frequency band 2, which contains words occurring between 0.001 and 0.01 times per million words in modern written English. More about OED's frequency bands
Frequency data is computed programmatically, and should be regarded as an estimate.
Frequency of zenith sector, n., 1770–2010
* Occurrences per million words in written English
Historical frequency series are derived from Google Books Ngrams (version 2), a data set based on the Google Books corpus of several million books printed in English between 1500 and 2010.
The overall frequency for a given word is calculated by summing frequencies for the main form of the word, any plural or inflected forms, and any major spelling variations.
For sets of homographs (distinct entries that share the same word-form, e.g. mole, n.¹, mole, n.², mole, n.³, etc.), we have estimated the frequency of each homograph entry as a fraction of the total Ngrams frequency for the word-form. This may result in inaccuracies.
Smoothing has been applied to series for lower-frequency words, using a moving-average algorithm. This reduces short-term fluctuations, which may be produced by variability in the content of the Google Books corpus.
Decade
Frequency per million words
1770
0.0073
1780
0.014
1790
0.025
1800
0.027
1810
0.029
1820
0.035
1830
0.037
1840
0.037
1850
0.033
1860
0.022
1870
0.018
1880
0.016
1890
0.01
1900
0.0074
1910
0.0054
1920
0.0045
1930
0.0042
1940
0.004
1950
0.0042
1960
0.0042
1970
0.0051
1980
0.0054
1990
0.0059
2000
0.0057
2010
0.0056
Originally published as part of the entry for zenith, n. & adj.