Aberration--A shift of direction (or location) from the one predicted by a simple calculation. Abberation of starlight--a small shift in the observed position of stars, due to the Earth's orbital velocity.

Absolute temperature--temperature in degrees centigrade (also known in this case as "degrees Kelvin" K°) measured from the absolute zero of -273.1° C, the temperature at which all atomic and molecular motions are expected to cease.

Absorption lines Dark lines interrupting a continuous color spectrum, caused by a cool gas between the light source and the observer. Cool gas absorbs light in the same frequencies as it emits when hot, e.g. double yellow line of sodium. Such dark lines in the Sun's spectrum were discovered by Joseph Frauenhofer.

Acceleration -- Rate at which velocity changes (negative acceleration--slowing down--is also known as deceleration). Acceleration is a vector quantity.

Algebra A branch of mathematics. Originally (and still in high school context), it refers to the art of calculating with unknown quantities, represented by letters. Modern algebra has expanded this to manipulating symbols represented by letter, following certain rules which may differ from the ones applying to numbers, e.g. vector algebra, matrix algebra etc.

Algebraic equation A relation involving unknown numbers, which is satisfied only for a certain value (or values) of those numbers. Finding those values is "solving the equation."

Algebraic expansion As used here, representing an algebraic expression involving a small quantity by a sum of terms which rapidly decrease. By omitting the small term, an approximate solution may be obtained. In an iteration, the procedure is repeated again and again, increasing the accuracy.

Algebraic formula A relation between undefined quantities, each representing a variable. For instance, after t seconds, the velocity v of a stone thrown vertically with velocity u is, in meters/second, (neglecting air resistance) v = g * t + u where * marks multiplication and g = 9.81 is the acceleration due to gravity. Unlike n equation, the formula has no specific solution: but if you replace g, t and u by the appropriate numbers, it will give the value of v.

Algebraic identity A relation between symbols (usually letters) representing unknown numbers, which is always correct, regardless of the numbers the symbols represent. For instance, if a^2 is the second power (a squared) and * denotes multiplication, then (a^2)-(b^2)=(a+b)*(a-b). Sometimes distinguished by replacing the ymbol = with one containing three parallel lines.

Angle of attack--in the theory of airplane wings, the angle between the wing profile (roughly, measured along its bottom) and the wing's motion relative to the surrounding air.

Anomaly -- in orbital motion, one of the angles which gauges the motion of a planet or satellite around its orbit, increasing by 360^o every revolution. The true anomaly f equals the polar angle f in polar coordinates with origin at the center of the motion (e.g. Sun or Earth). The mean anomaly is a related angle which increases in direct proportion to the time elapsed (the true anomaly does not--the motion is faster near the center). The eccentric anomaly is an auxiliary angle used in relating true anomaly (which is observed) and mean anomaly (which is calculated).

Aphelion -- the point in a planet's orbit furthest from the Sun (Helios is Greek for Sun). See perihelion, apogee.

Apogee -- the point in a satellite's orbit furthest away from Earth (see perigee, aphelion).

Apollo (project)--the US mission to land humans on the Moon and bring them back safely.

Apparent motion -- The observed motion of a heavenly body across the celestial sphere, assuming the Earth is at the sphere's center and is standing still.

Astronomical unit (AU). Mean Sun-Earth distance, used as scaling distance in the solar system. Using Kepler's laws, it is easy to derive distances in this system as measured in astronomical units. Independent measurement of one distance in that system then fixes the magnitude of all the rest.

Atlas -- An early liquid-fueled rocket, used by US astronauts and still in use for unmanned launches. Because of its lightweight construction it uses no staging, but only drops two of its engines.

Azimuth and elevation -- Two angles which give the direction of a surveyor's telescope (theodolite). Azimuth is the rotation angle of the telescope around a vertical axis, measured (counterclockwise from above) from due north, a direction whose azimuth is zero degrees. Elevation is the angle the telescope is lifted above the horizontal plane.
[In 3-dimensional polar coordinates centered on the instrument, azimuth is f, elevation is 90^o-q; the direction of straight up has elevation 90^o but q = 0].

Ballistic pendulum -- A device often used for measuring the energy of motion of a bullet, adapted by Goddard to measure the thrust of small rockets with various nozzles. For a bullet is is a heavy block of wood or sand-filled box, hanging by a string; the bullet is weighed, then fired into the pendulum, and the distance the pendulum rises allows the bullet's velocity to be deduced.

Angle of attack

Binomial Theorem--A formula first derived by Newton, giving (1+z) ^a, the result of raising 1 + z to an arbitrary power a, as a sequence of form

where the terms A_i (i = 1,2,3...) are given by the formula and where a can be positive, negative, fractional or whole. When the magnitude of z is less than 1, the higher powers get smaller and smaller and the formula can be made as precise as one wishes by including enough of them (for z of small magnitude, 1-2 terms are sufficient), although the result is never exact. For magnitudes of z equal to 1 or more, the formula only holds for values of a which are positive whole numbers. In that case, for any z, the result is exact and the sum of terms with powers of z does not go on arbitrarily but ends with z ^a.

