Aberration: - The image of and object seen in a telescope suffers from certain defects. These defects are called aberrations. For example chromatic aberration is the defect of seeing colures at the edges of the image.

Almanac: - A calendar of astronomical events published periodically giving position of planets, times of eclipses etc.

Anti-clockwise: - Direction of rotation of a body opposite to the hands of a clock.

Aperture: - The size of the opening (average diameter) through which light enters an optical instrument. In case of the telescope it is the diameter of the objective lens or mirror, which gathers light. The pupil of the eye is the effective aperture of the eye.

Arc: - This the term used to define the distance between two points as an angle measured in degrees, minutes and seconds .The two points are supposed to lie on the circumference of the circle with eyes as the center. The angle, which the segment of arc between the two points subtends at the center, defines the separations as an angle of arc. Thus Alpha Capricorn's has two stars separated by 6' 16'' of the arc.

Axis: - This is the line around which a body or a number of bodies revolve. Thus the line joining North and South poles is the Earth's axis. Axis also refers to the central line in the tube of a telescope or any line passing through the center of a lens or mirror and at right angles to it.

Asteroids: - Broken down planets or piece of rock is orbiting the sun in the space between the orbits of Jupiter and Mars. Also called Planetoids.

Barlow: - It is the name given to a single concave lens, which can be attached to the eyepiece of a telescope to increase its apparent magnification.

Binary: - A system of two stars rotating around a common axis.

Black Hole: - A dark region in the sky having a very high gravitational field. These are stars, which have collapsed into a core of high density and have such large mass that they attract all material in the neighborhood. Even light radiations cannot escape from them. Only the force of gravity they exert on neighboring objects identifies them.

Celestial Equator: - It is an imaginary circle drawn in the sky around the equator of the Earth. It is the circle of zero declination and is used as the base line for measuring the angles of declination of the stars.

Clock-wise: - The Direction of rotation if the hands of a clock.

Cluster: - A group of star located close to one another.

Collimation: - The process of aligning the axes of lenses and mirrors in a telescope.

Comet: - A moving object in the sky with a bright head or nucleus followed by one or more tails of transparent haze of light. Comets appear once in a while and make a sweep around the sun in a period of few weeks.

Concave: - A curved surface with a depression on the center. Surface of a hollow sphere when seen from inside the sphere.

Conjunction: - When solar system bodies (or one solar system body and another celestial body) appear closer to each other. Ex. – a star and a planet coming closer, a planet and a planet coming closer etc.

Constellations: - A group of stars defines a certain region in the sky. The group usually forms a shape and is given a name such as Leo, Sagittarius etc.

Convex: - A curved surface, which rises in the middle. The surface of a sphere when seen from outside.

Co-ordinates: - Distance or angles from fixed points, which specify the location of an object. For example the position of a star in the sky is given by its declination and Right Ascension and these are called star co-ordinates.

Craters: - Circular pits seen on the surface of Moon. These are formed by bombardment of heavy bodies from outer space.

Crescent: - When the dark side if Moon or a Planet faces the earth and only one edge is seen as bright, it is called Crescent.

Culmination: - Stars rising in the east and moving towards west are said to Culminate when they reach their highest Point in the sky. This happens when they cross the Meridian.

Declination: - It is the angle measured in degrees, minutes and seconds by which a star is North or South of the Celestial Equator. Declinations of the Equator is 0*; North declinations are positive, South declinations are negative. Pole star has a declination of + 90*.

Dispersion: - When white light passes through a glass prism it breaks down into seven rainbow colors. This Phenomenon is called dispersion.

Double stars: - Two stars seen so close to each other that they appear as one to the naked eye, from a double star. If the stars are at different distances from the earth but happen to be in the same line they are called Optical doubles, if they are actually close to each other they are called physical doubles or binaries.

Eclipse: - When a star of any heavenly body is covered under the shadow of another such body, it is said to be under eclipse. When sun is partly or fully covered by moon we have solar eclipse. If moon falls under the shadow of earth, it is lunar eclipse. If sun, moon or any other planet covers any star, it is said to be eclipsed.

Ecliptic: - It is an imaginary circle in the sky, which defines the plane in which the earth and other planets orbit round the sun. In effect it is the path of the sun drawn in the background of the stars in one year. Because of the inclination of the axis of the earth the ecliptic is not coincident with the equatorial but is at an angle of (23-1/2)* with it.

