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Azimuth circle, or Vertical circle, one of the great circles of the sphere intersecting each other in the zenith and nadir, and cutting the horizon at right angles.

The instrument is supported on a strong tripod, fitted with levelling screws; to this tripod is fixed the azimuth circle and a long vertical steel axis.

In fact the modern equatorial, and the altitude and azimuth circle are astrolabes in the strictest and oldest meaning of the term; and Tycho in one of his astrolabes came so near the modern equatorial that it may be taken as the first of the kind.

It is also provided with an azimuth circle or mirror and a shadow pin or style placed in the centre of the glass cover, by either of which the variable angle between the compass north and true north, called the “total error,” or variation and deviation combined, can be observed.

An arc of the horizon intercepted between the azimuth circle of a celestial object and the magnetic meridian.