  ## G l o s s a r y

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.

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

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 360o 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).

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.

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 90o-q; the direction of straight up has elevation 90o 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.

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

(1+z) a = 1 + A1z + A2z 2 + A3z 3 + ....

where the terms Ai (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.

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  numbers (its "cartesian coordinates") giving its distances from 2  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.

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 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 areound 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.

Declination -- See "right ascension and declination"

De Laval nozzle -- A device for efficiently converting the energy of a hot gas to kinetic energy of motion, originally used in some steam turbines and now used in practically all rockets. By constricting the outflow of the gas until it reaches the velocity of sound and then letting it expand again, an extremely fast jet is produced.

Diffraction grating A flat optical surface, transparent or reflecting, ruled with many parallel grooves at precisely spaced distances. The active parts are not the grooves but the flat sections left between them, which act like a large number of precisely spaced slits. The light waves passing those slits resonate with each other in a way which depends on wavelength, causing different wavelengths to be steered in different directions. The overall effect on light containing different wavelengths is like that of a glass prism: the intensity of the light deflected is much smaller than with a prism, but the ability to separate close colors is much better.

Drag--the air resistance encountered by a moving object. Drag is one of the four forces sensed by an airplane, the others being lift, thrust and weight.

Eccentricity -- Number between 0 and 1, gauging the elongation of elliptic orbit. The eccentricity e of the orbital ellipse is one of the "orbital elements" characterizing it.

Ecliptic -- A line around the middle of the celestial sphere, connecting the points occupied by the Sun over the year. The moon and the visible planets also appear to move very close to that line, which cuts the celestial equator at an angle of about 23.5o . See plane of the ecliptic.

Electromagnetic field (EM field)--the regions of space near electric currents, magnets, broadcasting antennas etc., regions in which electric and magnetic forces may act. Generally the EM field is regarded as a modification of space itself, enabling it to store and transmit energy. See also (below) "electromagnetic wave" and magnetic field.

Electromagnetic wave or "electromagnetic radiation"--a combination of oscillating magnetic and electric fields, spreading in wavelike fashion through space at a speed of about 300 000 km.sec. James Clerk Maxwell's theory in 1864 suggested that light was such a wave, and today we know that such waves include all forms of light--also infra-red and ultra-violet, as well as radio waves, microwaves, x-rays and gamma rays.

Electron--a lightweight particle, carrying a negative electric charge and found in all atoms. Electrons can be energized or even torn from atoms by light and by collisions, and they are responsible for many electric phenomena in solid matter and in plasmas. (About the discovery of the electron in 1897, click here.)

Ellipse -- A closed curve resembling a flattened circle (the shadow of a circle tilted towards the light is an ellipse). May be defined:

1.   As the collection of points whose distances (R1, R2) from two given points (the foci of the ellipse--in singular, focus) add up to the same sum.
2.    Or else , in polar coordinates (r,f), as the curve whose points satisfy a relation r = a(1 - e)/(1 + e cosf) where a is the semi-major axis, half the width in the direction through the two foci. One of the foci is then at the origin and e is the eccentricity, a number ranging from 0 (circle) to 1 (parabola).
3.    Or else, in cartesian coordinates with the origin halfway between the foci, as the curve of all points (x,y) whose coordinates satisfy (x/a)2 + (y/b)2 = 1

Energy -- Ability to perform work, i.e. to advance against resistance, for instance lift a body against gravity, or drag it against friction.

Epicycle -- A circle around a point which (in the simplest form of Ptolemy's system) moved steadily around the celestial sphere. Greek astronomers proposed that planets moved along epicycles around the Sun or around other points which circled around the sky; later additional corrections were added. The theory of epicycles was the earliest explanation for the irregular apparent motion of the planets--prograde (forward), then retrograde

Equatorial axis -- Among the two mutually perpendicular axes of a telescope, the one that points at the celestial pole. To keep a star in view, the telescope must be rotated around this axis at the same rate as the Earth turns.

Equilibrium (of forces) -- A situation when more than one force acts on a body, but because the sum of forces is zero, no motion results.

Equinox -- the time of the year (around March 21 and September 23) when the position of the Sun in the sky (following the ecliptic) crosses the celestial equator. To a good approximation, the length of the day and night are then equal, and the Sun rises exactly in the east and sets exactly in the west. . Equinox is viewed as the beginning of spring and fall.
The term is also used for each of the two points on the celestial sphere at which the ecliptic and the celestial equator intersect, i.e. the points occupied by the Sun at equinox.

