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ALTAZIMUTH, an astronomical telescope of a pattern sim ilar to a theodolite but on a much larger scale. It is used prin cipally to observe positions of the moon before first quarter and after last quarter, when the moon passes across the meridian in daylight. In order to secure more favourable nocturnal observa tions at such times it is necessary to have an instrument which can be turned out of the meridian but in other respects is used similarly to a meridian circle.