## true equator Quick reference

### A Dictionary of Astronomy (3 ed.)

...equator The great circle perpendicular to the direction of the true pole...

## mean equator Quick reference

### A Dictionary of Astronomy (3 ed.)

...equator The great circle perpendicular to the direction of the mean pole...

## wavefront Quick reference

### A Dictionary of Astronomy (3 ed.)

... An imaginary surface connecting points at which the electromagnetic waves from a source are in phase. The wavefront is perpendicular to the direction of...

## horizon Quick reference

### A Dictionary of Astronomy (3 ed.)

... The great circle formed by the intersection of the plane perpendicular to the observer’s zenith with the celestial sphere; also called the astronomical horizon...

## hour circle Quick reference

### A Dictionary of Astronomy (3 ed.)

...circle A great circle on the celestial sphere passing through a celestial object and the celestial poles; also known as meridian of hour angle . It is perpendicular to the celestial...

## tangential velocity Quick reference

### A Dictionary of Astronomy (3 ed.)

...velocity The component of a star’s velocity that is perpendicular to our line of sight (i.e. in the tangent plane); also known as transverse velocity . A star’s tangential velocity can be computed from its observed proper motion and measured...

## collimation error Quick reference

### A Dictionary of Astronomy (3 ed.)

...error The error in alignment between the optical axis of a telescope and the declination axis of its equatorial mount; the two should be exactly perpendicular if the telescope is to track stars...

## prime vertical Quick reference

### A Dictionary of Astronomy (3 ed.)

...vertical The great circle that passes through the observer’s zenith, perpendicular to the meridian. It intersects the horizon at the west and east points. The parts of the circle going from zenith through east and west are known as the prime vertical east and the prime vertical west...

## equator Quick reference

### A Dictionary of Astronomy (3 ed.)

... The intersection of the surface of a body with a plane perpendicular to the body’s axis of rotation. The plane of the equator passes through the body’s centre and divides the body into northern and southern hemispheres. All points on the equator are equidistant from the body’s poles of rotation. See also Celestial Equator ; Galactic Equator...

## pole Quick reference

### A Dictionary of Astronomy (3 ed.)

... The direction perpendicular to a given plane, such as the plane of the Earth’s equator or the plane of the ecliptic. For a rotating body, the poles lie at the ends of the body’s axis of rotation. See also celestial pole ; ecliptic pole ; galactic pole...

## Wollaston prism Quick reference

### A Dictionary of Astronomy (3 ed.)

...prism A device used for analysing and producing polarized light, used in some designs of polarimeter . It consists of two prisms of either quartz or calcite cemented together. The Wollaston prism divides incoming light into two linearly polarized beams with perpendicular planes. It is named after the British scientist William Hyde Wollaston ( 1766–1828...

## dipole antenna Quick reference

### A Dictionary of Astronomy (3 ed.)

...antenna An antenna consisting of a straight metal rod or wire broken in the centre to make two terminals. The terminals are connected to a feed which carries the signal to the receiver. The maximum sensitivity is in the direction perpendicular to the rods. See Folded Dipole ; Full-Wave Dipole ; Half-Wave Dipole...

## eccentric anomaly Quick reference

### A Dictionary of Astronomy (3 ed.)

...anomaly ( symbol E ) The angle between the periapsis of an orbit and a given point on a circle around the orbit, as seen from the centre of the orbit ( see diagram ). The point concerned is found by drawing a line perpendicular to the major axis through the actual position of the orbiting body until it cuts the circle around the orbit, as in the diagram. The diameter of the circle is equal to the major axis of the orbit, and its centre is also the orbit’s centre. The angle of eccentric anomaly is measured in the direction of orbital motion. See...

## conic section Quick reference

### A Dictionary of Astronomy (3 ed.)

...section A figure obtained by slicing a cone. There are four different types of conic section. If the cone is cut perpendicular to its axis, the resulting figure is a circle. If the cut is not perpendicular to the axis but still produces a closed curve, the curve is an ellipse. If the cone is cut parallel to one of its sloping sides, the resulting curve is a parabola, which is not closed. If the angle of cut is tilted still further, the open figure obtained is a hyperbola. An ellipse has an eccentricity less than 1; a circle is a special case of an...

## Yagi antenna Quick reference

### A Dictionary of Astronomy (3 ed.)

...antenna A directional aerial consisting of a dipole and a number of parallel rods arranged perpendicular to the line of sight. The rods in front of the dipole are termed directors , while the one behind is the reflector . The design is commonly used in television aerials, but in radio astronomy Yagi antennas are used as elements in interferometers. The design is named after the Japanese electrical engineer Hidetsugu Yagi ( 1886–1976...

## lift-to-drag ratio Quick reference

### A Dictionary of Space Exploration (3 ed.)

...lift-to-drag ratio ( L/D ) The figure obtained by dividing the lift by the drag. Lift is the force acting on a body, perpendicular to the direction of motion. Drag is the retarding force acting upon a body, parallel to the direction of motion. The ratio is an important part of aerodynamics, and is used to calculate the path of spacecraft and aircraft as they travel through the Earth's...

## flux Quick reference

### A Dictionary of Astronomy (3 ed.)

...of energy or number of particles passing through a given area of surface in unit time, usually per second. Luminous flux is the rate of flow of energy in the form of photons, measured in lumens; magnetic flux is a measure of the strength and extent of a magnetic field perpendicular to a surface, measured in webers; particle flux is the number of particles, for example in a stellar wind, passing through a unit area per...

## hexahedrite Quick reference

### A Dictionary of Astronomy (3 ed.)

... A class of iron meteorite, containing not more than 4–6% nickel. Hexahedrites consist of large cubic crystals of kamacite (an iron–nickel alloy) over 50 mm across, and they can be cleaved in three perpendicular directions along the faces of a hexahedron, hence their name. They display a pattern of fine lines called Neumann lines . The hexa-octahedrite meteorites are transitional between hexahedrites and octahedrites . In these meteorites the thickness of the kamacite plates (the band width ) varies from 3 to 50 mm, most of the kamacite grains...

## Magellanic Clouds Quick reference

### A Dictionary of Astronomy (3 ed.)

...naked eye in the southern hemisphere like detached portions of the Milky Way. They are named after the Portuguese explorer Ferdinand Magellan ( 1480–1521 ), who described them during his voyage around the world. Both Clouds are believed to orbit our Galaxy in a plane nearly perpendicular to its disk, and may eventually spiral into the Galaxy. See also Large Magellanic Cloud ; Magellanic Stream ; Small Magellanic cloud...

## rectangular coordinates Quick reference

### A Dictionary of Astronomy (3 ed.)

...coordinates A system for specifying the position of an object relative to two or three mutually perpendicular axes from some specified origin. The axes are identified as x, y , and (where a third axis is used) z . Cartesian coordinates are rectangular coordinates. In astronomy, rectangular coordinates are sometimes used for Solar System objects, with the length of each axis given in astronomical units. They are usually heliocentric or geocentric in origin, but other points of origin can be...