Explorer 1 -- The first US artificial satellite, launched 31 January 1958 by a 4-stage modified military rocket. Provided the earliest observations of the Earth's radiation belt.

Field The region in which a particular type of force can be observed; depending on the force, one can thus speak of a gravity field, magnetic field, electric field (or when the two are linked by fast oscillations, electromagnetic field) and nuclear field. The laws of physics suggest that fields represent more than a possibility of force being observed, but that they can also transmit energy and momentum, e.g. a light wave is a phenomenon completely defined by fields. For that reason a field is often viewed as a space which was modified by the sources of the force which the field represents.

Firmament -- The celestial sphere and the collection of stars whose position is fixed on it.

First point in Aries -- Another name for the position on the celestial sphere of the vernal equinox. It is called so because in ancient time that point was in Aries, a constellation of the zodiac. It is currently moving from Pisces to Aquarius.

Flare (Solar flare)--a rapid outburst on the Sun, usually in the vicinity of active sunspots. A sudden brightening (only rarely seen without special filters, isolating the red light of hydrogen) may be followed by the signatures of particle acceleration to high energies--x-rays, radio noise and often, a bit later, the arrival of high-energy ions from the Sun. Flares appear to be associated with rapid energy releases high above the photosphere, apparently from the magnetic fields of sunspots. Their link to coronal mass ejections, which may also be powered by magnetic energy, is still unclear.

Fly-by maneuver (or swing-by maneuver) -- The encounter between a moving spacecraft and a moving planet or moon, affecting the spacecraft's motion like an elastic collision (in which no energy is lost to heat). Depending on the details of the encounter, the spacecraft can gain or lose appreciable amounts of energy, and appreciable changes in the direction of its motion can result.
Fly-by maneuvers with the Moon have been used to reach the L1 Lagrangian point; fly-by maneuvers with the planets have played an essential role in space missions exploring the solar system.

Force -- In mechanics, the cause of motion. It is a vector quantity, in the direction of the acceleration it causes.

Frequency (Often denoted by n, the Greek letter letter nu.) --the number of back-and-forth cycles per second, in a wave or wave-like process. Expressed this way, the frequency is said to be given in units of Hertz (Hz), named after the scientist who first produced and observed radio waves in the lab. Alternating current in homes in the US goes through 60 cycles each second, hence its frequency is 60 Hz; in Europe it is 50 cycles and 50 Hz.

g -- The symbol used for the acceleration due to gravity. At the Earth's surface it averages 9.81 meters/second2, directed towards the Earth's center. In common talk, "g forces" are stresses due to acceleration, e.g. on astronauts or payloads. In the same vein, "zero g" is the condition when no acceleration is sensed, because gravity is already fully employed supplying the centripetal force which holds the object in its orbit (or alternatively from the rotating frame of reference, because gravity is fully balanced by the centrifugal force).

Geodesy -- The study of the shape of the Earth, e.g. its deviations from an exact sphere.

Gnomon -- The part of a sundial which casts the shadow, usually a rod or fin pointed at the celestial pole.

Greenhouse effect The surface of the Earth is, on the average, in a state of equilibrium between heating and cooling: that is, on the average, the rate at which sunlight heats it equals the rate at which it loses heat.

If no atmosphere existed, all that loss would take place by infra-red radiation from the surface. The Earth's atmosphere, however, absorbs infra-red, which heats it up and slows down the escape of heat. The same process occurs in glass-covered greenhouses, whose panes let sunlight in but absorb the infra-red emitted back, keeping their interior warm even in winter. For that reason, the process is known as the "greenhouse effect."

Some gases which constitute only a small portion of the atmosphere--water vapor, CO2 (carbon dioxide) and CH4 (methane)--are major contributors to the greenhouse effect. Burning coal and oil in the last century has markedly increased the CO2 content of the atmosphere, which is why some scientists credit the warming trend experienced in the last decades of the 20th century to an increased "greenhouse effect."

Gregorian calendar -- Introduced in 1582 by Pope Gregory the 13th, this calendar modifies the Julian calendar for greater precision, decreeing that century years such as 1900 are not leap years, except if the number of centuries is divisible by 4 (e.g. 2000).

High Energy Particles--charged atomic particles moving rapidly, often at a significant fraction of the speed of light. They can penetrate matter, ionize the material which they traverse and emit energetic photons (e.g. of x-rays). See also solar energetic particles.