Black body radiation--light or other electromagnetic radiation emitted due to heat by a solid, liquid or dense gas, with no color of its own (hence "black"). Distinguished by a continuous distribution of spectral color, with its peak of emission shifting towards shorter wavelengths as the temperature increases--e.g. infra-red for a warm hand, red for a hot iron bar, yellow for the glowing filament in a lightbulb.

Black hole--an extremely compact object, collapsed by gravity which has overcome electric and nuclear forces. It is believed that stars appreciably larger than the Sun, once they have exhausted all their nuclear fuel, collapse to form black holes: they are "black" because no light escapes their intense gravity. Material attracted to a black hole, though, gains enormous energy and can radiate part of it before being swallowed up. Some astronomers believe that enormously massive black holes exist in the center of our galaxy and of other galaxies.

Boyle's law A fundamental law in the behavior of ideal gases: if temperature is kept constant, the density D of a gas varies proportionally to its pressure P. Actual gases usually approximate ideal ones pretty well as long as the temperature is far above their boiling point, e.g. nitrogen and oxygen in ordinary air.

Bulge of the Earth The extra extension of the Earth's equator, caused by the centrifugal force of the Earth's rotation, which slightly flattens the spherical shape of the Earth. The Earth's bulge causes the planes of satellite orbits inclined to the equator (but not polar) to slowly rotate around the Earth's axis.

Buoyancy--The lifting force acting in a fluid on bodies and regions less dense than their surroundings. The buoyancy of hot air--the force that also lifts hot-air balloons--is the main cause of weather-related flows in the Earth's atmosphere. Also see convection.

Calendar -- A system of marking days of the year, usually devised in a way to give each date a fixed place in the cycle of seasons.

Calorie -- Unit used in measuring the energy of heat or chemical energy. A "small" calorie is the heat needed to warm up one gram of water by 1 degree centigrade and equals about 4.18 joule. A "kilocalorie" or "big calorie" equals 1000 calories and is the unit usually used in describing the energy content of food.

Cartesian coordinates -- A system of uniquely marking the position of a point on a plane [or in 3-dimensional space] -- by 2 [3] numbers (its "cartesian coordinates") giving its distances from 2 [3] mutually perpendicular lines ("cartesian axes"). The distances and the axes to which they are parallel are usually marked (x,y) in a plane and (x,y,z) in space; the "origin" is the point at which the axes intersect.

Celestial coordinates -- see "right ascension and declination."

Celestial pole -- One of the two points in the sky around which the celestial sphere seems to rotate.

Celestial sphere -- An immense sphere surrounding Earth, to which the fixed stars seen at night appear to be attached. Although strictly speaking such a sphere does not exist, it is often used as a convenient tool for mapping the position of stars and other heavenly bodies. In a similar way, although it is clear that the apparent rotation of the celestial sphere is really due to the Earth rotating around its axis, that rotation is often used for convenient description of apparent motions such as the rising and setting of stars.

Center of gravity -- (CG), also known (more precisely) as center of mass. In a distributed mass, an appropriately defined "average location" of its parts. If the mass is a rigid (=undeforming) body subject to the earth's gravity, then if it is supported at the CG, it will stay balanced and not tilt to any side.
 In a system subject only to internal forces, the center of gravity always stays in the same spot; hence the Earth-Moon system rotates around its mutual center of gravity (not around the Earth's center), and a rocket flies forwards when it ejects a high-speed stream of gas backwards.

Centrifugal force -- A force which must be included in the calculation of equilibria between forces in a rotating frame of reference (e.g. rotating carrousel, rotating space station, rotating Earth). In the rotating frame, the forces on a body of mass m are in equilibrium (as evidenced by the body staying at the same place) only if all forces acting on it, plus a "centrifugal force" mv²/R directed away from the center of rotation, add up to zero. See Coriolis force.

Centripetal acceleration -- The acceleration associated with motion around a circle, directed to the center of the circle.

Centripetal force -- The force making a motion is a circle possible, always directed to the center of the circle. To make a (small) object of mass m move with velocity v around a circle of radius R, a centripetal force of magnitude mv²/R must be applied.

Chromosphere--a reddish layer in the Sun´s atmosphere, the transition between the photosphere and the corona

CME--see coronal mass ejection.

Chronometer An accurate clock used to derive the local longitude from the position of the Sun in the sky.

Circulation (atmospheric, also known as "global circulation"). Large scale flows of air in the atmosphere, distributing to higher latitudes (and ultimately returning it to space via radiation) the heat deposited by sunlight near the equator (ultimately returning it to space via radiation).

Climate The average pattern of weather variation at a certain location, throughout the year.

