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Encyclopedia > Meridian circle

## Contents

### 17th century

A meridian circle enabled the observer to determine simultaneously right ascension and declination, but it does not appear to have been much used for right ascension during the 17th century, the method of equal altitudes by portable quadrants or measures of the angular distance between stars with a sextant being preferred. These methods were very inconvenient and in 1690 Ole Rømer invented the transit instrument. Right ascension (RA; symbol Î±: Greek letter alpha) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. ... In astronomy, declination (dec) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. ... (16th century - 17th century - 18th century - more centuries) As a means of recording the passage of time, the 17th century was that century which lasted from 1601-1700. ... A sextant is a measuring instrument used to measure the angle of elevation of a celestial object above the horizon. ... Events Giovanni Domenico Cassini observes differential rotation within Jupiters atmosphere. ... Ole RÃ¸mer. ...

### 18th century

The transit instrument consists of a horizontal axis in the direction east and west resting on firmly fixed supports, and having a telescope fixed at right angles to it, revolving freely in the plane of the meridian: At the same time Rømer invented the altitude and azimuth instrument for measuring vertical and horizontal angles, and in 1704 he combined a vertical circle with his transit instrument, so as to determine both co-ordinates at the same time. 50 cm refracting telescope at Nice Observatory. ... Azimuth is the horizontal component of a direction (compass direction), measured around the horizon toward the East, i. ... Events Building of the Students Monument in Aiud, Romania. ...

## Structure

Several recent instruments have been made entirely of steel, which is much more rigid than brass. The centre of the axis is shaped like a cube, the sides of which form the basis of two cones which end in cylindrical parts. The pivots rest on V-shaped bearings, either let into the massive stone or brick piers which support the instrument or attached to metal frameworks bolted on the tops of the piers: In order to relieve the pivots from the weight of the instrument, which would soon destroy their figure, the cylindrical part of each end of the axis is supported by a hook supplied with friction rollers, and suspended from a lever supported by the pier and counterbalanced so as to leave only about 10 lb pressure on each bearing. Near each end of the axis is attached a circle or wheel (generally of 3 or 31 ft diameter) finely divided to 2 ft or 5 ft on a slip of silver let into the face of the circle near the circumference. The old steel cable of a colliery winding tower Steel is a metal alloy whose major component is iron, with carbon being the primary alloying material. ... Brass is the term used for alloys of copper and zinc in a solid solution. ... The word axis has several meanings: In mathematics, axis can mean: A straight line around which a geometric figure can be rotated. ... A cube (or regular hexahedron) is a three-dimensional Platonic solid composed of six square faces, with three meeting at each vertex. ... In common usage and elementary geometry, a cone (Greek: ÎºÏŽÎ½Î¿Ï‚) is a solid object obtained by rotating a right triangle around one of its two short sides, the cones axis. ... A right circular cylinder In mathematics, a cylinder is a quadric, i. ... A pivot is that on which something turns. ...

The graduation is read off by means of microscopes, generally four for each circle at 90° from each other, as by taking the mean of the four readings the eccentricity and the accidental errors of graduation are to a great extent eliminated.' In the earlier instruments by Pistor and Martins the microscopes were fixed in holes drilled through the pier, but afterwards they let the piers be made narrower, so that the microscopes could be at the sides of them, attached to radial arms starting from near the bearings of the axis. This is preferable, as it allows of the temporary attachment of auxiliary microscopes for the purpose of investigating the errors of graduation of the circle, but the plan of the Repsolds and of Simms, to make the piers short and to let the microscopes and supports of the axis be carried by an iron framework, is better still, as no part of the circle is '

The most notable exception was the transit instrument and vertical circle of the Pulkovo Observatory, specially designed by the elder Struve for fundamental determinations. This instrument differs in many particulars from others: the important principle of symmetry in all the parts (scrupulously followed in all others) is quite discarded; there is only one circle; and the instrument cannot be reversed. There is a similar instrument at the Cape observatory. On Reichenbach's circles there were verniers instead of microscopes, and they were attached to an alidade circle, the immovability of which was tested by a level. The Pulkovo Space Observatory (ÐŸÑƒÌÐ»ÐºÐ¾Ð²ÑÐºÐ°Ñ Ð°ÑÑ‚Ñ€Ð¾Ð½Ð¾Ð¼Ð¸ÌÑ‡ÐµÑÐºÐ°Ñ Ð¾Ð±ÑÐµÑ€Ð²Ð°Ñ‚Ð¾ÌÑ€Ð¸Ñ in Russian), the principal space observatory of the Russian Academy of Sciences, located 19 km south of Saint Petersburg on...

At transmission at ao,000 volts Glow Lamps Storage Batteries Arcs exposed to radiation from the pier, which may cause strain and thereby change the angular distance between various parts of the circle. Each microscope is furnished with a micrometer screw, which moves a frame carrying a cross, or better two close parallel threads of spider's web, with which the distance of a division line from the centre of the field can be measured, the drum of the screw being divided to single seconds of arc (0.1" being estimated), while the number of revolutions are counted by a kind of comb in the field of view.

## References

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain. EncyclopÃ¦dia Britannica, the 11th edition The EncyclopÃ¦dia Britannica Eleventh Edition (1910â€“1911) is perhaps the most famous edition of the EncyclopÃ¦dia Britannica. ... The public domain comprises the body of all creative works and other knowledge—writing, artwork, music, science, inventions, and others—in which no person or organization has any proprietary interest. ...

Results from FactBites:

 Meridian (246 words) In the sky, a meridian is an imaginary great circle on the celestial sphere that is perpendicular to the local horizon. On the earth, a meridian is a 'straight line' on the earth's surface between the North Pole and the South Pole (in fact, half of a great circle). Considering the meridian that passes through Greenwich, England, to be zero degrees of longitude, or the prime meridian, the others are numbered by how many degrees they are from that one where they cross the equator.
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