no measurable parallax and must be very distant.

The startling discovery that stars are not necessarily permanent, that
new stars may appear, and possibly that old ones may disappear, had
upon him exactly the same effect that a similar occurrence had upon
Hipparchus 1,700 years before. He felt it his duty to catalogue all
the principal stars, so that there should be no mistake in the
future. During the construction of his catalogue of 1,000 stars he
prepared and used accurate tables of refraction deduced from his own
observations. Thus he eliminated (so far as naked eye observations
required) the effect of atmospheric refraction which makes the
altitude of a star seem greater than it really is.

Tycho Brahe was able to correct the lunar theory by his observations.
Copernicus had introduced two epicycles on the lunar orbit in the hope
of obtaining a better accordance between theory and observation; and
he was not too ambitious, as his desire was to get the tables accurate
to ten minutes. Tycho Brahe found that the tables of Copernicus were
in error as much as two degrees. He re-discovered the inequality
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 established an altazimuth at Greenwich
Observatory with this special object, to get observations of the moon
in all phases.] He also discovered other lunar equalities, and wanted
to add another epicycle to the moon's orbit, but he feared that these
would soon become unmanageable if further observations showed more new
inequalities.

But, as it turned out, the most fruitful work of Tycho Brahe was on
the motions of the planets, and especially of the planet Mars, for it