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sire was to get the tables accurate to ten
minutes. Tycho Brahe found that the ta-
bles of Copernicus were in error as much
as two degrees. He re-discovered the in-
equality called ”variation” by observing the
moon in all phases–a thing which had not
been attended to. [It is remarkable that in
the nineteenth century Sir George Airy es-
tablished an altazimuth at Greenwich Ob-
177