Ellipse: - An elongated circle. Orbits of certain planets and comets around the sun are not circular but oblong or elliptical.

Equator: - It is a line drawn on the surface of the earth such that all points on it are equidistant from the North Pole and South Pole. The equator passes through Africa, between North and South America and a little south of India and Sri Lanka.

Equitorial: - Another name for Celestial Equator (see Celestial Equator).

Equinox: - It is the point at which the two imaginary great circles, the Celestial equator and the Ecliptic cross each other. There are two such points. The first called the Vernal Equinox is the point with Right Accession 0h 0m; the other is Autumnal Equinox with R.A 12h 0m. The sun is at Vernal Equinox on 21 st March when it crosses the equatorial plane from South to North.

Eye-piece: - A component of the telescope through which the image is seen magnified. Eye-piece is usually a combination if two lenses and determines the magnification and field of view of a telescope. Different eye-pieces can be used with the same telescope to obtain larger magnification or field of view.

Focal Length: - It is the distance at which parallel rays of light are converged to form an image by a lens or mirror and the focal length of the eye-piece together determine the overall magnification of the instrument.

Galaxy: - A vast system of stars, such as our Milky Way, which form a family or group quite far away from another such system. The individual stars we see belong to our Milky Way. But we also see other galaxies at distances several thousand times greater than the farthest star of our galaxy.

Galactic: - Objects belonging to our own galaxy, or Milky Way.

Helium: - A light gas similar to Hydrogen. In a nuclear fusion hydrogen gets converted to helium releasing tremendous energy as heat. This process is going on in the sun and is the source of solar energy.

Hemisphere: - A half globe. Northern hemisphere of our earth refers to all countries North of equator.

Hyperbola: - A geometrical figure which describes the path of comets, which enter our solar system only once and never return again.

Latitude: - It is one of the co-ordinates to locate places on earth. It is measured as an angle north or south of equator, North pole being at 90* latitude.

Light Year: - It is the measure of astronomical distances. The distance light travels at a speed if 186270 miles per second or 300,000 km/sec in one year is one light year. The actual distance is approximately 5.88 million-million miles or 9.46 million-million kilometers.

Longitude: - It is one of the co-ordinates used to locate places on the surface of earth. It is measured as an angle East or west of Greenwich, England. Greenwich being at 0* longitude.

Lunar: - Refers to the Moon. Thus lunar eclipse means eclipse of the moon.

Magnification: - In a telescope an object is seen larger than with the naked eye. The number of times the image is larger than the naked eye view is called magnification. The magnification if a telescope is calculated by dividing the focal length of the objective by the focal length of the eye-piece.

Magnitude: - It is the order in which the brightness of different stars is graded. The brightest are grouped in 1 st magnitude, the fainter ones being 2nd, 3rd, 4th magnitude etc. Each successive magnitude number represents 2-1/2 times brighter than the 1 st magnitude are given 0th magnitude; still brighter one and some planets have negative magnitude numbers. Depending upon the actual light intensity measured magnitude is given in fractions also. Thus the brightest star Sirius has a magnitude of –1.58.

Meridian: - It is an imaginary circle drawn in the sky from North to South and passing through the Zenith of the observer. Half of this circle lying on the other side of the earth is called Antemeridian.

Messier objects: - These refer to star, galaxies, nebulae etc first observed and catalogued by Messier. These objects are denoted by M-numbers such as M-57, which is the Ring Nebula in Lyra. There are 107 objects listed in Messier's catalogue.

Meteors: - Small material objects racing in the outer space sometimes enter the earth's atmosphere and appear as shooting stars. They are called Meteors or Meteorites.

Milky Way: - A hazy patch extending across the sky, seen on a clear night as a faint cloud. It actually consists of consists of millions of faint stars belonging to our galaxy. As the galaxy is in the shape of a disc, sitting on the earth the edge of the disc appears as Milky Way.

Nebulae: - Hazy patches in the sky representing gaseous material or a group of large number of stars. Some nebulae are part if our galaxy, but others represent other complete galaxies, far off universe.

Nova: - Nova means new and the term was used for stars which\where though to have newly created. Actually these are stars which undergo internal explosion and thereby increase in brightness suddenly settle down to their original brightness.