Ice ages -- Times in the geological past when great glaciers extended far into Europe, Asia and America.

Inertial force A force which must be added to the equations of motion when Newton's laws are used in a rotating or otherwise accelerating frame of reference. Some call it a "fictional force" because when the same motion is solved in the frame of the "outside world&," these forces do not appear.

Infra-red radiation (or infra-red light). The region of the electromagnetic spectrum adjacent to that of visible light, but with longer wavelengths (0.65-10 micrometers, typical). Infra-red radiation is emitted by hot objects and by excited molecules. See also greenhouse effect.

Ion--usually, an atom from which one or more electrons have been torn off, leaving a positively charged particle. "Negative ions" are atoms which have acquired one or more extra electrons, and clusters of atoms can also become ions.

Ionization--the process by which a neutral atom, or a cluster of such atoms, becomes an ion. This may occur, for instance, by absorbtion of light ("photoionization") or by a collision with a fast particle ("impact ionization"). Also, certain molecules (such as table salt or sodium chloride, NaCl) are formed by natural ions (like Na+ and Cl-) held together by their electric attraction, and they may fall apart when dissolved in water (which weakens the attraction), enabling the solution to conduct electricity.

Iteration--The repetition of a process of calculation again and again, each time improving the accuracy of the result. For an example of iteration (with "Kepler's Equation") see here

Jet Propulsion Lab -- An outgrowth of the Guggenheim Aeronautical Laboratory of Caltech, in Pasadena (near Los Angeles, California). JPL was the center of US rocket development in World War II and was founded by Theodore Von Karman and Frank Malina. Today it is the focus of NASA's exploration of the planets and of distant space.

Joule -- (pronounced like "jewel"). Unit of energy: the ability to overcome one Newton along 1 meter (assuming g = 10 meter/sec2, it is also the energy required to lift 1 kg by 0.1 meters). Named for James Prescott Joule, one of the first to measure the "rate of exchange" between mechanical energy and heat.

Julian Calendar -- Introduced in 46 BC by the Roman ruler Julius Ceasar, this calendar assumes a year of 365.25 days, and uses a cycle in which 3 "ordinary" years of 365 days are followed by a "leap year" with 366 days. Leap years are the years whose number is divisible by 4.

Kepler's laws --
Three laws of planetary motion, published by Johannes Kepler using accurate observations by Tycho Brahe and shown by Isaac Newton to be a direct result of his theory of gravitation and his laws of motion:

1. Planets move in ellipses, with the Sun at one focus.
2. The line connecting the planets to the Sun sweeps equal areas in equal times.
3. The square of a planet's orbital period is proportional to the cube of its mean distance from the Sun.

1st law
: This corrected the simpler model of Copernicus, which assumed circles. More accurately, the focus is at the center of gravity of the Sun and orbiting body (discounting other planets) and non-periodic motions along parabolas or hyperbolas are also possible.
2nd law: The second law expresses the way a planet speeds up when approaching the Sun and the way it slows down when drawing away.
3rd law: The third law gives the exact relation by which planets move faster on orbits which are closer to the Sun, e.g. Venus moves faster than Earth (see retrograde motion). For a more precise formulation, "mean distance" should be replaced by semimajor axis.

Kilowatt-hour -- (KWH). The amount of energy supplied by one kilowatt (1000 watt) for 1 hour (3600 seconds), equal to 3 600 000 joule. Electric bills are usually figured by the number of KWHs consumed.

Kinetic energy -- Energy stored in the motion of a mechanical system--e.g. by a rolling car, or a turning flywheel.

Lagrangian points -- In a system of two large bodies (Sun-Earth or Earth-Moon), these are the points where a small third body will keep a fixed position relative to the other two. Named for French astronomer Louis Lagrange (1736-1813) who first studied them and who showed there existed 5 such points. In the Sun-Earth system only two are important, both on the Earth-Sun line--the L1 point 236 Earth radii sunward of Earth, and the L2 point at a similar distance on the night side. The L1 point is a good "early warning" outpost intercepting shocks and particles emitted by the Sun and its vicinity has been occupied by several spacecraft. Altogether five Lagrangian points exist in the Earth-Sun or Earth-Moon system.

Latitude and longitude -- Two angles which specify a location on Earth. If a line is drawn from the Earth's center to the given location, then latitude is the angle between that line and its projection on the plane of the Earth's equator (latitude also equals 90o- q, where the "co-latitude" q is the angle between the line and the axis of the Earth).

