An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A → transit circle that defines the position of the → Greenwich Meridian since the first observation was taken with it in 1851. Airy's transit circle lies at longitude 0°, by definition, and latitude 51° 28' 38'' N.

Named after Sir George Biddell Airy (1801-1892), Astronomer Royal, at the Royal Observatory in Greenwich from 1835 to 1881. Airy transformed the observatory, installing some of the most advanced astronomical apparatus of his day and expanded both staff numbers and their workload; → transit; → circle.

A circle on the celestial sphere that has equal altitude over the Earth's surface and lies parallel to the horizon. Also called almucantar, circle of altitude, parallel of altitude.

→ altitude; → circle.

One of great circles of the → celestial sphere which passes through the → zenith, → nadir, and the star, cutting the horizon at right angles. Same as → vertical circle.

→ azimuth; → circle.

An instrument which was an improved form of the → reflecting circle, used for measuring angular distances. In Borda's version the arm carrying the telescope was extended right across the circle. The telescope and a clamp and tangent screw were at one end, and the half-silvered horizon glass at the far end from the eye. In practice, with the index arm clamped, the observer first aims directly at the right hand object and by reflection on the left, moving the telescope arm until this is achieved. He then frees the index arm, sights directly on the left hand object with the telescope arm clamped, and moves the index arm until the two coincide again. The difference in the readings of the index arm is twice the angle required, so that the final sum reading must be divided by twice the number of double operations. Borda's instrument greatly contributed to the French success in measuring the length of the meridional arc of the Earth's surface between Dunkirk and Barcelona (1792-1798). The operation carried out by Jean Baptiste Delambre (1749-1822) and Pierre Méchain (1744-1804) was essential for establishing the meter as the length unit.

After the French physicist and naval officer Jean-Charles de Borda (1733-1799), who made several contributions to hydrodynamics and nautical astronomy. Borda was also one of the most important metrological pioneers; → circle.

A closed curve lying in a plane and so constructed that all its points are equally distant from a fixed point in the plane.

From O.Fr. cercle, from L. circulus "small ring," dim. of circus "ring," from or akin to Gk. kirkos "a circle," from PIE *kirk- from base *(s)ker- "to turn, bend," related to Pers. carx "wheel, everything revolving in an orbit, circular motion, chariot."

Parhun "circle" in Mod.Pers. classical texts, from Proto-Iranian *pari-iâhana- "girdle, belt," from pari-, variant pirâ-, → circum-, + iâhana- "to girdle," cf. Av. yâh- "to girdle." The Pers. word pirâhan "shirt" is a variant of parhun. Gk. cognate zone "girdle."
Dâyeré, from Ar.

A small circle on the celestial sphere parallel to the horizon. The locus of all points of a given altitude. Also called → almucantar, → altitude circle, → parallel of altitude.

→ circle; → altitude.

1) A circle of the celestial sphere, parallel to the ecliptic.
2) A circle on the terrestrial surface parallel to the equator, along which longitude is measured.

→ circle; → latitude.

1) A great circle of the celestial sphere, from the pole to the ecliptic at right angles to the plane of the ecliptic.
2) A great circle on the terrestrial surface that meets the North and South poles and connects all places of the same longitude.

→ circle; → longitude.

A circle which passes through all three vertices of a triangle Also "Circumscribed circle".

→ circum-; → circle.

Two circles if they have the same size.

→ congruent; → circle.

For a telescope with an → equatorial mounting, a graduated circle attached to the → declination axis that shows the → declination to which the telescope is pointing.

→ declination; → circle.

The apparent path of an object in the sky during one day, due to Earth's rotation.

→ diurnal; → circle.

For a → triangle with two sides extended in the direction opposite their common → vertex, a circle that lies outside the triangle and is tangent to the three sides (two of them extended). The center of the excircle, called the → excenter, is the point of intersection of the bisector of the interior angle and the bisector of the exterior angles at the other two vertices.

→ ex-; → circle.

A circle on a sphere whose plane passes through the center of the sphere.

→ great; → circle.

A great circle passing through an object and the → celestial poles intersecting the → celestial equator at right angles.

→ hour; → circle.

A telescope with a graduated vertical scale, used to measure the declinations of heavenly bodies and sometimes to determine the time of meridian transits.

→ meridian; → circle.

The circle that touches a curve (on the concave side) and whose radius is the radius of curvature.

→ osculating; → circle.

An imaginary parallel circle on the celestial sphere or on the Earth at a distance of 23°.5 from either poles.

→ polar; → circle.

The path of either → celestial poles around the → ecliptic pole due to the → precession of equinox. It takes about 26,000 years for the celestial pole to complete path.

→ precessional; → circle.

Constructing a square whose area equals that of a given circle. This was one of the three geometric problems of antiquity. It was finally proved to be an impossible problem when π was proven to be transcendental by Lindemann in 1882. Same as → squaring the circle.

→ quadrature; → circle.