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Making a set of setting circles required a lot of precision crafting on a dividing engine. Setting circles usually had a large diameter and when combined with a vernier scale could point a telescope to nearly an arc minute of accuracy. In the 20th century setting circles were replaced with electronic encoders on most research telescopes.
The term transit theodolite, or transit for short, refers to a type of theodolite where the telescope is short enough to rotate in a full circle on its horizontal axis as well as around its vertical axis. It features a vertical circle which is graduated through the full 360 degrees and a telescope that could "flip over" ("transit the scope").
A large German equatorial mount on the Forststernwarte Jena 50cm Cassegrain reflector telescope. An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis, called polar axis, parallel to the Earth's axis of rotation. [1] [2] This type of mount is used for astronomical telescopes and cameras.
The time is read off on the equatorial ring; in the example below, the center bar is twisted until a sunray passes through a small hole and falls on the horizontal equatorial ring. Traveller's sundial or universal ring dial, a portable version of astronomical rings, closed for carrying.
In the largest telescopes, the mass and cost of an equatorial mount is prohibitive and they have been superseded by computer-controlled altazimuth mounts. [5] The simple structure of an altazimuth mount allows significant cost reductions, in spite of the additional cost associated with the more complex tracking and image-orienting mechanisms. [6]
Boundary is a circle. All parallels and meridians are circular arcs. Usually clipped near 80°N/S. Standard world projection of the NGS in 1922–1988. c. 150: Equidistant conic = simple conic: Conic Equidistant Based on Ptolemy's 1st Projection Distances along meridians are conserved, as is distance along one or two standard parallels. [3] 1772
Horizontal coordinates are very useful for determining the rise and set times of an object in the sky. When an object's altitude is 0°, it is on the horizon. [ a ] If at that moment its altitude is increasing, it is rising, but if its altitude is decreasing, it is setting.
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