To define longitude, imagine a large number of planes ("meridional planes") all of which contain the axis of the Earth. Assuming the equator is a circle, divide it into 360 degrees and fractions of degrees: then each meridional plane can be labeled by the angle at its intersection of the equator, and the longitude of a point is the angle f marking the meridional plane on which it sits. Longitude is similar to the angle f of 3-dimensional polar coordinates or to right ascension, but is measured from a zero longitude chosen as the longitude of the Greenwich observatory near London, Great Britain.

Law of areas -- Another name for Kepler's 2nd law.

Lift--the lifting force on a flying object (in particular, a wing or an aircraft), due to its motion relative to the surrounding air. Lift is one of the four forces sensed by an airplane, the others being drag, thrust and weight.

Liquid fueled rockets -- Rockets in which a liquid fuel (kerosene, liquid hydrogen) is combined in a combustion chamber with a liquid oxidizer (usually liquid oxygen, also fuming nitric acid or hydrogen peroxide). Very efficient and controllable, such rockets are generally used in spaceflight. Unlike solid fueled rockets, they can be shut off by remote command, simply by closing off their fuel line.

Magnetic field--a region in which magnetic forces can be observed. See "electromagnetic field," a more general field also including electric forces.

Magnetic field lines--lines in space, used for visually representing magnetic fields. At any point in space, the local field line points in the direction of the magnetic force which an isolated magnetic pole at that point would experience. In a plasma, magnetic field lines also guide the motion of ions and electrons, and direct the flow of some electric currents.

Magnetic storm--A large-scale disturbance of the magnetosphere, often initiated by the arrival of an plasma cloud originating at the Sun.
A magnetic storm is marked by the injection of an appreciable number of ions from the tail regions of the magnetosphere into ]the near-Earth magnetosphere, a process accompanied by increased auroral displays. The injected particles cause a world-wide drop in the equatorial magnetic field, taking perhaps 12 hours to reach its greatest intensity, followed by a more gradual recovery.

Magnetosphere -- The outermost environment of Earth, dominated by the Earth's magnetic field. The magnetosphere is the site of the radiation belt and many intricate phenomena. See solar wind.

Mass -- The mass of a body can be loosely defined as the amount of matter it contains. That is expressed in two ways:

1. inertial mass, the resistance of the matter to acceleration or deceleration, as given by the factor m in Newton's 2nd law F = ma
2. gravitational mass, the force exerted on the matter by gravity ("weight"), given near the surface of Earth by F = mg.
According to all experiments, the two are equal, causing all bodies subject to gravity only (near the surface of the Earth) to have the same acceleration a = g.

Metonic Calendar -- Named for the Athenian astronomer Meton, it is based on the moon, counting each cycle of the phases of the Moon as one month. Days are kept approximately in step with the seasons by including 7 leap years of 13 months in each cycle of 19 years. Used by the Chinese and the Jews.

Microwaves Electromagnetic waves longer than infra-red but shorter than radio, with typical wavelength 0.1-10 centimeters.

Milankovich theory -- Theory by which ice ages were caused by slow changes of the motion of the Earth in space, including the coupling between the 26 000 year cycle of the precession of the equinoxes and the annual variation of the Earth-Sun distance.

Momentum (plural: momenta). The momentum of a moving object is the product (result of multiplication) of its mass and velocity; like velocity, momentum is a vector. The law of conservation of momentum states that when two or more objects interact--a cannon fires a shell, a rocket shoots out a fast jet of hot gas, a bowling ball scatters a group of pins--the total vector sum of their momenta is unchanged. That, too, is an equivalent formulation of Newton's laws.

Muslim Calendar -- Based on a year of 12 months, each corresponding to one cycle of the Moon, but without the Metonic correction. Its months migrate through the seasons.

Neutron A particle found in the nuclei of atoms, similar to a proton but with no electric charge. Among light nuclei (helium, carbon, nitrogen), the ones that are most stable contain equal numbers of protons and neutrons. In heavier elements, the most stable ones have majority of neutrons, growing with mass. Varieties of nuclei also exist ("isotopes") which have other ratios between their numbers of protons and neutrons, but when the departure from the "most stable ratio" becomes large, neutrons can convert to protons + electrons (or vice versa), producing one form of radioactivity.