Color--a quality of light, depending on its wavelength. Spectral color of an emission of light is its place in the rainbow spectrum. Perceived color (or visual color) is the quality of light emission as conveyed by the human eye, combining the impressions of 3 types of light-sensitive cells which the eye contains. Perceived color can be the response to certain combinations of spectral colors, e.g. brown responds to green and red (or blue, yellow and red).

Comet--a body of dust, frozen water and gases falling sunward from the outer regions of the solar system. Comets become visible when they approach the Sun, as sunlight evaporates their upper layers and creates long tails of dust and ions. Comets are believed to be remnants of the formation of the solar system; some of them (like Halley's comet) are diverted by the attraction of planets into orbits of relatively short periods around the Sun.

Component of vector--When a vector is resolved into a sum of vectors in specified directions, each of those vector is the component of the given vector in the specified direction.

Conic Sections -- The family of curves generated by planes intersecting with a cone. Several cases are distinguished, depending on the angle between the plane and the axis of the cone. Precise definitions exist for each, but in general terms, when the plane is:

--Perpendicular to the axis, the curve is a circle.
--Moderately inclined to the axis, the curve is an ellipse.
--Parallel to one of the straight lines which generate the cone, the curve is a parabola.
--Even more steeply inclined, the curve is a hyperbola.

Conservation of Energy A fundamental law of physics (and chemistry): the total sum of energy in a "closed system"--one which does not interact with others around it--stays unchanged as time advances.

Conservation of momentum--A fundamental law of motion, equivalent to Newton's laws: in a system of bodies (=objects), the (vector) sum of all momenta cannot change due to any internal interactions.

Constellation -- A named grouping of fixed stars, e.g. Orion or the Big Dipper.

Convection A circulating flow in a fluid, carrying heat away from its source. Convection in the atmosphere carries heat from the sun-warmed ground to higher layers, where it is radiated away into space; the lower levels do not radiate efficiently because of the greenhouse effect. Atmospheric convection is the engine that drives the Earth's weather. Convection is also believed to occur in a certain depth range below the Sun's surface, helping carry away heat from the Sun's core region.

Copernican System -- A theory of planetary motions, proposed by Copernicus, according to which all planets move in circular orbits around the Sun, the ones closer to the Sun moving faster, with the Earth itself a planet orbiting between Venus and Mars.

Coriolis force -- A force which must be included in the calculation of motion in a rotating frame of reference, if the body moves in such a way that its rotation velocity changes. In general, it tends to preserve that part of its velocity. The Coriolis force is responsible for the swirling of hurricanes and large weather systems--for air flowing into a region of low pressure, counterclockwise north of the equator, clockwise south of the equator (reverse directions for air flowing out of a high pressure region). See centrifugal force.

Corona--the outermost layer of the Sun´s atmosphere, visible to the eye during a total solar eclipse; it can also be observed through special filters and best of all, by X-ray cameras aboard satellites. The corona is very hot, up to 1-1.5 million degrees centigrade, and is the source of the solar wind

Coronal hole--an area in the Sun's corona that appears dark when viewed in the far UV or in the long-wavelength end of the x-ray range. Coronal holes seem associated with sources of fast solar wind, probably because their field lines do not curve back to the Sun. Over most of the Sun their shapes are changeable and irregular, but the Sun's polar regions seem to contain "permanent" coronal holes.

Coronal mass ejection (CME)--a huge cloud of hot plasma, occasionally expelled from the Sun. It may accelerate ions and electrons and may travel through interplanetary space as far as the Earth´s orbit and beyond it, often preceded by a shock front. When the shock reaches Earth, a magnetic storm may result.

Crab nebula --a cloud-like nebula observed in the Crab constellation, the remnant of a supernova explosion observed in China in 1054. It contains a very rapidly rotating (and hence, young) pulsar, which is probably the remnant of the supernova. The emissions of radio waves and light from this nebula suggest the presence of high energy particles.

Crater (impact crater). A generally round depression created by the impact of a large compact mass on a planet or moon. Impact craters mark the Moon, also Mercury and Mars, and many satellites in the solar system. Notable ones on Earth include Meteor Crater in Arizona and Manicougan lake in Canada.

Cross staff A cross-shaped device to measure the elevation of the Sun or a star above the horizon, or the angle between the directions of two heavenly objects. The staff is aimed halfway between the objects, then the cross-piece is slid until aiming points at its end cover the two objects. Widely used for navigation at sea in the 15-17th century, later displaced by the more convenient sextant.

Deceleration (spell with one "c"!) Negative acceleration, slowing down of motion.

Declination -- One of the two angles uses to specify location on the celestial sphere. Declination is like latitude, but unlike latitude, it is measured from the north pole. The pole has declination 0, the equator 90 degrees, the southern celestial pole 180 degrees. See right ascension and declination

De Laval nozzle -- A device for efficiently converting t