Nucleus: - It is the core of an atom containing massive particles called Protons and Neutrons. If smashed, the element into another one. The term nucleus also is used to define the bright head of a comet.

Objective: - It is the large lens of mirror if a telescope which faces the sky and forms a real image of the object in view. This image is then seen magnified through the eye-piece. The size of the objective determines the size and capacity of the telescope. Thus 3-inch reflector is a telescope using a concave mirror of 3-inches diameter as objective.

Occultation: - When two celestial body hides another body, not because of its shadow but physically. In case of hiding due to shadow, it is called Eclipse.

Optical double: - Two stars almost in one line as seen from earth and appearing as a double star or Binary. The stars are otherwise unrelated to each other.

Opposition: - When a solar system body rises from the east at the same time when the sun sets in the east (the most favorable time to watch that object), the solar system body is said to be in opposition. Also, the object is visible throughout the night, at midnight it is above the earth. Ex. (rising of full moon)

Orbit: - The paths of planets and satellites, which revolve around a large parent body, are called their orbits. Orbits are usually circular or closed elliptical paths.

Pen-umbra: - It is that part of the shadow of an object under which the source of light is only partially covered. When earth passes through the pen-umbra of moon's shadow we see a partial solar eclipse.

Phases: - Refer to different shapes of moon and planets when they are seen from the earth as partly illuminated such is crescent moon, half moon etc.

Planets: - Heavenly bodies, which move around the sun, are called planets. Including the earth, there are nine planets known so far. Moon and other bodies, which revolve around other planets, are called satellites.

Planetoids: - Same as Asteroids (See as Asteroids).

Pulsar: - A star or distant galaxy which emits systematic pulses of ratio waves.

Quasar: - Quasi-stellar objects. This term is used for stars or distant objects, which emit high intensity of radio-active or other electromagnetic radiation such as radio waves, X-rays etc.

Retina: - The sensitive surface inside the eye on which light image is formed. A large number of optic nerves sensitive to light are situated in this region.

Red Giants: - Stars which are too bright for their temperature. These stars are enormously large in size but consist of very light material, which shines at low temperature.

Reflector: - A telescope which uses a concave mirror as the objective glass to face the sky.

Refractor: - A telescope which uses a convex lens as the objective glass to face the sky.

Resolve: - The capacity to see distinctly and individually objects very close to each other.

Resolving Power: - The minimum separation between two objects seen distinctly as two. The resolving power of the naked eye is about 4 minutes of arc. A small telescope with a 2-inch diameter objective has resolving power of better than 2 seconds of arc.

Retrograde: - When a planet reverses its path as seen from the earth, it is said to be in retrograde motion. This happens in case of Jupiter, Saturn etc when the earth passes across them at a faster rate.

Right Ascension: - It is a co-ordinate to specify the position of a star in its circle of declination that is the circle the star describes during the 24-hours rotation of the earth. It is measured in hours, minutes and seconds and is counted from the positions the sun occupies on 21 st of March.

Satellite: - A heavenly body moving around a planet. Our Moon or moons of Jupiter and Saturn are all satellites. The artificial rockets, which go into orbit round the earth, are also called satellites.

Spectrum: - White light when dispersed by a prism breaks down into a number of colors, which are called its spectrum. An analysis of the spectrum gives information about the chemical composition of the source of light.

Spectrometer: - An instrument to see and analysis the spectrum of light received from a specified source such as star.

Sun-spots: - Small dark spots appear on the bright surface of the sun when seen by a telescope. These sun-spots moves across the surface indicating rotation of sun. The spots are not permanent but appear and disappear periodically. When appearing they cause magnetic disturbances on earth.

Super-nova: - An explosive star. Some stars undergo an internal explosion of very high intensity so that their brightness increases several thousand times. This lasts for a few days only when they can be seen in daytime. Such phenomenon occurs very rarely-once in 300 years or so.

Transit: - It is a type of Eclipse. When any inner planet (Mercury and Venus) cast its shadow on sun, visible from earth as a small dot moving across sun is called transit.

Variable stars: - Stars which increase and decrease in brightness periodically. In some cases this is due to two stars revolving in such a manner that one eclipses the other. But many stars actually change their intrinsic brightness in a regular cycle of a few weeks to a few years.

White Dwarfs: - Tiny stars of extremely high temperature. These are stars, which are collapsing under their own gravity and are giving the last brilliance.