Neutron star A star (approximately sun-sized or larger), a remnant of a supernova explosion, in which gravity has caused all matter to collapse to a giant nucleus, composed only of neutrons. The collapse is also expected to greatly amplify any magnetic field present in the pre-collapse star, as well as speed up enormously any rate of rotation. It is believed that pulsars, pulsating radio sources with very precise pulsation periods, are neutron stars of radius about 10 km and rotation period about 1 second. Their magnetic axis spins and beams radio waves, in a way similar to the way a lighthouse beams its light. We detect pulsars when the Earth is in one of the directions swept by the beams.

Newton -- Unit of force, the force which, when applied to one kilogram mass, causes an acceleration of 1 meter/sec2.

Newton's laws of motion -- Three laws which form the foundation of classical mechanics, i.e. of the theory of ordinary motions (not motions on an atomic scale, covered by quantum mechanics, and not at velocities close to that of light, covered by relativity). The laws introduce the concepts of force and mass and state (in modern terms)

1. In the absence of forces, an object ("body") at rest stays at rest, and an object moving in a straight line with constant velocity persists in doing so.

2. A (small) body subject to a force accelerates; the acceleration is in the direction of the force and proportional to its magnitude, and inversely proportional to the mass of the body: F = ma.

3. Forces are produced in pairs, in opposite directions and equal magnitudes.

Newton's laws (2) and (3) in Mach's formulation reduce to:" When two small bodies act on each other, they accelerate in opposite directions and the ratio of their accelerations is always the same."

Nuclear forces The short-range forces acting on protons and neutrons in atomic nuclei. Two types actually exist, the "strong force" which holds nuclei together, and the "weak force" which determines the ratio between the numbers of protons and neutrons.

Nuclear fusion The process of releasing energy by combining hydrogen atoms to form helium, or more generally, to combine light nuclei into heavier ones. Nuclear fusion appears to be the source of the energy of the Sun and of stars.

Nucleus (atomic; plural: nuclei). The small concentration of protons and neutrons, positively charged, at the center of atoms. The nuclei of atoms are positively charged and contain by far most of their mass (all but about 0.05% or less).

Orbit -- The path of a body in space, generally under the influence of gravity.

Orbital elements -- Variables which characterize the motion of an orbiting body. For a planet or satellite in an elliptic orbit, 6 orbital elements exist: the semi-major axis gives its size, eccentricity its shape and mean anomaly its position along the orbit, at the given time. The three other elements are three angles which give the orientation in space of its orbital plane, e.g. that plane's inclination (to the plane of the Earth's equator or the ecliptic,depending on choice of coordinates).

Orbital period -- The length of time required for a body to complete one full (closed) orbit.

Particle--in general, a charged component of an atom, that is, an ion or electron.

Perigee -- the point of a satellite's orbit closest to Earth (see perihelion, apogee).

Perihelion -- The point in a planet's orbit when it is closest to the Sun (Helios is Greek for Sun). See aphelion, perigee

Photon --colloquially, a "particle of light." Although light spreads as an electromagnetic wave, it can be created or absorbed only in discrete amounts of energy, known as photons. The energy of a photon is greater the shorter the wavelength--smallest for radio waves, increasingly larger for microwaves, infra-red radiation, visible light and ultra-violet light. It is largest for x-rays and gamma rays.

Photosphere--The layer of the Sun from which all visible light reaches us. The Sun is too hot to have a solid surface and the photosphere consists of a plasma at about 5500 degrees centigrade.

Plane of the ecliptic -- (also called "the ecliptic" for short) The orbital plane of the Earth around the Sun. The line of the ecliptic on the celestial sphere is formed by the intersection of the plane of the ecliptic with that sphere. The reason the major planets and Moon appear in the sky close to the ecliptic is that the solar system is flat, and its orbital planes are very close to each other. We observe their motion (very nearly) edge-on.

Planets -- Celestial bodies such as the Earth which orbit the Sun (and by extension, similar orbiters around distant stars). Counting from the Sun outwards, planets visible to the eye are Mercury, Venus, (Earth), Mars, Jupiter and Saturn. The telescope also sees the more distant Uranus, Neptune and Pluto, as well as smaller asteroids (most of them inside the Jupiter orbit) and Kuiper objects (in the outer solar system). See also retrograde motion

Plasma --a gas containing free ions and electrons, and therefore capable of conducting electric currents. A "partially ionized plasma" such as the Earth's ionosphere is one that also contains neutral atoms.

Polar Coordinates -- An alternative system of marking a point on a plane by its radial distance (r) from an "origin" and a polar angle (f). Polar coordinates in 3-dimensional space use (r) and two polar angles (q,f) giving the direction from the origin to the point.
When 3-dimensional polar coordinates overlap a cartesian (x,y,z) system, q is the angle between the line to the origin and the z-axis, while f is the angle (counter-clockwise when viewed from +z) between the projection of that line onto the (x,y) plane and the x-axis. Concerning (q,f), see also latitude and longitude, declination and right ascension, azimuth and elevation.

Polaris (Pole Star, North Star) -- A fairly bright star, the last star in the tail (or handle) of the constellation of the Little Dipper (Ursa Minor). Currently located within a fraction of a degree from the celestial north pole, the point around which the celestial sphere appears to rotate. In the northern hemisphere, the direction towards Polaris is very nearly due north.

Potential energy -- Energy stored in the set-up of a mechanical system--e.g. by a weight able to descend (in the presence of gravity), or by a compressed spring.

Power -- The rate at which energy is supplied. See watt.

Precession -- A modern term, derived from the precession of the equinoxes and meaning a motion around a cone of the rotation axis of a spinning body.

Precession of the Equinoxes -- A slow motion of the axis of the Earth around a cone, one cycle in about 26000 years. As a result, the celestial pole moves around a circle in the sky, and in ancient times, for instance, was quite far from Polaris. Discovered by Hipparchus around 130 BC as a slow shift of the vernal equinox around the ecliptic (i.e. around the zodiac).

Prominence A cloud of cooler plasma extending high above the Sun"s visible surface, rising above the photosphere into the corona.

Propeller pitch--the angle at which the propeller blade (or part of it) "bites" into the air, its angle of attack.

Proton --an ion of hydrogen and one of the fundamental building blocks from which atomic nuclei are made.

Ptolemy's System -- The explanation given by ancient Greek astronomers to the motion of planets around the sky, described in a book by the Greek Ptolemy, around 150 AD. It regarded Earth as the center of the universe and assumed the motion of planets was a superposition of circular motions (see epicycles).

Pulsar. See neutron star

Pythagoras, theorem of -- A proved assertion in geometry, that in a right-angled triangle which has sides of length (a, b, c), if c is the long side facing the right angle, then a2 + b2 = c2

• In the narrow sense, some type of electromagnetic wave: radio, microwave, light (infra-red, visible or ultra-violet), x-rays or gamma rays are all types of electromagnetic radiation.
• Colloquially, an abbreviation of "ionizing radiation" meaning any spreading emission which can penetrate matter and ionize its atoms. That includes x-rays and gamma rays, but also high-energy ions and electrons emitted by radioactive substances, accelerated by laboratory devices or encountered in space (e.g. the "radiation belt" and "cosmic rays," also known as the "cosmic radiation").

Radioactivity --Instability of some atomic nuclei, causing them to change spontaneously to a lower energy level or to modify the number of protons and neutrons they contain. The 3 "classical" types of radioactive emissions are (1) alpha particles, nuclei of helium (2) beta-rays, fast electrons and (3) gamma-rays, high-energy photons.

Radio waves--Electromagnetic waves of relatively low frequency.

Reaction force -- The added force implied by the lack of motion (equilibrium) when an applied force exists (e.g. gravity).

Re-entry (atmospheric re-entry) -- The return of a spacecraft from orbit to Earth, in which the kinetic energy of the orbital motion is converted into heat. Since that heat is sufficient to melt the spacecraft, if the spacecraft is to land intact, the heat must be safely dissipated. Heat-resistant shields of various types are used, and the reentry is at a shallow angle, to stretch out the process.

Retrograde motion -- Temporary reversal of the apparent motion of a planet along the ecliptic. Caused because (by Kepler's 3rd law) a planet moves faster the closer it is to the Sun, so that (for instance) Jupiter appears to move backward when the faster-moving Earth overtakes it.

Right angle -- The angle formed when two straight lines intersect and the 4 angles at their crossing are all equal. When measured in degrees it equals 90o.

Right ascension and declination -- Two angles marking the position of a star on the celestial sphere. Imagine a line from the observer to the star, and draw its projection (like a shadow) onto the celestial equator. Declination d is the angle between the line and its projection (d = 90o - q, where q is the angle to the direction to the celestial pole); it is negative south of the equator. RA is the angle between the projection and the direction to the vernal equinox or first point in Aries.

Rocket -- A device shooting out a fast jet of gas, in order to produce a force in the opposite direction. See center of gravity, also Newton's laws of motion in Mach's formulation.

Rotation axis of the Earth -- The imaginary line around which the Earth turns. Its inclination of about 23.5o to the ecliptic is the reason for the seasons of the year.

Saturn V -- The biggest rocket built to date, weighing 2700 tons fully loaded. It was used to launch NASA's Moon mission and the Skylab space station.

Second law of thermodynamics -- A fundamental law of energy exchange, one of whose formulations is "no process is possible whose only net effect is the flow of heat from a cold body to a hot one." A consequence of this is that in any system only part of the heat energy can be converted to other forms; the rest of the heat flows to lower temperature.

Semimajor axis -- a property of an ellipse, equal to half its greatest width, as measured along the line connecting its two foci. The semi-major axis of an orbital ellipse is one of the "orbital elements" characterizing it, and is directly related to the energy of the motion.

Shock--A sudden transition at the front of fast flow of plasma or gas, when that flow moves too fast for the undisturbed gas to move out of its way. Also occurs when a steady fast flow hits a magnetic or solid obstacle.

Solar activity A general term for those processes and changes on the Sun that rise and fall with the sunspot cycle, e.g. flares.

Solar cycle (or sunspot cycle)--an irregular cycle, averaging about 11 years in length, during which the number of sunspots (and of their associated outbursts) rises and then drops again. Like the sunspots, the cycle is probably magnetic in nature, and the polar magnetic field of the Sun also reverses each solar cycle.

Solar energetic particles--high energy particles occasionally emitted from active areas on the Sun, associated with solar flares and coronal mass ejections. The Earth's magnetic field keeps them out of regions close to Earth (except for the polar caps) but they can pose a hazard to space travelers far from Earth.

Solar wind -- A fast outflow of hot gas in all directions from the upper atmosphere of the Sun ("solar corona"), which is too hot to allow the Sun's gravity to hold on to its gas. Its composition matches that of the Sun's atmosphere (mostly hydrogen) and its typical velocity is 400 km/sec, covering the distance from Sun to Earth in 4-5 days. The solar wind confines the Earth's magnetic field inside a cavity known as the magnetosphere and supplies energy to phenomena in the magnetosphere such as polar aurora ("northern lights") and magnetic storms.

Solid fueled rockets -- Rockets which burn a solid mixture of fuel and oxidizer, and have no separation between combustion chamber and fuel reservoir. Gunpowder is such a mixture and was the earliest rocket fuel. They are somewhat less efficient than the best liquid fuel rockets, but are preferred for military use because they need no lengthy preparation and are easily stored in ready-to-fly condition. They are also used in auxiliary rockets that help heavily loaded liquid-fuel rockets (Space Shuttle, Delta) lift off and go through the first stage of their flight.

Solstice -- The time of the year when the Sun's position is the sky is most distant from the celestial equator. To a good approximation, north of the equator the day (around June 21) and the night (around December 21) are at their longest at the summer and winter solstices, and that is when those seasons are assumed to begin (the dates themselves, however, are known as midsummer day and midwinter day, respectively). Summer north of the equator coincides with winter south of it (and vice versa), and solstice names are also interchanged there.

Spectral line A narrow range of spectral color, emitted (or absorbed) by a specific atom (or molecule).The energy of its photon corresponds to the difference between two energy levels of the atom, and such photons are emitted when the atom "falls" from the higher level to the lower one.

Spectrum In the original meaning, the spread of colors seen in the rainbow, covering all pure colors the eye can see. Spectrum of a substance, e.g. of an atomic element, is the collection of spectral lines emitted by it.

Sputnik ("satellite") -- The first artificial Earth satellite, orbited by the Soviet Union on October 7, 1957, using Korolev's R-7 rocket.

Staging of a rocket -- The placing of smaller rockets on top of larger ones, increasing the lifting ability of the combined set-up.

Stellar evolution (stellar=of a star). The different phases in the lifetime of a star, from its formation out of gas and dust, to the time after its nuclear fuel is exhausted. Based on observations of stars at various stages of their evolution, astronomers have developed a general theory of stellar evolution, by which the Sun is a typical "main sequance" star, in the middle of its evolutionary lifespan

Sundial -- A device for telling time of day by the shadow which sunlight produces on the instrument. See gnomon.

Sunspot--An intensely magnetic area on the Sun's visible face. For unclear reasons, it is slightly cooler than the surrounding photosphere (perhaps because the magnetic field somehow interferes with the outflow of solar heat in that region) and therefore appears a bit darker. Sunspots tend to be associated with violent solar outbursts of various kinds.

Supernova (More accurately, type II supernova.) When a star burns up all its fuel, it collapses and the released gravitational energy blows off its top layers, creating a supernova explosion. What remains of the star depends on its mass. Low-mass stars crush their atoms and become white dwarfs, about as big as Earth. High mass stars collapse into black holes whose gravity prevents any light from escaping. Stars with masses between those extremes collapse into neutron stars, consisting of extreme dense nuclear matter held together by gravity and nuclear force, with a radius of the order of 10 km.

Sweepback--the angle by which the wing of an airplane is swept back, measured from the direction perpendicular to the fuselage.

Synchronous orbit -- The circular orbit above the equator at a distance of 6.6 Earth radii, in which a spacecraft has an orbital period of 24 hours. Such satellites stay above the same spot on Earth and are therefore ideally suited for transmitting communications and broadcasts.

Thrust--the force acting on a rocket or an airplane, produced by the action of its motor and pulling it forward. In an airplane, thrust is one of the four forces sensed by an airplane, the others being lift, drag and weight

Ultraviolet (UV)--electromagnetic radiation resembling visible light, but of shorter wavelength. UV cannot be seen by the eye, and much of it is absorbed by ozone, a variant of oxygen, at altitudes of 30-40 km. Satellite telescopes, however, can and do view stars and the Sun in UV, and even in the extreme UV (EUV), the range between UV and X-rays.

Unit vector A vector of unit length. Vectors have both magnitude and direction, but in some calculations it is convenient to separate the two. Denoting vector by an underline, a vector V can be represented by two factors multiplying each other, a unit vector Vu giving just the direction, and a magnitude V, i.e., the vector is V=VuV.

V2 -- Abbreviation of "Vergeltungwaffe 2" (vengeance weapon 2), a 12-ton German rocket carrying a 1-ton explosive charge, used in World War II, starting in 1944. The V2 had a range of around 200 miles, used a liquid-fuel rocket and was the first large military rocket.

Vector -- A quantity having both magnitude and direction, e.g. displacement, velocity, acceleration and force. Vectors are added when, for instance, one moves in a frame that itself is moving too (e.g. swims across a flowing river). Vectors are added like arrows, end to end, and the sum (for two) is the vector from the tail of the first vector to the tip of the second.

Vector resolution--The representation of a given vector as the sum of vectors in given directions. See componenet

Velocity -- Rate of position change, a vector quantity.

Vernal equinox -- The spring equinox. The term is also used for the point occupied by the Sun at that time, one of the two intersections on the celestial spher, between the ecliptic and the celestial equator. Also known as first point in Aries.

Watt -- Unit of power, the rate at which energy is supplied. One watt is the power which supplies 1 joule per second, 1 kilowatt = 1000 watts. A grown human climbing stairs (e.g.) supplies about 100 watt; 1 horsepower = 736 watt. Named for James Watt, inventor of the modern steam engine.

Wave A disturbance spreading in space, obeying a certain "wave equation." Sound waves, ocean waves and electromagnetic waves are some of the examples; other, more complicated types of waves can spread in plasmas.

Wavelength (Often denoted by l, the Greek letter letter lambda.) The distance between two crests of a propagating wave of a single frequency n . If v is the velocity at which the wave advances, v=ln.

Wave number A term used for the inverse of the wavelength, i.e. for 1/l

Weight -- The force exerted on mass by gravity.

Weightlessness (or "zero g") the condition when no force (such as weight) is sensed. Occurs in orbit or free fall, when gravity already produces its full acceleration and can produce no further effect.

X-1 -- A rocket-powered research airplane, the first to fly faster than sound, on 14 October 1947.

X-rays--electromagnetic waves of short wavelength, capable of penetrating some thickness of matter. Medical x-rays are produced by letting a stream of fast electrons come to a sudden stop at a metal plate; it is believed that X-rays emitted by the Sun or stars also come from fast electrons.

Zodiac -- Twelve constellations dividing the ecliptic into approximately equal parts. Each month the Sun is in a different constellation of the zodiac.

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